Efficacy of one or two doses of Ty21a Salmonella typhi vaccine in entericcoated capsules in a controlled field trial .t
Robert E. Black , Myron M. Levine*, Catterine Ferreccio *~, Mary Lou Clements *t, Claudio Lanata*, James Rooney*, Rene Germanier ~ and the Chilean Typhoid Committee ~ Typhoid fever remains an important public health problem in many areas o f the worm and an effective, non-reactogenic vaccine would be useful to control this disease. An attenuated Salmonella typhi strain ( Ty21a), which has shown promise in previous trials, was evaluated in a controlled field trial in Santiago, Chile. In this trial, 82 543 schoolchildren were randomly assigned to receive one or two doses o f Ty21a vaccine in enteric-coated capsules or placebo. The enteric-coated vaccine formulation was well tolerated and practical for mass oral immunization. In the first two years o f surveillance, 213 cases o f bacteriologically-confirmed typhoid fever were found in schoolchildren participating in the trial; annual rates in the placebo group were 139 and 227 per 100 000. Vaccine efficacy in the first two years after vaccination was 59% for two doses and 29% for one dose; no ejficacy was found 3-5 years after vaccination. These results indicate that it will be necessary to identify a vaccine formulation and schedule for Ty21a S. typhi that is practical and provides high level protection for > 2 years.
Keywords:Salmonella typhi; vaccine;enteric-coatedcapsule; typhoid fever Introduction Typhoid fever remains an important endemic disease in many countries. An effective vaccine is highly desirable for both indigenous populations and travellers to high risk areas 1'2. Currently available parenteral whole cell Salmonella typhi vaccines have been shown to provide 60-70% protection against typhoid fever in trials in endemic areas, but they commonly cause local and systemic reactions, limiting their acceptability 3. An attenuated S. typhi strain, Ty21a, has recently shown promise as an oral typhoid fever vaccine '~. Initial studies on North American volunteers demonstrated that the Ty21a vaccine was safe and provided substantial protection against induced typhoid fever 5. A placebocontrolled field trial in Egypt indicated that three doses, given after neutralization of gastric acid with sodium bicarbonate, were non-reactogenic and resulted in a high level of protective efficacy for at least 3 years 6. A commercial formulation that was marketed after the Egyptian field trial, consisting of two gelatine capsules of sodium bicarbonate and a third capsule of lyophilized vaccine, was evaluated in Chile, in comparison to an enteric-coated capsule containing the vaccine v. *Center for Vaccine Development, University of Maryland School of Medicine, Baltimore, MD, USA. tCurrent address: Department of International Health, School of Hygiene and Public Health, The Johns Hopkins University, Baltimore, MD, USA. tMinistry of Health, Santiago, Chile, South America. ~Swiss Serum and Vaccine Institute, Berne, Switzerland. ~'Drs A. Schuster, H. Rodriguez, J.M. Borgono, I. Prenzel, H. Lobos, Ministry of Health, Santiago, Chile, South America. (Received 22 July 1988; revised 12 July 1989; accepted 12 July 1989) 0264-410X/90/080081-O4$03.00 ,~') 199013utterworth & Co. (Publishers) Ltd
The enteric-coated vaccine provided significantly better efficacy (67% for at least 3 years) than the gelatine capsule formulation, thus reducing the logistical difficulties of administering the vaccine. In this trial, three doses of enteric-coated vaccine administered within 1 week provided protection equal to a schedule that increased the interval between doses to 21 days. To further simplify vaccine administration, it would be desirable to provide immunization with one or two doses of vaccine rather than with three. Thus, the protective efficacy of one or two doses of Ty21a S. typhi vaccine in enteric-coated capsules was evaluated in a large-scale placebo-controlled field trial.
Methods The vaccine consisted of lyophilized attenuated S. typhi, strain Ty21a. The vaccine was prepared and tested for purity, safety and potency by the Swiss Serum and Vaccine Institute, as previously described 4'6. For each vaccine dose, 2 - 5 x 109 viable Ty21a S. typhi were contained in a gelatine capsule and lacquered with a hydroxyprophy-methyl-cellulose-phthalate8. With this enteric-coating the capsules were able to withstand at least 2h in artificial gastric juice (pH 1.5), but would open within 10 rain in artificial intestinal fluid (pH > 6). The field trial was undertaken in Santiago, Chile where typhoid fever annual incidence rates exceeded 150 per 100000 for the previous 6 years and where more than half of the cases occur in children 5-19 years old 9-12. The Northern Area of Santiago with a population of 598 000 was selected because it had high reported rates of typhoid fever and a good health infrastructure.
