Travel Medicine and Infectious Disease (2014) 12, 3e4
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevierhealth.com/journals/tmid
EDITORIAL
Looking for TB in the sky: Money well spent? Since the 1990s, a handful of cases of airline passengers traveling with tuberculosis, including drug-resistant forms, have generated international headlines (“Killer TB Patient Sorry for Flying Home”) [1e3]. However, there are reasons to believe that the risk of acquiring tuberculosis during air travel is actually low, even when the exposure is to a person with extensive pulmonary disease. When exposed to a person with untreated active tuberculosis, the probability of becoming infected is mostly determined by three factors: duration of exposure, infectiousness of the source case, and the environment where exposure occurs [4]. The infectiousness of the case is determined by the radiologic and microbiologic extent of the disease, including the presence of laryngeal involvement, and is reduced by effective treatment or wearing a mask [4]. The likelihood of infection is determined by the concentration of airborne tuberculosis organisms; anything that removes viable organisms reduces that risk. Hence ventilation or ultraviolet light will reduce airborne concentrations of the tuberculosis-containing droplets generated by an active case [5]. Wearing personal protective masks or respirators can also lower the risk of acquiring infection, but is not considered here as few travelers fly with N95 respirators. Commercial jet aircraft are well-ventilated. When cruising, the air exchanges per hour are higher than the minimum recommended by the CDC for tuberculosis infection control purposes in the majority of health-care settings and will remove 99.9% of airborne tuberculosis organisms within 20 min [6,7]. Furthermore, the ventilation systems of commercial jet aircraft are designed such that air flow in the passenger cabin is predominantly top-to-bottom, with minimum air movement between rows [6]. These high levels of ventilation of commercial aircraft should substantially lower the risk of transmission from a passenger with active tuberculosis to other passengers. Given the low risk of tuberculosis transmission during air travel, it is reasonable to assess the value of tuberculosis contact investigations (TBCI) of airline passengers. Two papers. In this issue of Travel Medicine and Infectious Disease address the economics of United States tuberculosis airline contact investigation policies [8,9] Marienau and
colleagues used results of 131 air travel associated TBCI conducted by the CDC between 2007 and 2009, in which outcomes data were available for 758 passenger-contacts, to estimate lower and upper limits of transmission risk. They used this data to compare the potential yield, risks, benefits and costs of the protocol for flight-related TBCI used by the U.S. Centers for Disease Control (CDC) up to 2011, with the protocol adopted in 2011. The current protocol restricts flight-related TBCI for drug-susceptible tuberculosis to cases that are both smear and cavitypositive. In their analysis of the CDC dataset, the proportion of passengers with suspected flight-related transmission events (i.e. those with new latent tuberculosis infection, ‘LTBI’) was not higher in the sub-group of contacts of smear-positive cavitary cases. This held true even if inflight transmission was defined only as contacts with tuberculin skin test conversions. This could have been because some of the LTBI detected was not due to in-flight transmission, but rather represented pre-existing LTBI, especially likely for travelers who were older and/or foreign born. Misclassification of pre-existing LTBI as due to in-flight transmission would reduce the potential to find differences in its prevalence between travelers exposed to smear and cavity-positive cases and less contagious cases, so that only large differences would be detectable. The fact that no difference could be detected at all, suggests the risk of in-flight transmission was low even for smear and cavity-positive tuberculosis. The authors also demonstrated that if they assumed the upper limit of transmission risk, both 2008 and 2011 protocols would be cost-effective, at all assumed levels of cost for contact investigation, and rate of progression to active tuberculosis. However, these results are not very informative, as the true average in-flight transmission risk is unlikely to be close to this upper limit [10]. When the lower estimate of in-flight transmission risk was used, neither protocol was cost-effective under most assumptions of cost for contact investigations, and risk of progression to active disease. As a result of these findings, the authors concluded the restricted protocol adopted by the CDC in 2011 is justified
1477-8939/$ - see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tmaid.2013.12.001
4 and appropriate. Erring on the side of caution, the 2011 protocol allows for investigating contacts of smearnegative or non-cavitary passengers when there are reasons to suspect a higher degree of infectiousness [11]. It may also be prudent to pursue investigations in situations where boarded passengers remain grounded for a substantial amount of time, as ventilation is much lower prior to flight [10]. Based on the authors’ analyses, it appears the restricted protocol will result in a negligible number of unidentified cases from in-flight transmission. The newer protocol is also financially sound, as it should reduce expenditures on airline passenger TBCI by 50%. These savings can be reinvested in other, higher impact public health interventions.
