Influenza virus prevalence in asymptomatic and symptomatic subjects during pandemic and postpandemic periods

Abstract

In 2009, Influenza A(H1N1)pdm09 had a major effect on global health, causing thousands of deaths. However, the results of asymptomatic infection in transmission has not been well studied. We analyzed asymptomatic influenza infection in individuals with different risk factors for acquiring influenza in a university hospital. Respiratory samples from 100 children with respiratory symptoms, their asymptomatic caregivers, 100 asymptomatic HIV-infected patients, and 100 health care workers collected during 2009-2011 were tested for influenza by real time real-time polymerase chain reaction. Influenza infection rate in symptomatic children was 44% and in asymptomatic adults it was 8.3% (P<.01). Children older than age 5 years had a 3.4 times greater chance of being infected during influenza season than younger children. Among the asymptomatic group, influenza was more frequent in caregivers (14%; P=.032) and a higher rate (31.8%) was observed if a child was infected during the 2009 pandemic. Contact with an infected child was an important risk factor for influenza acquisition (odds ratio, 3.8; 95% confidence interval, 1.1-13.2) among caregivers. The mean cluster of differentiation 4 T-cell count of HIV-positive patients infected with influenza was not statistically different from uninfected patients (P= .29). Asymptomatic household contacts of symptomatic children may play an important role in community transmission and a more proactive intervention should be considered during future pandemics.