BACKGROUND: Anti-viral prophylaxis is used to prevent the transmission of 
influenza. We studied serological confirmation of 2009 Influenza A (H1N1) 
infections during oseltamivir prophylaxis and after cessation of prophylaxis.
METHODS: Between 22 Jun and 16 Jul 09, we performed a cohort study in 3 
outbreaks in the Singapore military where post-exposure oseltamivir ring 
chemoprophylaxis (75 mg daily for 10 days) was administered. The entire cohort 
was screened by RT-PCR (with HA gene primers) using nasopharyngeal swabs three 
times a week. Three blood samples were taken for haemagglutination inhibition 
testing--at the start of outbreak, 2 weeks after completion of 10 day 
oseltamivir prophylaxis, and 3 weeks after the pandemic's peak in Singapore. 
Questionnaires were also administered to collect clinical symptoms.
RESULTS: 237 personnel were included for analysis. The overall infection rate of 
2009 Influenza A (H1N1) during the three outbreaks was 11.4% (27/237). This 
included 11 index cases and 16 personnel (7.1%) who developed four-fold or 
higher rise in antibody titres during oseltamivir prophylaxis. Of these 16 
personnel, 8 (3.5%) were symptomatic while the remaining 8 personnel (3.5%) were 
asymptomatic and tested negative on PCR. Post-cessation of prophylaxis, an 
additional 23 (12.1%) seroconverted. There was no significant difference in mean 
fold-rise in GMT between those who seroconverted during and post-prophylaxis 
(11.3 vs 11.7, p = 0.888). No allergic, neuropsychiatric or other severe 
side-effects were noted.
CONCLUSIONS: Post-exposure oseltamivir prophylaxis reduced the rate of infection 
during outbreaks, and did not substantially increase subsequent infection rates 
upon cessation. Asymptomatic infections occur during prophylaxis, which may 
confer protection against future infection. Post-exposure prophylaxis is 
effective as a measure in mitigating pandemic influenza outbreaks.