Seroprevalence of Antibodies to Pandemic (H1N1) 2009 Influenza Virus Among Hospital Staff in a Medical Center in Taiwan

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Abstract
Background: The pandemic (H1N1) 2009 influenza emerged in April 2009 and spread rapidly and broadly all over the world. In addition to specific antiviral agents, massive vaccination is thought to be the most effective way of controlling the transmission. To understand the prevaccination status of certain risk groups, this study compared the baseline sero-prevalence of antibodies to the pandemic (H1N1) 2009 influenza virus among hospital staff with different contact risks and that of the general population. Methods: A total of 295 serum samples from hospital staff and 244 control serum samples from people who came for physical check-up (control group) were collected between October 2009 and November 2009 before the massive vaccination campaign. The hospital staff was divided into first-line risk personnel (group 1) and second-line risk personnel (group 2) according to their potential contact risks. Hemagglutination-inhibition (HI) tests were conducted to determine the individual serological status. The seropositive rate (SPR, defined as the proportion with HI titer ≥ 1:40) of antibodies to H1N1 influenza virus and its geometric mean titer (GMT) were calculated and compared among the different groups. Results: The mean ages and sex ratio (% male) of the hospital staff and control groups were 36.9 ± 10.6 years and 52.0 ± 12.6 years, and 24.4% and 57.6%, respectively. The SPR of the antibodies to H1N1 influenza virus of the hospital staff was significantly higher than that of the control group (20.0% vs. 2.9%, p < 0.001). Furthermore, the SPR antibodies to H1N1 influenza virus of group 1 were significantly higher than that of group 2 (30.8% vs. 12.6%, p < 0.001). However, the GMT of antibodies to H1N1 influenza virus of the hospital staff was not significantly different from that of the control group (p = 0.925). Conclusion: The SPR of antibodies against the pandemic (H1N1) 2009 virus in the hospital staff was higher than that in the general population, reflecting a higher contact risk. Prevaccination surveillance of the immune status of different risk groups may help to prioritize which groups should be vaccinated first.