Variation in severity of respiratory syncytial virus infections with subtype K e n n e t h M, M c C o n n o c h i e , MD, MPH, C a r o l i n e B, Hall, MD, E d w a r d E. Walsh, MD, a n d Klaus J. R o g h m a n n , PhD From the Departments of Pediatrics and Internal Medicine, Rochester General Hospital and Strong Memorial Hospital, University of Rochester School of Medicine, Rochester, New York Two major subtypes of respiratory syncytial virus have been identified. This study assessed the hypothesis that A-subtype infections were more severe than B-subtype infections a m o n g the 157 infants hospitalized in two hospitals in Rochester, N.Y., during two winters. Severity was measured both by specific clinical observations and by a severity index that was d e r i v e d empirically. A m o n g all subjects, several clinical observations suggested that A-subtype infections w e r e m o r e s e v e r e . For e x a m p l e , m e c h a n i c a l ventilation was required in 12.6% of those with A-subtype c o m p a r e d with 1.6% of those with B-subtype infection (relative risk = 7 . 8 8 ; p = 0 . 0 1 ) . A m o n g high-risk infants (infants with underlying conditions or a g e 3 months or less at admission), c a r b o n d i o x i d e tension greater than 45 mm Hg was found in 37.0% of those with A-subtype c o m p a r e d with 12.0% of those with B-subtype infection (relative risk = 3 . 0 8 ; p = 0 . 0 4 ) . In discrete multivariate (Iogit) analysis, effects of subtype (odds ratio = 6.59; p 72/min Pulse >174/min LOS >5 days Sao2 45 pH