Measuring the impact of influenza vaccination in the Netherlands using retrospective observational primary care, hospitalisation and mortality data

We aimed to estimate the impact of influenza vaccination in the Netherlands using general practitioner medical records for 2011-2020. We found that vaccinees had higher consultation rates for influenza-like-illness, acute respiratory infections, and pneumonia, as well as antibiotic use, hospitalisations, and several control diagnoses (i.e. illnesses for which there was no a priori expectation that influenza vaccination would play a protective effect). We found similar rates for respiratory mortality and lower all-cause mortality in the vaccinees versus non-vaccinees, mainly driven by the 75+ age group. These results expand, but are fairly consistent with those of previous investigations, and highlight the difficulty of using registry data to assess the impact of vaccination, because of underlying differences between vaccinees and non-vaccinees. Whether these biases also play a role for hospitalisations and mortality remains unclear. Our findings support the implementation of randomized studies to assess the impact of influenza vaccination.