Vaccine, Vol. 8, February 1990
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Ty21a S. typhi vaccine efficacy: R.E. Black et al.
Approximately 80% of the health visits in this area are to health facilities of the National Health Service. In early 1982, informational pamphlets explaining the vaccine trial and informed consent notices were sent to the parents of all schoolchildren (aged 5-22 years) in the area; parents returned signed consent notices if they wished their child to participate. The 3655 school classes in the area were randomized into three groups: one group to receive two doses o; the enteric-coated vaccine, one group to receive one dose of vaccine and one of placebo, and one group to receive two doses of placebo. The two doses were given 1 week apart. A placebo, identical in appearance, was provided and the bottles containing vaccine and placebo were marked with the codes A and B. On 26 May and 2 June 1982, the days of mass vaccination, lists of consenting children in assigned classrooms and bottles of vaccine or placebo were issued to 225 staff members of the Northern Area clinics, who served as vaccinators. Vaccine was carried to the schools in refrigerated containers and administered to the children by the vaccinators. Children were not permitted to eat for 1 h before or after vaccination. Children were not allowed to bite the capsules, but were instructed to swallow them with water. During June 1982, vaccinators provided the assigned vaccine or placebo to children who were absent on the two previous vaccination days. The vaccinators recorded the type of capsule (A or B) swallowed by the child on the class list and these lists were used to prepare a computer file of all schoolchildren in the area. This file was used to calculate the number of children by age who received one or two doses of vaccine or placebo. The vaccine received by the children was not available to study workers and investigators until one year of surveillance was completed. The code was broken at that point for a preliminary analysis by the investigators; however, throughout the entire study, the vaccine received by the children remained unaccessible to surveillance workers or clinicians in Santiago. After the vaccinations were completed, surveillance for typhoid fever was increased by enhancing the awareness of the disease among physicians and nurses at area health clinics and hospitals. This was done by conducting medical conferences at area hospitals, sending letters to private physicians and laboratories, and visiting each of the 13 Northern Area health centres and the Children's Hospital (Hospital Roberto del Rio), at least weekly, to review typhoid fever surveillance with the medical and nursing personnel. Furthermore, notices and identification cards were sent to the parents of all schoolchildren in the area, including those who refused vaccination, to inform them of the symptoms of typhoid fever and of the availability of free diagnostic services at the area clinics. Bacteriologic evaluation of suspected typhoid fever was intensified in the area by obtaining blood and bone marrow specimens from suspected typhoid fever patients for culture. Suspect cases seen as outpatients in the 13 area clinics had two blood cultures taken, at least 30 min apart. Patients with suspected typhoid fever admitted to the Hospital Roberto del Rio had three blood cultures and a bone marrow culture obtained before treatment was begun. Whole blood (3ml) and 1-2ml of bone marrow were inoculated into 50 ml of brain heart infusion broth with 0.05% sodium polyanetholsulphonate. Blood cultures from the clinics were transported daily to the bacteriology laboratory of Hospital Roberto del Rio for processing by standard methods 13't4. S. typhi (194
82
Vaccine, Vol. 8, February 1990
strains) isolated from children participating in the trial were phage typed by the Institute of Public Health in Santiago, Chile and kindly confirmed by the Central Public Health Laboratory, London, U K ~5. Only patients with enteric fever confirmed by isolation of S. typhi from blood or bone marrow during the five years following vaccination (1 July 1982 to 30 June 1987), were considered typhoid fever cases for this analysis. Patients with enteric fever and with blood or bone marrow positive for S. paratyphi B were considered cases of S. paratyphi B illness. The study was done in a double-blind manner with the code kept by the World Health Organization ( W H O ) in Geneva until the first analysis 1 year following vaccination. Results were analysed using the X2 test a6, analysis of log odds ratios ~7, and t test with variance calculated for cluster samples 18.
