Abstract
BackgroundMost reports of influenza vaccine effectiveness consider current-season vaccination only.AimWe evaluated a method to estimate the effect of influenza vaccinations (EIV) considering vaccination history.MethodsWe used a test-negative design with well-documented vaccination history to evaluate the average EIV over eight influenza seasons (2011/12–2018/19; n = 10,356). Modifying effect was considered as difference in effects of vaccination in current and previous seasons and current-season vaccination only. We also explored differences between current-season estimates excluding from the reference category people vaccinated in any of the five previous seasons and estimates without this exclusion or only for one or three previous seasons.ResultsThe EIV was 50%, 45% and 38% in people vaccinated in the current season who had previously received none, one to two and three to five doses, respectively, and it was 30% and 43% for one to two and three to five prior doses only. Vaccination in at least three previous seasons reduced the effect of current-season vaccination by 12 percentage points overall, 31 among outpatients, 22 in 9–65 year-olds, and 23 against influenza B. Including people vaccinated in previous seasons only in the unvaccinated category underestimated EIV by 9 percentage points on average (31% vs 40%). Estimates considering vaccination of three or five previous seasons were similar.ConclusionsVaccine effectiveness studies should consider influenza vaccination in previous seasons, as it can retain effect and is often an effect modifier. Vaccination status in three categories (current season, previous seasons only, unvaccinated) reflects the whole EIV.
Highlights
The World Health Organization recommends annual influenza vaccination in the target population [1].People vaccinated against influenza in one season are more likely to be vaccinated in the one [2]; those in the target population frequently accumulate influenza vaccines over the years.Influenza vaccines received in previous seasons may retain a notable protective effect [3,4,5]
We considered the reference values to be the effect of influenza vaccinations (EIV) estimates obtained from the ‘full model’ with the vaccination status in six categories: current-season vaccination and three to five prior doses, current-season vaccination and one to two prior doses, currentseason vaccination and no prior doses, three to five prior doses and no current-season vaccination, one to two prior doses and no current-season vaccination, and unvaccinated in the current and five previous seasons as the reference category [6,7,8]
We reduced the number of vaccination categories by combining all individuals vaccinated in the current season regardless of their vaccination history, and these were compared with subjects unvaccinated in the current and five previous seasons
Summary
The World Health Organization recommends annual influenza vaccination in the target population [1]. Influenza vaccines received in previous seasons may retain a notable protective effect [3,4,5]. IVE studies usually aim at assessing how each seasonal vaccine composition has worked against the circulating influenza viruses in real-life conditions, when frequently repeated vaccination is the most common scenario. Most of these studies consider only the current-season vaccination [12], and those that consider previous vaccines usually do so as a secondary analysis [3,13]. Most reports of influenza vaccine effectiveness consider current-season vaccination only
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