A systematic review 1 including 7 studies (10 trials) with a total of 5117 subjects in the field studies and 52 trials with a total of 12,341 subjects in serologic studies was abstracted in DARE. The study question was whether the protection rate decreases if vaccinations are repeated annually. The pooled protection-rate difference in field studies was close to 0, thus detecting no difference between single and multiple vaccinations. In the serologic studies, 9 trials were significantly in favour of single vaccination, and 7 trials were in favour of multiple vaccination.
Comment: The quality of evidence is downgraded by indirectness of evidence (mostly surrogate (serological) outcomes).
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