Seroprevalence and Risk Factors for the 2009 Influenza A (H1N1) Virus among Japanese Healthcare Workers

Background: The H1N1 influenza pandemic emerged in April 2009 and spread rapidly and broadly all over the world. Health care workers (HCWs) are at an increased risk of influenza because of their contact with patients. Large-scale vaccination is thought to be the most effective way of controlling the transmission of influenza among HCWs. However, there were severe shortages in the amount of vaccines available during the pandemic.

Objective: This study aimed to determine the risk factors associated with H1N1 seropositivity in HCWs.

Methods: In this study, serial serological samples from HCWs (n = 461) were collected from September 14 to October 4, 2009 and December 14, 2009 to January 15, 2010. Hemagglutination inhibition was measured in serum samples collected before and after H1N1 vaccination. In addition, the geometric mean titer (GMT) was calculated and compared among the different risk groups.

Results: Doctors and nurses had the highest risk of seropositivity (odds ratio [OR], 5.25; 95% confidence interval [CI], 1.21–22.7). Among HCWs, pediatricians, emergency workers, and internal medicine and resident physicians, had an increased risk of seropositivity (adjusted OR, 1.77; 95% CI, 1.01–3.10). Further, HCWs who had high titers of antibodies against seasonal influenza had an increased risk of seropositivity (adjusted OR, 3.92; 95% CI, 1.68–9.18). These data concurred with the GMT data. After H1N1 vaccination, only 60% of HCWs (242/405) acquired protective antibodies against H1N1. High baseline titers increased the possibility of acquiring protective antibodies (adjusted OR, 6.59; 95% CI, 3.40–12.8). Further, HCWs who had a high titer of antibodies against seasonal influenza had an increased probability of acquiring protective antibodies (adjusted OR, 2.29; 95% CI, 1.36–3.88).

Conclusions: The seropositivity and GMT of the antibodies against the H1N1 virus in the HCWs, particularly those belonging to departments like Pediatrics, Emergency room, and Internal Medicine, were higher than that in the general population and the other hospital staff. This finding suggested that the former group of HCWs had a higher contact risk. Prevaccination surveillance of the immune status of different risk groups may help identify the groups that should be vaccinated first. HCWs should be vaccinated against the H1N1 influenza virus as soon as possible because they are at high risk of contracting H1N1.