Could influenza transmission be reduced by restricting mass gatherings? Towards an evidence-based policy framework.

INTRODUCTION: Mass gatherings (MG) may provide ideal conditions for influenza transmission. The evidence for an association between MG and influenza transmission is reviewed to assess whether restricting MG may reduce transmission. METHODS: Major databases were searched (Pubmed, EMBASE, Scopus, CINAHL), producing 1706 articles that were sifted by title, abstract, and full-text. A narrative approach was adopted for data synthesis. RESULTS: Twenty-four papers met the inclusion criteria, covering MG of varying sizes and settings, and including 9 observational studies, 10 outbreak reports, 4 event reports, and a quasi-experimental study. There is some evidence that certain types of MG may be associated with increased risk of influenza transmission. MG may also "seed" new strains into an area, and may instigate community transmission in a pandemic. Restricting MGs, in combination with other social distancing interventions, may help reduce transmission, but it was not possible to identify conclusive evidence on the individual effect of MG restriction alone. Evidence suggests that event duration and crowdedness may be the key factors that determine the risk of influenza transmission, and possibly the type of venue (indoor/outdoor). CONCLUSION: These factors potentially represent a basis for a policy-making framework for MG restrictions in the event of a severe pandemic.