Introduction Droplet spread and direct contact are crucial factors in influenza transmission. Mass gatherings (MG) seem to provide ideal conditions for transmission. This systematic review examined the evidence for an association between MG and influenza transmission, and assessed whether restricting MG is effective at reducing or preventing transmission.
Methods The published literature to July 2010 was reviewed by searching major databases (Pubmed, EMBASE, Scopus, and CINAHL), producing a combined list of 1670 articles that was sifted for relevance by title, abstract, and full-text assessment. A narrative approach was adopted for data synthesis.
Results 18 papers met the inclusion criteria, covering MG of varying sizes and settings, and including 5 observational studies, nine outbreak reports, three event surveillance reports, and a quasi-experimental study. There is evidence that some MG are associated with increased risk of influenza transmission. MG can also be “seeding” events for introducing new strains to an area, and may instigate community transmission in the early stages of a pandemic. Restriction of MG, in combination with other social distancing interventions, may help reduce transmission, but evidence is lacking on the effect of MG restriction alone.
Conclusion The evidence suggests that crowd density and event duration may be the key characteristics of an MG that determine the risk of influenza transmission. The type of venue (indoor or outdoor) also seems crucial. These considerations could potentially underlie the development of evidence-based, risk-dependent, policy-making frameworks for restricting MG in pandemic situations. Voluntary rather than legislated restrictions may be suitable.
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