Number needed to vaccinate to prevent hospitalizations of pregnant women due to inter-pandemic influenza in Sweden, 2003-2009.

BACKGROUND: The evidence of increased risk of severe disease for healthy pregnant women due to inter-pandemic influenza consists mainly of observational studies of health service utilization in USA and Canada. However, these results can be context dependent and estimates in a European setting are sparse. For policy purposes we therefore decided to elucidate the potential value of vaccination in Sweden. MATERIALS AND METHODS: We conducted a retrospective, register-based study of hospitalizations due to inter-pandemic influenza or respiratory infection attributable to influenza in pregnant women in Sweden. With aggregated data from 2003 to 2009 we assessed the number needed to vaccinate (NNV) to prevent one such hospitalization. RESULTS: We included on average 96,000 pregnant women/year and identified 9-48 hospitalizations/season fulfilling the case definition. Assuming 80% vaccine effectiveness the NNV was >1,900 pregnant women. The estimate is higher than those found in the USA, Canada, and UK. The difference may be explained by differing methods to estimate NNV, but also differences in propensity to hospitalize and the basic health status of the pregnant women. CONCLUSIONS: Because of the increased risk associated with influenza A(H1N1)pdm09, vaccination is presently offered to all pregnant women in Sweden, but vaccination against other inter-pandemic influenza types seems disputable. The study illustrates the context dependence of preventive health measures and points to the need for national NNV estimates and international harmonization of study methods for comparisons between countries.