Results Of the 137697 available schoolchildren, 91 954 (67%), whose parents consented, participated in the vaccination and were randomized to one of the three vaccine groups. Of the participating children, 82 543 received all of the assigned doses and were included in the analysis (Table 1); 29% of the children were 5-9 years old, 40% 10-14 years and 31% > 14 years. The enteric-coated vaccine was well tolerated with no adverse effects noted. In the year following vaccination, 260 bacteriologicallyconfirmed cases of typhoid fever were identified in schoolchildren in the Northern Area; 139 of these were in children who received the assigned vaccines in the field trial. The latter cases were included in the analysis of vaccine efficacy. S. typhi isolated were of phage types usually found in Santiago (type 46, 41%, type El 31°/o) 19 and the frequency of various phage types was similar in all vaccine groups. Children in the placebo group had a typhoid fever incidence rate of 227 per 100 000 in the first year after vaccination. In comparison, children who received two doses or one dose of the oral vaccine had rates of 109 and 170, respectively (Table 1). Two doses of vaccine resulted in 52% efficacy and one dose in 25% efficacy in the year following vaccination. There was no difference in vaccine efficacy depending on the age of the child at the time of vaccination. In the second year after vaccination, the incidence of typhoid fever in the children who received placebo was 139 per 100 000 and the efficacy with two doses was 7 ! % and with one dose 35% (Table I). Taking together the first 2 years after vaccination, the efficacy with two doses was 59% (Confidence Interval [-CI] 41-71%, p 0 . 0 5
did not differ significantly from those in the placebo group.
Discussion This large-scale field evaluation of Ty21a S. typhi attenuated oral vaccine confirmed previous experience that the vaccine strain is safe and well tolerated 5-~. It further demonstrated that the enteric-coated formulation is highly practical and efficient for mass oral immunization of schoolchildren. Two doses of 109 Ty21a S. typhi vaccine in entericcoated capsules provided 59% protection from typhoid fever for 2 years after vaccination in a highly endemic setting, while one dose provided only 29% efficacy. However, the protection, even with two doses, was short-lived, dropping to insignificant levels by the third year after vaccination. Two doses of the vaccine also appeared to confer some protection against enteric fever caused by S. paratyphi B; a similar finding of partial protection against paratyphoid fever was noted in the Ty21a field trial in the western area of Santiago 7. It is known that immunity to Ty2 la is dependent on a smooth lipopolysaccharide antigen 4"5. Since S. paratyphi B shares O antigen 12 with S. typhi 2°, the immune response to
this antigen of Ty21a S. typhi may explain the cross protection against S. paratyphi B. The level of protection against typhoid fever from two doses of the vaccine during the first 24 months of surveillance in this field trial is less than that observed in the Egyptian field trial of the vaccine 6, but not substantially different from the 67% efficacy found with three doses of enteric-coated vaccine in Santiago 7. However, the duration of protection is clearly shorter than that found in the trial in the western area of Santiago, where the same level of efficacy has now been shown to persist for at least 4 years 2t. The long duration of high-level protection has been explained by a presumed activation of cell-mediated immunity, the mechanism by which attenuated Salmonella vaccines are believed to protect against pathogenic Salmonella, an intracellular pathogen 2~. The relatively short-lived protection in this trial suggests that the cellular immune system may not have been suffi~,iently stimulated, and perhaps other mechanisms, such as SIgA intestinal antibodies may have been operative. Although volunteer studies and several field trials have established the biological activity of Ty21a live oral typhoid fever vaccine, additional field trials will be needed to identify a practical vaccine formulation and schedule
V a c c i n e , Vol. 8, F e b r u a r y 1990
83
Ty21a S. b/phi vaccine efficacy: R.E. Black et al.
that improves both the level and duration of protection. The higher efficacy found in the Egypt field trial suggests that ingestion of a liquid vaccine (reconstituted lyophilate) after appropriate neutralization of gastric acid may be a more effective method for giving the vaccine. Further studies of similar formulations in areas with endemic typhoid fever are indicated. However, it remains to be demonstrated if fewer than three doses of this formulation will provide protection of sufficient duration and, thus, enhance the role of this vaccine in public health efforts to control typhoid fever.
6
7
8 9 10
Acknowledgements The authors are grateful for the assistance of the field trial staff, including Viviana Sotomayor, Leonor Atroza, Gloria Berrios, Cecilia Rivera, Maria Rosa Aguirre and Irma Canepa and to the health workers and laboratory personnel of the Northern Area, Santiago. The field trial was supported by grants from the World Health Organization, the Pan American Health Organization and by a contract from the US Army Medical Research and Development Command (DAMD 17-C-1115). Informed consent was obtained from the parents of all participating children. The study was approved by the ethical review committees of the University of Maryland School of Medicine, the World Health Organization, the Pan American Health Organization, and the Ministry of Health, Santiago, Chile.
