H1N1 Hemagglutinin-Inhibition Seroprevalence in Emergency Department Health Care Workers After the First Wave of the 2009 Influenza Pandemic

STUDY OBJECTIVE:The 2009 H1N1 pandemic (H1N1pdm) virus has been associated with high rates of asymptomatic infections. Existing influenza infection control policies do not address potential transmission through exposure to asymptomatic infected individuals in health care settings. We conducted a seroprevalence study of H1N1pdm infection to determine whether health care workers (HCWs) in the emergency department showed increased evidence of infection during the first wave of the pandemic than that previously reported in adults in the community. METHODS:Blood samples and demographic and clinical data were collected from eligible emergency department HCWs. Subjects' sera were tested for presence of antibodies specific for seasonal H1N1 and H1N1pdm viruses by hemagglutination-inhibition assay. RESULTS:One hundred eight subjects were enrolled, of which 20 (18.5%) were seropositive for H1N1pdm and 52 (48%) for seasonal H1N1. The median age of H1N1pdm-seropositive subjects was 32 years (range, 24-59 years). Of H1N1pdm-seropositive subjects, 35% were asymptomatic. Rates of H1N1pdm detection in HCWs (18.5%) were significantly higher than those observed previously in an identical age cohort in the community (2.6%, n = 262). CONCLUSIONS:The higher serodetection rates in adults observed in the current study suggest potentially significantly more frequent infections in HCWs than in the general population. Further investigations are needed to ascertain the relative incidence of influenza infections in HCWs and non-HCWs, to study influenza transmission by asymptomatic infected subjects and ascertain the burden of such transmission in health care settings.