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 Organization (WHO) reported the emergence of a novel influenza A virus (2009 influenza A[H1N1]). Singapore detected its first imported cases of 2009 influenza A(H1N1) in late May 2009. “Virological surveillance documented sustained community 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 Chen, PhD, of Tan Tock Seng Hospital, Singapore, and colleagues conducted a study using blood specimens to determine antibody 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 four 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 facilities (n = 300) from June 22, 2009, to October 15, 2009.
Titers (the concentration of the antibodies) were expressed as the reciprocal of the highest dilution of serum where hemagglutination (the clumping or clustering of red blood cells caused by certain viruses or antibodies) was prevented. A four-fold or greater increase in antibody titers between any of the three serological samples was defined as evidence of H1N1 seroconversion (the development of detectable antibodies in the blood directed against an infectious agent).