Acceptable protective efficacy of influenza vaccination in young military conscripts under circumstances of incomplete antigenic and genetic match

Commercial inactivated parenteral influenza vaccines reduced febrile (great er than or equal to 38 degreesC) respiratory illness by 53% (95% CL: 41-63% ) during a 3 week outbreak in 1998 when A/Sydney/5/97(H3N2)-like influenza viruses were shown to be the predominant etiological agents and an older an tigenic variant, A/Nanchang/933/95, served as the vaccine virus. The calcul atory efficacy for preventing virologically diagnosed influenza infections was 57% (95% CF: 40-68%). The study population consisted of 1374 young male military conscripts. Vaccination coverage on a voluntary basis was 67%. Va ccination was ineffective in preventing febrile illness during a second epi demic wave lasting 2 weeks when mainly adenoviruses were shown to have been circulating in the garrison. Out of the 36 nasopharyngeal aspirates positi ve for influenza A by antigen detection. 18 A/Sydney/5/97-like strains (10 from non-vaccinated and eight From vaccinated subjects) and two A/Nanchang/ 933/95-like strains (both from non-vaccinated subjects) were isolated in MD CK cell cultures. Intraepidemic variation was detected among the A/Sydney/5 /97-like field strains in their HA1 sequences and reactivity in HI tests, b ut no evidence was obtained that this variation would have been of signific ance to the virus in breaking through the vaccination-induced immunity. (C) 2001 Elsevier Science Ltd. All rights reserved.