The impact of national vaccination policy changes on influenza incidence in the Netherlands

Background: We assessed the impact of two major modifications of the Dutch National Influenza Prevention Programme - the introduction in 1997 of free-of-charge vaccination to persons aged ≥65 years and to high-risk groups (previously only advised, and not free of charge), and the lowering of the eligible age to 60 years in 2008 - on the estimated incidence of influenza infection leading to influenza-like illness (ILI). Methods: Additive negative-binomial segmented regression models were fitted to ILI data from GP sentinel surveillance in two-eight-season intervals (1993/4 to 2000/1, 2004/5 to 2011/12, comparing pre- and post-policy-change periods within each interval), with laboratory virological reporting of samples positive for influenza or other ILI-causing pathogens as covariates. Results: For the 2008 policy change, there was a significant step decrease in influenza contribution considering all ages (=-111 per 100 positives; 95% CI: -162, -65·0), <60 years and 60-64 years age groups (B = -92·1 per 100; 95% CI: -134, -55·5; B = -5·2; 95% CI: -10·3, -1·2, respectively). There was no evidence for a decrease associated with the 1997 policy change targeting the ≥65 years age group. Conclusions: In the Netherlands, a 56% reduction in influenza contribution was associated with the 2008 policy targeting 60-64 year-olds, but there was no effect of the earlier policy targeting ≥65-year-olds, for whom vaccination coverage was already rising before the policy change.