Evaluation and sensitivity analysis of the effectiveness of unadjuvanted A(H1N1)pdm09 vaccine in hospital workers using multi-center cohort study in Korea

• We evaluated the effectiveness of the A(H1N1)pdm09 vaccine in hospital workers.
• Crude vaccine effectiveness (VE) was 81.8%.
• Adjusted VE was 81.9%.
• The vaccine effectively protected the hospital workers against the A(H1N1)pdm09
• VE estimates were sensitive to varying adjustment or assumptions (75.4–93.3%).

The A(H1N1)pdm09 vaccine was developed and administered to hospital workers at first during the peak of the outbreak in Korea. The present multicenter cohort study was conducted to evaluate whether the A(H1N1)pdm09 vaccine effectively protected hospital workers. The study vaccine contained 15 μg of A/California/7/2009 NYMC X-179A(H1N1) without adjuvant, Greenflu-S™. Participants were requested in December 2009 and April 2010 to answer a questionnaire about whether they had been confirmed to be A(H1N1)pdm09 patients based on the real-time reverse transcription-polymerase chain reaction (RT-PCR) assay and had received the A(H1N1)pdm09 vaccine. The vaccination and incidence rates of the A(H1N1)pdm09 were 96.4% (8769/9097), and 0.6% (57/9097), respectively. The crude vaccine effectiveness (VE) was calculated to be 81.8% [95% confidence interval (CI); 61.0–91.5%], while the effectiveness adjusted for hospital, immunization with seasonal trivalent influenza vaccine (TIV) in 2009–10, contact with A(H1N1)pdm09 patients, pre-existing disease, smoking history, and date of follow-up start was 81.9% (95% CI; 52.8–93.1%). When we defined insufficiently immunized cases as unvaccinated, instead of deleting them from the cohort, the adjusted VE increased up to 93.3%. In conclusion, the unadjuvanted monovalent A(H1N1)pdm vaccine was highly protective in hospital workers against laboratory-confirmed A(H1N1)pdm09 infections. VE estimates were sensitive to varying adjustment or assumptions (75.4–93.3%), which suggest the necessity of careful analysis of VE.