11
12
13
14
15
16 17
References 1
2
3
4
5
84
Institute of Medicine. The prospects for immunizing against Salmonella typhi. In: New Vaccine Development: Establishing Priorities. Volume II. Diseases of Importance in Developing Countries. National Academy Press, Washington, DC, 1986, p. 319 Taylor, D.N., Pollard, R.A. and Blake, P.A. Typhoid in the United States and the risk to the international traveler. J. Infect. Dis. 1983, 148, 599 Ashcroft, M.T., Morrison, R.J. and Nicholson, C.C. Controlled field trial in British Guyana school children of heat-killed-phenolized and acetone-killed-lyophilizedtyphoid vaccines. Am. J. Hyg. 1964,79, 196 Germanier, R. and Furer, E. Isolation and characterization of Gal E mutant Ty21a of Salmonella typhi: a candidate strain for a live, oral typhoid vaccine. J. Infect. Dis. 1975, 131,553 Gilman, R.H., Hornick, R.B., Woodward, W.E., DuPont, H.L., Snyder, M.J., Levine, M.M. and Libonati, J.P. Evaluation of a UDP-glucose-
Vaccine, Vol. 8, F e b r u a r y 1990
18
19
20 21
22
4-epimeraseless mutant of Salmonella typhi as a live oral vaccine. J. Infect. Dis. 1977, 136, 717 Wahdan, M.H., Serie, C,, Cerisier, Y., Sallam, S. and Germanier, R. A controlled field trial of live Salmonella typhi strain Ty21a oral vaccine against typhoid: three-year results. J. Infect. Dis. 1982, 145, 292 Levine, M.M., Ferreccio, C., Black, R.E. and Germanier, R. and Chilean Typhoid Committee. Large-scale field trial of Ty21a live oral typhoid vaccine in enteric-coated capsule formulation. Lancet 1987, i, 1050 Osal, A. (Ed.) Remington's Pharmaceutical Sciences. 16th Edn. Mack Publishing Co., Easton, PA, 1980, p. 1590 Ristori, C. Epidemiologia de la fiebre tifoidea en Chile. Bol de Vigilancia Epidemiol de Ministerio de Salud, Chile, 1981, 8, 8 Medina, E. and Yrarrazaval, M. Fiebre tifoidea en Chile: consideraciones epidemiologicas. Rev. Med. Chile 1983, 111,609 Ferreccio, C., Levine, M.M., Mantero~a, A., Rodriguez, G., Rivara, I., Prenzel, I. et al. Benign bacteremia caused by Salmonella typhi and paratyphi in children younger than two years. J. Pediat. 1984, 104, 899 Levine, M.M., Black, R.E., Ferreccio, C., Clements, M.L., Lanata, C., Sears, S. et al. Interventions to Control Endemic Typhoid Fever: Field Studies in Santiago, Chile. PAHO Co-publication Series 41=1. PAHO, Washington, DC, 1985, p. 37 laenberg, H.D., Washington, J.A., Balows, A. and Sonnenwirth, A.C. Collection, handling and processing of specimens. In: Manual of Clinical Microbiology (Eds Lennette, E,, Balows, A., Hausler, W.J. and Truant, J.P.) Am. Soc. Microbiol., Washington, DC, 1980, Ch. 6 Martin, W.J. and Washington, J.A. Enterobacteriaceae. In: Manual of Clinical Microbiology (Eds Lennette, E., Balows, A., Hausler, W.J. and Truant, J.P.) Am. Soc. Microbiol., Washington, DC, 1980, Ch. 6 Anderson, E.S. and Williams, R.E.O. Bacteriophage typing of enteric pathogens and staphylococci and its use in epidemiology. J. Clin. Path. 1956, 9, 94 Snedecor, G.W. and Cochran, W.F. Statistical Methods. The Iowa State University Press, Ames, 1980 Fleiss, J.L. Statistical Methods for Rates and Proportions. John Wiley & Sons, New York, 1973, p.46 Serfling, R.E. and Sherman, I.L. Attribute Sampling Methods for Local Health Departments. US Government Printing Office, Washington, DC, 1965, p. 115 International Federation for Enteric Phage-typing (WEPT). The geographical distribution of Salmonella typhi and Salmonella paratyphi A and B phage types during the period 1 January 1970 to 31 December 1973. J. Hyg. Camb. 1982, 88, 231 Edwards, P.R. Kauffmann-White Schema Center for Disease Control, Atlanta, GA, 1963 Levine, M.M., Ferreccio, C., Black, R.E, Tacket, C., Germanier, R. and Chilean Typhoid Committee. Progress in vaccines against typhoid fever. Rev. Infect. Dis. 1989, 11, $552 Lindberg, A.A. and Robertsson, J.A. Salmonella typhimurium infection in calves: cell-mediated and humoral immune reactions before and after challenge with live virulent bacteria in calves given live or inactivated vaccines. Infect. Immun. 1983, 41,751
Robert E. Black , Myron M. Levine*, Catterine Ferreccio *~, Mary Lou Clements *t, Claudio Lanata*, James Rooney*, Rene Germanier ~ and the Chilean Typhoid Committee ~ Typhoid fever remains an important public health problem in many areas o f the worm and an effective, non-reactogenic vaccine would be useful to control this disease. An attenuated Salmonella typhi strain ( Ty21a), which has shown promise in previous trials, was evaluated in a controlled field trial in Santiago, Chile. In this trial, 82 543 schoolchildren were randomly assigned to receive one or two doses o f Ty21a vaccine in enteric-coated capsules or placebo. The enteric-coated vaccine formulation was well tolerated and practical for mass oral immunization. In the first two years o f surveillance, 213 cases o f bacteriologically-confirmed typhoid fever were found in schoolchildren participating in the trial; annual rates in the placebo group were 139 and 227 per 100 000. Vaccine efficacy in the first two years after vaccination was 59% for two doses and 29% for one dose; no ejficacy was found 3-5 years after vaccination. These results indicate that it will be necessary to identify a vaccine formulation and schedule for Ty21a S. typhi that is practical and provides high level protection for > 2 years.
