Variation in those infected by H1N1

An analysis of blood samples taken before, during and after an epidemic wave of influenza A(H1N1) in Singapore in 2009 finds variation in infection risks and antibody levels, with younger age groups and military personnel having higher infection rates than other groups, according to a study in the April 14 issue of JAMA.

On April 24, 2009, the World Health Organiza¬tion (WHO) reported the emergence of a novel in¬fluenza A virus (2009 influenza A[H1N1]). Singapore detected its first im¬ported cases of 2009 influenza A(H1N1) in late May 2009. "Virological surveillance documented sustained commu¬nity transmission from the latter half of June 2009, followed by a single epidemic wave peaking in the first week of August and subsiding by September 2009," the authors write.

Mark I. C. Chen, Ph.D., of Tan Tock Seng Hospital, Singapore, and colleagues conducted a study using blood specimens to determine an¬tibody levels against 2009 influenza A(H1N1) as a marker of infection in different population groups to compare the risk of infection in these groups and to investigate risk factors for infection. The study included serological samples from 4 distinct groups: general population (n = 838), military personnel (n = 1,213), staff from an acute care hospital (n = 558), and staff as well as residents from long-term care fa¬cilities (n = 300) from June 22, 2009, to October 15, 2009.

Titers (the concentration of the antibodies) were ex¬pressed as the reciprocal of the high¬est dilution of serum where hemagglutination (the clumping or clustering of red blood cells caused by certain viruses or antibodies) was prevented. A 4-fold or greater increase in antibody titers between any of the 3 serological samples was defined as evidence of H1N1 seroconversion (the development of detectable antibodies in the blood directed against an infectious agent).

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 par¬ticipants 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 ob¬servations," 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 de¬sired."

"In conclusion, our study shows wide variation in serologically determined in¬fection rates by cohorts and age groups, suggesting that context-specific risks of infection need to be taken into ac¬count and that interventions need to be tailored to the population at risk. Al¬though it appears that a large propor¬tion of the Singapore adult population remain susceptible to the 2009 influ¬enza A(H1N1) virus after the first epi¬demic wave, for a significant second wave to occur, a sufficient number of susceptible children may also be re¬quired for efficient transmission. These and other factors will need to be con¬sidered in the determination of opti¬mal pandemic vaccination strategies for influenza A(H1N1)."

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