References [1] Luscombe R. Killer tuberculosis patient sorry for flying home. Guardian June 1, 2007. [2] Kolata G. First documented case of tuberculosis passed on airliner is reported by the U.S. The New York Times; March 15, 1995. [3] McKay B. Dangerous tuberculosis patient detained on U.S. Border. Wall St J March 1, 2013. [4] Rieder HL, International Union against Tuberculosis and Lung Disease. Epidemiologic basis of tuberculosis control. Paris: International Union Against Tuberculosis and Lung Disease; 1999. [5] Nardell EA. The role of ventilation in preventing nosocomial transmission of tuberculosis. Int J Tuberc Lung Dis 1998 Sep; 2(9 Suppl. 1):S110e7.
Editorial [6] World Health Organization. Tuberculosis and air travel: guidelines for prevention and control. 3rd ed. Geneva: World Health Organization; 2008. [7] Jensen PA, Lambert LA, Iademarco MF, Ridzon R, CDC. Guidelines for preventing the transmission of Mycobacterium tuberculosis in health-care settings, 2005. MMWR Recomm Rep 2005 Dec 30;54(RR-17):1e141. [8] Marienau KJ, Cramer EH, Coleman MS, Marano N, Cetron MS. Flight related tuberculosis contact investigations in the United States: comparative risk and economic analysis of alternate protocols. Travel Med Infect Dis 2014 Oct 8;12(1):54e62. [9] Coleman MS, Marienau KJ, Marano N, Marks SM, Cetron MS. Economics of United States tuberculosis airline contact investigation policies: a return on investment analysis. Travel Med Infect Dis 2013. 10.016. [10] Ko G, Thompson KM, Nardell EA. Estimation of tuberculosis risk on a commercial airliner. Risk Anal 2004 Apr;24(2): 379e88. [11] Public health interventions involving travelers with tuberculosiseU.S. ports of entry, 2007e2012. MMWR Morb Mortal Wkly Rep 2012 Aug 3;61(30):570e3.
Faiz Ahmad Khan Dick Menzies* Montreal Chest Institute, McGill University, Canada *Corresponding author. Montreal Chest Institute, Room K1.24, 3650 St. Urbain St., Montreal, PQ H2X 2P4, Canada. Tel.: þ1 514 934 1934x32128, þ1 514 934 1934x32129; fax: þ1 514 843 2083. E-mail address: [email protected] (D. Menzies)
4 December 2013
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevierhealth.com/journals/tmid
EDITORIAL
Looking for TB in the sky: Money well spent? Since the 1990s, a handful of cases of airline passengers traveling with tuberculosis, including drug-resistant forms, have generated international headlines (“Killer TB Patient Sorry for Flying Home”) [1e3]. However, there are reasons to believe that the risk of acquiring tuberculosis during air travel is actually low, even when the exposure is to a person with extensive pulmonary disease. When exposed to a person with untreated active tuberculosis, the probability of becoming infected is mostly determined by three factors: duration of exposure, infectiousness of the source case, and the environment where exposure occurs [4]. The infectiousness of the case is determined by the radiologic and microbiologic extent of the disease, including the presence of laryngeal involvement, and is reduced by effective treatment or wearing a mask [4]. The likelihood of infection is determined by the concentration of airborne tuberculosis organisms; anything that removes viable organisms reduces that risk. Hence ventilation or ultraviolet light will reduce airborne concentrations of the tuberculosis-containing droplets generated by an active case [5]. Wearing personal protective masks or respirators can also lower the risk of acquiring infection, but is not considered here as few travelers fly with N95 respirators. Commercial jet aircraft are well-ventilated. When cruising, the air exchanges per hour are higher than the minimum recommended by the CDC for tuberculosis infection control purposes in the majority of health-care settings and will remove 99.9% of airborne tuberculosis organisms within 20 min [6,7]. Furthermore, the ventilation systems of commercial jet aircraft are designed such that air flow in the passenger cabin is predominantly top-to-bottom, with minimum air movement between rows [6]. These high levels of ventilation of commercial aircraft should substantially lower the risk of transmission from a passenger with active tuberculosis to other passengers. Given the low risk of tuberculosis transmission during air travel, it is reasonable to assess the value of tuberculosis contact investigations (TBCI) of airline passengers. Two papers. In this issue of Travel Medicine and Infectious Disease address the economics of United States tuberculosis airline contact investigation policies [8,9] Marienau and