Keywords:Salmonella typhi; vaccine;enteric-coatedcapsule; typhoid fever Introduction Typhoid fever remains an important endemic disease in many countries. An effective vaccine is highly desirable for both indigenous populations and travellers to high risk areas 1'2. Currently available parenteral whole cell Salmonella typhi vaccines have been shown to provide 60-70% protection against typhoid fever in trials in endemic areas, but they commonly cause local and systemic reactions, limiting their acceptability 3. An attenuated S. typhi strain, Ty21a, has recently shown promise as an oral typhoid fever vaccine '~. Initial studies on North American volunteers demonstrated that the Ty21a vaccine was safe and provided substantial protection against induced typhoid fever 5. A placebocontrolled field trial in Egypt indicated that three doses, given after neutralization of gastric acid with sodium bicarbonate, were non-reactogenic and resulted in a high level of protective efficacy for at least 3 years 6. A commercial formulation that was marketed after the Egyptian field trial, consisting of two gelatine capsules of sodium bicarbonate and a third capsule of lyophilized vaccine, was evaluated in Chile, in comparison to an enteric-coated capsule containing the vaccine v. *Center for Vaccine Development, University of Maryland School of Medicine, Baltimore, MD, USA. tCurrent address: Department of International Health, School of Hygiene and Public Health, The Johns Hopkins University, Baltimore, MD, USA. tMinistry of Health, Santiago, Chile, South America. ~Swiss Serum and Vaccine Institute, Berne, Switzerland. ~'Drs A. Schuster, H. Rodriguez, J.M. Borgono, I. Prenzel, H. Lobos, Ministry of Health, Santiago, Chile, South America. (Received 22 July 1988; revised 12 July 1989; accepted 12 July 1989) 0264-410X/90/080081-O4$03.00 ,~') 199013utterworth & Co. (Publishers) Ltd
The enteric-coated vaccine provided significantly better efficacy (67% for at least 3 years) than the gelatine capsule formulation, thus reducing the logistical difficulties of administering the vaccine. In this trial, three doses of enteric-coated vaccine administered within 1 week provided protection equal to a schedule that increased the interval between doses to 21 days. To further simplify vaccine administration, it would be desirable to provide immunization with one or two doses of vaccine rather than with three. Thus, the protective efficacy of one or two doses of Ty21a S. typhi vaccine in enteric-coated capsules was evaluated in a large-scale placebo-controlled field trial.
Methods The vaccine consisted of lyophilized attenuated S. typhi, strain Ty21a. The vaccine was prepared and tested for purity, safety and potency by the Swiss Serum and Vaccine Institute, as previously described 4'6. For each vaccine dose, 2 - 5 x 109 viable Ty21a S. typhi were contained in a gelatine capsule and lacquered with a hydroxyprophy-methyl-cellulose-phthalate8. With this enteric-coating the capsules were able to withstand at least 2h in artificial gastric juice (pH 1.5), but would open within 10 rain in artificial intestinal fluid (pH > 6). The field trial was undertaken in Santiago, Chile where typhoid fever annual incidence rates exceeded 150 per 100000 for the previous 6 years and where more than half of the cases occur in children 5-19 years old 9-12. The Northern Area of Santiago with a population of 598 000 was selected because it had high reported rates of typhoid fever and a good health infrastructure.
