Background:
There is limited research on the impact of the yearly influenza vaccination programs in the eligible population. This study aimed to estimate the number of primary care medically attended influenza-confirmed cases (MAICC) among the population aged ≥65 years averted by influenza vaccination programme in Portugal during five seasons in the pre-COVID pandemic period (2015/16 to 2019/20).

Methods:
We compared the number of observed MAICC to the estimated number that would have occurred in a population without seasonal influenza vaccination (N). To estimate N, we used: i) number of MAICC estimated from national influenza surveillance systems, ii) vaccine coverage (VC) collected in a national telephone survey, iii) influenza vaccine effectiveness (IVE) estimates weighted by the proportion of virus circulation each season in Portugal. We estimated the number of MAICC averted (NAE) by the influenza vaccination programme per 100.000 inhabitants and number needed to vaccinate to prevent one MAICC. We used Monte-Carlo simulations to estimate 95% uncertainty intervals (UI).

Results:
Comparing with results from 2015/16 to 2017/18 (NAE ranged 24 to 44 per 100.000 inhab) the season 2018/19 showed the highest NAE (62.3 per 100.000 inhab) attributed to the influenza vaccination programme. In 2019/20 season the vaccination strategy averted approximately 11.7 per 100.000 inhab (95%UI: 6.0 to 20.9) events and it was necessary to vaccinate 549 (95%UI: 436 to 742) to prevent one MAICC in primary care.

Conclusion:
The influenza vaccination strategy had consistent and positive benefit, with more pronounced impact in 2018/19 season. This results were mainly due to a combination of a higher vaccination coverage assumed for 2018/19 (60.8%) and one of the highest vaccine effectiveness (34.8% vs. previous study range 8.5% to 40.6%). To maximize its impact, efforts should be conducted to increase the vaccine coverage. In addition, the surge for more effective vaccines should be maintained.