colleagues used results of 131 air travel associated TBCI conducted by the CDC between 2007 and 2009, in which outcomes data were available for 758 passenger-contacts, to estimate lower and upper limits of transmission risk. They used this data to compare the potential yield, risks, benefits and costs of the protocol for flight-related TBCI used by the U.S. Centers for Disease Control (CDC) up to 2011, with the protocol adopted in 2011. The current protocol restricts flight-related TBCI for drug-susceptible tuberculosis to cases that are both smear and cavitypositive. In their analysis of the CDC dataset, the proportion of passengers with suspected flight-related transmission events (i.e. those with new latent tuberculosis infection, ‘LTBI’) was not higher in the sub-group of contacts of smear-positive cavitary cases. This held true even if inflight transmission was defined only as contacts with tuberculin skin test conversions. This could have been because some of the LTBI detected was not due to in-flight transmission, but rather represented pre-existing LTBI, especially likely for travelers who were older and/or foreign born. Misclassification of pre-existing LTBI as due to in-flight transmission would reduce the potential to find differences in its prevalence between travelers exposed to smear and cavity-positive cases and less contagious cases, so that only large differences would be detectable. The fact that no difference could be detected at all, suggests the risk of in-flight transmission was low even for smear and cavity-positive tuberculosis. The authors also demonstrated that if they assumed the upper limit of transmission risk, both 2008 and 2011 protocols would be cost-effective, at all assumed levels of cost for contact investigation, and rate of progression to active tuberculosis. However, these results are not very informative, as the true average in-flight transmission risk is unlikely to be close to this upper limit [10]. When the lower estimate of in-flight transmission risk was used, neither protocol was cost-effective under most assumptions of cost for contact investigations, and risk of progression to active disease. As a result of these findings, the authors concluded the restricted protocol adopted by the CDC in 2011 is justified
1477-8939/$ - see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tmaid.2013.12.001
4 and appropriate. Erring on the side of caution, the 2011 protocol allows for investigating contacts of smearnegative or non-cavitary passengers when there are reasons to suspect a higher degree of infectiousness [11]. It may also be prudent to pursue investigations in situations where boarded passengers remain grounded for a substantial amount of time, as ventilation is much lower prior to flight [10]. Based on the authors’ analyses, it appears the restricted protocol will result in a negligible number of unidentified cases from in-flight transmission. The newer protocol is also financially sound, as it should reduce expenditures on airline passenger TBCI by 50%. These savings can be reinvested in other, higher impact public health interventions.
References [1] Luscombe R. Killer tuberculosis patient sorry for flying home. Guardian June 1, 2007. [2] Kolata G. First documented case of tuberculosis passed on airliner is reported by the U.S. The New York Times; March 15, 1995. [3] McKay B. Dangerous tuberculosis patient detained on U.S. Border. Wall St J March 1, 2013. [4] Rieder HL, International Union against Tuberculosis and Lung Disease. Epidemiologic basis of tuberculosis control. Paris: International Union Against Tuberculosis and Lung Disease; 1999. [5] Nardell EA. The role of ventilation in preventing nosocomial transmission of tuberculosis. Int J Tuberc Lung Dis 1998 Sep; 2(9 Suppl. 1):S110e7.
Editorial [6] World Health Organization. Tuberculosis and air travel: guidelines for prevention and control. 3rd ed. Geneva: World Health Organization; 2008. [7] Jensen PA, Lambert LA, Iademarco MF, Ridzon R, CDC. Guidelines for preventing the transmission of Mycobacterium tuberculosis in health-care settings, 2005. MMWR Recomm Rep 2005 Dec 30;54(RR-17):1e141. [8] Marienau KJ, Cramer EH, Coleman MS, Marano N, Cetron MS. Flight related tuberculosis contact investigations in the United States: comparative risk and economic analysis of alternate protocols. Travel Med Infect Dis 2014 Oct 8;12(1):54e62. [9] Coleman MS, Marienau KJ, Marano N, Marks SM, Cetron MS. Economics of United States tuberculosis airline contact investigation policies: a return on investment analysis. Travel Med Infect Dis 2013. 10.016. [10] Ko G, Thompson KM, Nardell EA. Estimation of tuberculosis risk on a commercial airliner. Risk Anal 2004 Apr;24(2): 379e88. [11] Public health interventions involving travelers with tuberculosiseU.S. ports of entry, 2007e2012. MMWR Morb Mortal Wkly Rep 2012 Aug 3;61(30):570e3.
Faiz Ahmad Khan Dick Menzies* Montreal Chest Institute, McGill University, Canada *Corresponding author. Montreal Chest Institute, Room K1.24, 3650 St. Urbain St., Montreal, PQ H2X 2P4, Canada. Tel.: þ1 514 934 1934x32128, þ1 514 934 1934x32129; fax: þ1 514 843 2083. E-mail address: [email protected] (D. Menzies)
4 December 2013