Vaccine, Vol. 8, February 1990
81
Ty21a S. typhi vaccine efficacy: R.E. Black et al.
Approximately 80% of the health visits in this area are to health facilities of the National Health Service. In early 1982, informational pamphlets explaining the vaccine trial and informed consent notices were sent to the parents of all schoolchildren (aged 5-22 years) in the area; parents returned signed consent notices if they wished their child to participate. The 3655 school classes in the area were randomized into three groups: one group to receive two doses o; the enteric-coated vaccine, one group to receive one dose of vaccine and one of placebo, and one group to receive two doses of placebo. The two doses were given 1 week apart. A placebo, identical in appearance, was provided and the bottles containing vaccine and placebo were marked with the codes A and B. On 26 May and 2 June 1982, the days of mass vaccination, lists of consenting children in assigned classrooms and bottles of vaccine or placebo were issued to 225 staff members of the Northern Area clinics, who served as vaccinators. Vaccine was carried to the schools in refrigerated containers and administered to the children by the vaccinators. Children were not permitted to eat for 1 h before or after vaccination. Children were not allowed to bite the capsules, but were instructed to swallow them with water. During June 1982, vaccinators provided the assigned vaccine or placebo to children who were absent on the two previous vaccination days. The vaccinators recorded the type of capsule (A or B) swallowed by the child on the class list and these lists were used to prepare a computer file of all schoolchildren in the area. This file was used to calculate the number of children by age who received one or two doses of vaccine or placebo. The vaccine received by the children was not available to study workers and investigators until one year of surveillance was completed. The code was broken at that point for a preliminary analysis by the investigators; however, throughout the entire study, the vaccine received by the children remained unaccessible to surveillance workers or clinicians in Santiago. After the vaccinations were completed, surveillance for typhoid fever was increased by enhancing the awareness of the disease among physicians and nurses at area health clinics and hospitals. This was done by conducting medical conferences at area hospitals, sending letters to private physicians and laboratories, and visiting each of the 13 Northern Area health centres and the Children's Hospital (Hospital Roberto del Rio), at least weekly, to review typhoid fever surveillance with the medical and nursing personnel. Furthermore, notices and identification cards were sent to the parents of all schoolchildren in the area, including those who refused vaccination, to inform them of the symptoms of typhoid fever and of the availability of free diagnostic services at the area clinics. Bacteriologic evaluation of suspected typhoid fever was intensified in the area by obtaining blood and bone marrow specimens from suspected typhoid fever patients for culture. Suspect cases seen as outpatients in the 13 area clinics had two blood cultures taken, at least 30 min apart. Patients with suspected typhoid fever admitted to the Hospital Roberto del Rio had three blood cultures and a bone marrow culture obtained before treatment was begun. Whole blood (3ml) and 1-2ml of bone marrow were inoculated into 50 ml of brain heart infusion broth with 0.05% sodium polyanetholsulphonate. Blood cultures from the clinics were transported daily to the bacteriology laboratory of Hospital Roberto del Rio for processing by standard methods 13't4. S. typhi (194
82
Vaccine, Vol. 8, February 1990
strains) isolated from children participating in the trial were phage typed by the Institute of Public Health in Santiago, Chile and kindly confirmed by the Central Public Health Laboratory, London, U K ~5. Only patients with enteric fever confirmed by isolation of S. typhi from blood or bone marrow during the five years following vaccination (1 July 1982 to 30 June 1987), were considered typhoid fever cases for this analysis. Patients with enteric fever and with blood or bone marrow positive for S. paratyphi B were considered cases of S. paratyphi B illness. The study was done in a double-blind manner with the code kept by the World Health Organization ( W H O ) in Geneva until the first analysis 1 year following vaccination. Results were analysed using the X2 test a6, analysis of log odds ratios ~7, and t test with variance calculated for cluster samples 18.
