Safety and effectiveness of dose-sparing strategies for intramuscular seasonal influenza vaccine: a rapid scoping review

BACKGROUND: The objective of this rapid scoping review was to identify studies of dose-sparing strategies for administration of intramuscular seasonal influenza vaccines in healthy individuals of all ages. METHODS: Comprehensive literature searches were executed in MEDLINE, Embase and the Cochrane library. The grey literature was searched via international clinical trial registries for relevant studies published in English in the last 20 years. We included studies in healthy humans of any age that used any dose-sparing strategy to administer intramuscular seasonal influenza vaccines. Title/abstract and full-text screening were carried out by pairs of reviewers independently. Data extraction was conducted by a single reviewer and verified by a second reviewer. Our outcomes were influenza infections, intensive care unit admission, pneumonia, hospitalisations, adverse events and mortality. Results were summarised descriptively. RESULTS: A total of 13 studies with 10 351 participants were included in the review and all studies were randomised controlled trials (RCTs) conducted between 2006 and 2019. The most common interventions were the trivalent influenza vaccine (n=10), followed by the quadrivalent influenza vaccine (n=4). Nine studies included infants/toddlers 6–36 months old and one of these studies also included children and adolescents. In these nine studies, no clinical effectiveness outcomes were reported. Of the four adult studies (≥18 years), two studies reported on effectiveness outcomes, however, only one RCT reported on laboratory-confirmed influenza. CONCLUSIONS: Due to the low number of studies in healthy adults and the lack of studies assessing confirmed influenza and influenza-like illness, there remains a need for further evaluation.