Association between monovalent influenza A (H1N1) pdm09 vaccine and pneumonia among the elderly in the 2009-2010 season in Japan: A case-control study

Abstract

We investigated the association between monovalent influenza A (H1N1) pdm09 (H1N1pdm) vaccine and pneumonia in elderly people. Study design was a hospital-based, matched case-control study. Cases comprised patients ≥ 65 years old who had been newly diagnosed with pneumonia. For each case, 2 controls were defined as individuals with other diseases (not pneumonia) who were matched by sex, age, entry date, and the visited hospital. Study period was the interval from 1 September 2009 until 30 September 2010. Because a pandemic of influenza A (H1N1) occurred during study period, we analyzed selected subjects who had enrolled during the influenza A (H1N1) pandemic. We calculated the odds ratios (ORs) and 95% confidence intervals (CIs) for pneumonia in H1N1pdm-vaccinated subjects compared with unvaccinated subjects using a conditional logistic regression model to assess the association between H1N1pdm vaccine and pneumonia. The subjects during the period of the influenza A (H1N1) pandemic were 20 cases and 40 controls. Subjects who had received H1N1pdm vaccine showed a significantly decreased OR for pneumonia (OR = 0.10, 95% CI = 0.01-0.98) compared with unvaccinated subjects. In conclusion, H1N1pdm vaccination may have prevented pneumonia among the elderly during the 2009-2010 influenza A (H1N1) pandemic in Japan.

Keywords: ADL, activities of daily living; CI, confidence interval; COPD, chronic obstructive pulmonary disease; H1N1pdm, monovalent influenza A (H1N1) pdm09; OR, odds ratio; TIV, trivalent seasonal influenza vaccine; effectiveness; elderly people; matched case-control study; monovalent influenza A (H1N1) pdm09 vaccine; pneumonia.

Figure 1.

(A) Study period and weekly cases of influenza in Japan from week 35 of 2009 to week 39 of 2010. (B) Study period and monthly reports of isolation/detection of influenza viruses in Japan from August 2009 to September 2010.