Background: Most deaths in the 1918 influenza pandemic were caused by secondary bacterial pneumonia.
Methods: We performed a systematic review and reanalysis of studies of bacterial vaccine efficacy (VE) in preventing pneumonia and mortality among patients with influenza during the 1918 pandemic.
Results: A meta-analysis of 6 civilian studies of mixed killed bacterial vaccines containing pneumococci identified significant heterogeneity among studies and estimated VE at 34% (95% confidence interval [CI], 19%-47%) in preventing pneumonia and 42% (95% CI, 18%-59%) in reducing case fatality rates among patients with influenza, using random-effects models. Using fixed-effect models, the pooled VE from 3 military studies was 59% (95% CI, 43%-70%) for pneumonia and 70% (95% CI, 50%-82%) for case fatality. Military studies showed less heterogeneity and may provide more accurate results than civilian studies, given the potential biases in the included studies. Findings of 1 military study using hemolytic streptococci also suggested that there was significant protection.
Conclusions: Despite significant methodological problems, the systematic biases in these studies do not exclude the possibilities that whole-cell inactivated pneumococcal vaccines may confer cross-protection to multiple pneumococcal serotypes and that bacterial vaccines may play a role in preventing influenza-associated pneumonia.