Abstract
BackgroundConflicting results regarding the impact of repeated vaccination on influenza vaccine effectiveness (VE) may cause confusion regarding the benefits of receiving the current season’s vaccine.MethodsWe systematically searched MEDLINE, Embase, PubMed, and Cumulative Index to Nursing and Allied Health Literature from database inception to August 17, 2016, for observational studies published in English that reported VE against laboratory-confirmed influenza for four vaccination groups, namely current season only, prior season only, both seasons, and neither season. We pooled differences in VE (∆VE) between vaccination groups by influenza season and type/subtype using a random effects model. The study protocol is registered with PROSPERO (registration number: CRD42016037241).ResultsWe identified 3435 unique articles, reviewed the full text of 634, and included 20 for meta-analysis. Compared to prior season vaccination only, vaccination in both seasons was associated with greater protection against influenza H1N1 (∆VE = 26%; 95% CI, 15% to 36%) and B (∆VE = 24%; 95% CI, 7% to 42%), but not H3N2 (∆VE = 10%; 95% CI, –6% to 25%). Compared to no vaccination for either season, individuals who received the current season’s vaccine had greater protection against H1N1 (∆VE = 61%; 95% CI, 50% to 70%), H3N2 (∆VE = 41%; 95% CI, 33% to 48%), and B (∆VE = 62%; 95% CI, 54% to 68%). We observed no differences in VE between vaccination in both seasons and the current season only for H1N1 (∆VE = 4%; 95% CI, –7% to 15%), H3N2 (∆VE = –12%; 95% CI, –27% to 4%), or B (∆VE = –8%; 95% CI, –17% to 1%).ConclusionsFrom the patient perspective, our results support current season vaccination regardless of prior season vaccination. We found no overall evidence that prior season vaccination negatively impacts current season VE. It is important that future VE studies include vaccination history over multiple seasons in order to evaluate repeated vaccination in more detail.
Highlights
Conflicting results regarding the impact of repeated vaccination on influenza vaccine effectiveness (VE) may cause confusion regarding the benefits of receiving the current season’s vaccine
When compared to vaccination in the prior season only, VE was higher for vaccination in both seasons for influenza H1N1 (ΔVE = 26%; 95% CI, 15% to 36%; I2 = 0%) and B (ΔVE = 24%; 95% CI, 7% to 42%; I2 = 44%), but not H3N2 (ΔVE = 10%; 95% CI, –6% to 25%; I2 = 33%) (Table 2, Figs. 2, 3, and 4)
We observed no statistically significant VE differences between vaccination in both seasons and vaccination in the current season only for influenza H1N1 (ΔVE = 4%; 95% CI, –7% to 15%; I2 = 0%), H3N2 (ΔVE = –12%; 95% CI, –27% to 4%; I2 = 52%), or B (ΔVE = –8%; 95% CI, –17% to 1%; I2 = 0%) (Table 2, Figs. 8, 9, and 10)
Summary
Conflicting results regarding the impact of repeated vaccination on influenza vaccine effectiveness (VE) may cause confusion regarding the benefits of receiving the current season’s vaccine. With increasing numbers of people being immunized against influenza annually, the impact of repeated vaccination has gained significant interest. Ramsay et al BMC Medicine (2017) 15:159 impacts current VE, having been repeatedly vaccinated in earlier years may be detrimental to the protection of older adults when they need it most. Studies from the 1970s and 1980s found inconsistent results regarding the impact of repeated vaccination [3, 4]. In 1999, a systematic review and meta-analysis of field studies, trials, and serologic studies found no evidence of negative impacts of repeated vaccination [5]. Some studies have found VE to be reduced in those who received repeated prior influenza vaccinations [6,7,8]
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