Baseline titers of 40 or more were observed in 2.6 percent of members of the community; 9.4 percent of military personnel; 6.6 percent of hospital staff; and 6.7 percent of participants from long-term care facilities. The researchers found that in participants with 1 or more follow-up serum samples, 29.4 percent of military personnel seroconverted compared with 13.5 percent of community members, 6.5 percent of hospital staff, and 1.2 percent of long-term care participants. Additional analysis indicated that having another household member who seroconverted remained associated with a higher likelihood of infection. "Our study also shows the variation in infection risks, with younger age groups and military personnel having much higher infection rates. The lower infection rates in older participants corroborate other epidemiological observations," the authors write.

The researchers also found that "only 13 percent of the community cohort seroconverted, which supports the case for targeted vaccination in populations for which protection is desired."

"In conclusion, our study shows wide variation in serologically determined infection rates by cohorts and age groups, suggesting that context-specific risks of infection need to be taken into account and that interventions need to be tailored to the population at risk. Although it appears that a large proportion of the Singapore adult population remain susceptible to the 2009 influenza A(H1N1) virus after the first epidemic wave, for a significant second wave to occur, a sufficient number of susceptible children may also be required for efficient transmission. These and other factors will need to be considered in the determination of optimal pandemic vaccination strategies for influenza A(H1N1)."

SOURCE JAMA