Results Of the 137697 available schoolchildren, 91 954 (67%), whose parents consented, participated in the vaccination and were randomized to one of the three vaccine groups. Of the participating children, 82 543 received all of the assigned doses and were included in the analysis (Table 1); 29% of the children were 5-9 years old, 40% 10-14 years and 31% > 14 years. The enteric-coated vaccine was well tolerated with no adverse effects noted. In the year following vaccination, 260 bacteriologicallyconfirmed cases of typhoid fever were identified in schoolchildren in the Northern Area; 139 of these were in children who received the assigned vaccines in the field trial. The latter cases were included in the analysis of vaccine efficacy. S. typhi isolated were of phage types usually found in Santiago (type 46, 41%, type El 31°/o) 19 and the frequency of various phage types was similar in all vaccine groups. Children in the placebo group had a typhoid fever incidence rate of 227 per 100 000 in the first year after vaccination. In comparison, children who received two doses or one dose of the oral vaccine had rates of 109 and 170, respectively (Table 1). Two doses of vaccine resulted in 52% efficacy and one dose in 25% efficacy in the year following vaccination. There was no difference in vaccine efficacy depending on the age of the child at the time of vaccination. In the second year after vaccination, the incidence of typhoid fever in the children who received placebo was 139 per 100 000 and the efficacy with two doses was 7 ! % and with one dose 35% (Table I). Taking together the first 2 years after vaccination, the efficacy with two doses was 59% (Confidence Interval [-CI] 41-71%, p 0 . 0 5
did not differ significantly from those in the placebo group.
Discussion This large-scale field evaluation of Ty21a S. typhi attenuated oral vaccine confirmed previous experience that the vaccine strain is safe and well tolerated 5-~. It further demonstrated that the enteric-coated formulation is highly practical and efficient for mass oral immunization of schoolchildren. Two doses of 109 Ty21a S. typhi vaccine in entericcoated capsules provided 59% protection from typhoid fever for 2 years after vaccination in a highly endemic setting, while one dose provided only 29% efficacy. However, the protection, even with two doses, was short-lived, dropping to insignificant levels by the third year after vaccination. Two doses of the vaccine also appeared to confer some protection against enteric fever caused by S. paratyphi B; a similar finding of partial protection against paratyphoid fever was noted in the Ty21a field trial in the western area of Santiago 7. It is known that immunity to Ty2 la is dependent on a smooth lipopolysaccharide antigen 4"5. Since S. paratyphi B shares O antigen 12 with S. typhi 2°, the immune response to
this antigen of Ty21a S. typhi may explain the cross protection against S. paratyphi B. The level of protection against typhoid fever from two doses of the vaccine during the first 24 months of surveillance in this field trial is less than that observed in the Egyptian field trial of the vaccine 6, but not substantially different from the 67% efficacy found with three doses of enteric-coated vaccine in Santiago 7. However, the duration of protection is clearly shorter than that found in the trial in the western area of Santiago, where the same level of efficacy has now been shown to persist for at least 4 years 2t. The long duration of high-level protection has been explained by a presumed activation of cell-mediated immunity, the mechanism by which attenuated Salmonella vaccines are believed to protect against pathogenic Salmonella, an intracellular pathogen 2~. The relatively short-lived protection in this trial suggests that the cellular immune system may not have been suffi~,iently stimulated, and perhaps other mechanisms, such as SIgA intestinal antibodies may have been operative. Although volunteer studies and several field trials have established the biological activity of Ty21a live oral typhoid fever vaccine, additional field trials will be needed to identify a practical vaccine formulation and schedule
V a c c i n e , Vol. 8, F e b r u a r y 1990
83
Ty21a S. b/phi vaccine efficacy: R.E. Black et al.
that improves both the level and duration of protection. The higher efficacy found in the Egypt field trial suggests that ingestion of a liquid vaccine (reconstituted lyophilate) after appropriate neutralization of gastric acid may be a more effective method for giving the vaccine. Further studies of similar formulations in areas with endemic typhoid fever are indicated. However, it remains to be demonstrated if fewer than three doses of this formulation will provide protection of sufficient duration and, thus, enhance the role of this vaccine in public health efforts to control typhoid fever.
6
7
8 9 10
Acknowledgements The authors are grateful for the assistance of the field trial staff, including Viviana Sotomayor, Leonor Atroza, Gloria Berrios, Cecilia Rivera, Maria Rosa Aguirre and Irma Canepa and to the health workers and laboratory personnel of the Northern Area, Santiago. The field trial was supported by grants from the World Health Organization, the Pan American Health Organization and by a contract from the US Army Medical Research and Development Command (DAMD 17-C-1115). Informed consent was obtained from the parents of all participating children. The study was approved by the ethical review committees of the University of Maryland School of Medicine, the World Health Organization, the Pan American Health Organization, and the Ministry of Health, Santiago, Chile.
