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Impact of influenza vaccination programmes among the elderly population on primary care, Portugal, Spain and the Netherlands: 2015/16 to 2017/18 influenza seasons.

Machado, A, Mazagatos, C., Dijkstra, F., Kislaya, I., Gherasim, A., McDonald, S.A., Kissling, E., Valenciano, M., Meijer, A., Hooiveld, M., Nunes, B., Larrauri, A. Impact of influenza vaccination programmes among the elderly population on primary care, Portugal, Spain and the Netherlands: 2015/16 to 2017/18 influenza seasons. Eurosurveillance: 2019, 24(45)

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Background
To increase the acceptability of influenza vaccine, it is important to quantify the overall benefits of the vaccination programme.

Aim
To assess the impact of influenza vaccination in Portugal, Spain and the Netherlands, we estimated the number of medically attended influenza-confirmed cases (MAICC) in primary care averted in the seasons 2015/16 to 2017/18 among those ≥65 years.

Methods
We used an ecological approach to estimate vaccination impact. We compared the number of observed MAICC (n) to the estimated number that would have occurred without the vaccination programme (N). To estimate N, we used:
(i) MAICC estimated from influenza surveillance systems,
(ii) vaccine coverage,
(iii) pooled (sub)typespecific influenza vaccine effectiveness estimates for seasons 2015/16 to 2017/18, weighted by the proportion of virus circulation in each season and country.

We estimated the number of MAICC averted (NAE) and the prevented fraction (PF) by the vaccination programme.

Results
The annual average of NAE in the population ≥65 years was 33, 58 and 204 MAICC per 100,000 in Portugal, Spain and the Netherlands, respectively. On average, influenza vaccination prevented 10.7%, 10.9% and 14.2% of potential influenza MAICC each season in these countries. The lowest PF was in 2016/17 (4.9–6.1%) with an NAE ranging from 24 to 69 per 100,000.

Conclusions
Our results suggest that influenza vaccination programmes reduced a substantial number of MAICC. Together with studies on hospitalisations and deaths averted by influenza vaccination programmes, this will contribute to the evaluation of the impact of vaccination strategies and strengthen public health communication.