Elsevier

Vaccine

Volume 32, Issue 52, 12 December 2014, Pages 7135-7140
Vaccine

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

https://doi.org/10.1016/j.vaccine.2014.10.027Get rights and content
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Highlights

  • Few inter-pandemic influenza hospitalizations/season among pregnant women in Sweden.

  • For all tested scenarios the number needed to vaccinate was >1,900 pregnant women.

  • Number needed to vaccinate may vary, even between northern European countries.

Abstract

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.

Abbreviations

NNV
number needed to vaccinate
ECDC
European Centre for Disease Prevention and Control
RIRI
respiratory infection that can possibly be related to influenza
NBHW
National Board of Health and Welfare
GAM
generalized additive regression model
VE
vaccine effectiveness

Keywords

Influenza
Policy
Respiratory tract infections
Pregnancy
Vaccination

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