11
12
13
14
15
16 17
References 1
2
3
4
5
84
Institute of Medicine. The prospects for immunizing against Salmonella typhi. In: New Vaccine Development: Establishing Priorities. Volume II. Diseases of Importance in Developing Countries. National Academy Press, Washington, DC, 1986, p. 319 Taylor, D.N., Pollard, R.A. and Blake, P.A. Typhoid in the United States and the risk to the international traveler. J. Infect. Dis. 1983, 148, 599 Ashcroft, M.T., Morrison, R.J. and Nicholson, C.C. Controlled field trial in British Guyana school children of heat-killed-phenolized and acetone-killed-lyophilizedtyphoid vaccines. Am. J. Hyg. 1964,79, 196 Germanier, R. and Furer, E. Isolation and characterization of Gal E mutant Ty21a of Salmonella typhi: a candidate strain for a live, oral typhoid vaccine. J. Infect. Dis. 1975, 131,553 Gilman, R.H., Hornick, R.B., Woodward, W.E., DuPont, H.L., Snyder, M.J., Levine, M.M. and Libonati, J.P. Evaluation of a UDP-glucose-
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18
19
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4-epimeraseless mutant of Salmonella typhi as a live oral vaccine. J. Infect. Dis. 1977, 136, 717 Wahdan, M.H., Serie, C,, Cerisier, Y., Sallam, S. and Germanier, R. A controlled field trial of live Salmonella typhi strain Ty21a oral vaccine against typhoid: three-year results. J. Infect. Dis. 1982, 145, 292 Levine, M.M., Ferreccio, C., Black, R.E. and Germanier, R. and Chilean Typhoid Committee. Large-scale field trial of Ty21a live oral typhoid vaccine in enteric-coated capsule formulation. Lancet 1987, i, 1050 Osal, A. (Ed.) Remington's Pharmaceutical Sciences. 16th Edn. Mack Publishing Co., Easton, PA, 1980, p. 1590 Ristori, C. Epidemiologia de la fiebre tifoidea en Chile. Bol de Vigilancia Epidemiol de Ministerio de Salud, Chile, 1981, 8, 8 Medina, E. and Yrarrazaval, M. Fiebre tifoidea en Chile: consideraciones epidemiologicas. Rev. Med. Chile 1983, 111,609 Ferreccio, C., Levine, M.M., Mantero~a, A., Rodriguez, G., Rivara, I., Prenzel, I. et al. Benign bacteremia caused by Salmonella typhi and paratyphi in children younger than two years. J. Pediat. 1984, 104, 899 Levine, M.M., Black, R.E., Ferreccio, C., Clements, M.L., Lanata, C., Sears, S. et al. Interventions to Control Endemic Typhoid Fever: Field Studies in Santiago, Chile. PAHO Co-publication Series 41=1. PAHO, Washington, DC, 1985, p. 37 laenberg, H.D., Washington, J.A., Balows, A. and Sonnenwirth, A.C. Collection, handling and processing of specimens. In: Manual of Clinical Microbiology (Eds Lennette, E,, Balows, A., Hausler, W.J. and Truant, J.P.) Am. Soc. Microbiol., Washington, DC, 1980, Ch. 6 Martin, W.J. and Washington, J.A. Enterobacteriaceae. In: Manual of Clinical Microbiology (Eds Lennette, E., Balows, A., Hausler, W.J. and Truant, J.P.) Am. Soc. Microbiol., Washington, DC, 1980, Ch. 6 Anderson, E.S. and Williams, R.E.O. Bacteriophage typing of enteric pathogens and staphylococci and its use in epidemiology. J. Clin. Path. 1956, 9, 94 Snedecor, G.W. and Cochran, W.F. Statistical Methods. The Iowa State University Press, Ames, 1980 Fleiss, J.L. Statistical Methods for Rates and Proportions. John Wiley & Sons, New York, 1973, p.46 Serfling, R.E. and Sherman, I.L. Attribute Sampling Methods for Local Health Departments. US Government Printing Office, Washington, DC, 1965, p. 115 International Federation for Enteric Phage-typing (WEPT). The geographical distribution of Salmonella typhi and Salmonella paratyphi A and B phage types during the period 1 January 1970 to 31 December 1973. J. Hyg. Camb. 1982, 88, 231 Edwards, P.R. Kauffmann-White Schema Center for Disease Control, Atlanta, GA, 1963 Levine, M.M., Ferreccio, C., Black, R.E, Tacket, C., Germanier, R. and Chilean Typhoid Committee. Progress in vaccines against typhoid fever. Rev. Infect. Dis. 1989, 11, $552 Lindberg, A.A. and Robertsson, J.A. Salmonella typhimurium infection in calves: cell-mediated and humoral immune reactions before and after challenge with live virulent bacteria in calves given live or inactivated vaccines. Infect. Immun. 1983, 41,751
