Low attack rate of novel influenza A (H1N1) virus infection among healthcare workers: a prospective study in a setting with an elaborated containment plan

This study aimed to determine incidence rates of novel influenza A (H1N1) infection among healthcare personnel with different exposure risks during the 2009 H1N1 pandemic. From August 2009 until April 2010, 66 healthcare workers from a 410 bed teaching hospital in Amsterdam were monitored. The following three different exposure groups were created: a high- (n = 26), intermediate- (n = 20), and low-risk group (n = 20). Throat swabs were collected each week and analyzed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) in order to detect the H1N1 virus. Blood was drawn at study enrollment and once monthly thereafter, and serum specimens were tested with an H1N1-specific hemagglutination-inhibition serologic assay. Influenza-like signs and symptoms were assessed weekly. One of 26 high-risk group participants proved H1N1 positive once by RT-PCR. This corresponds to an incidence rate in the high-risk group of 5.7/1,000 person weeks (95% CI 0-17/1,000). None of the intermediate- and low-risk group participants proved H1N1 positive by RT-PCR. Significant antibody titer rises in convalescent sera were demonstrated in three participants: one was a confirmation of the case that had proved H1N1 positive by RT-PCR; the others occurred in two asymptomatic participants belonging to the low- and high-risk groups. An influenza-like illness was assumed in four participants from the high- (n = 1), intermediate- (n = 1) and low-risk (n = 2) groups; these findings were not confirmed by positive results from either diagnostic test. This study demonstrates a low incidence rate of influenza A (H1N1) infection among healthcare workers during the 2009 H1N1 pandemic in a setting with high hygiene standards.