What is the role of superspreading events in influenza pandemics?
Interesting study from 2014 about the impact of mass gatherings on an influenza pandemic, commissioned by the Department of Health and produced by Public Health England:
The observational studies and the quasi-experimental study together provided evidence that mass gatherings are associated with a risk of influenza transmission, particularly within the specialised setting of Hajj. New evidence supports the findings that respiratory viruses (including influenza) are transmitted at specialised events such as the Hajj; however specific evidence for pandemic influenza does not suggest widespread transmission occurs. Four strands of evidence emerged from the outbreak reports.
First, they provided further evidence linking mass gatherings with influenza transmission. Second, the evidence suggests that crowd density and event duration may be the key characteristics of a mass gathering that determine the risk of influenza transmission. There is some new evidence that ships provide an example of a specialised setting that may be a risk factor for influenza outbreaks, particularly out of season. It is therefore argued that the mass gathering definition should be expanded to include civilian and military ships.
Third, some evidence suggests that mass gatherings can be “seeding” events leading to the introduction of new strains of influenza into the host geographical area, or instigating community transmission in the early stages of a pandemic. Finally, evidence from the historical outbreak analyses suggests that, when implemented in combination with other social distancing interventions such as isolation measures and school closures, restriction of mass gatherings may significantly help to reduce influenza transmission.
It was not possible to tease out from the data available any conclusive evidence on the individual effect of restriction of mass gatherings. However there was no new evidence to support hypotheses that restrictions can be effective at reducing influenza transmission in these settings. Surveillance reports from multi-location sports championships (including World Cup tournaments, the Winter Olympiad and the Asian Youth Games) provided no evidence of significant influenza transmission at such events, and one new study supported this view.
Third, some evidence suggests that mass gatherings can be “seeding” events leading to the introduction of new strains of influenza into the host geographical area, or instigating community transmission in the early stages of a pandemic. Finally, evidence from the historical outbreak analyses suggests that, when implemented in combination with other social distancing interventions such as isolation measures and school closures, restriction of mass gatherings may significantly help to reduce influenza transmission.
It was not possible to tease out from the data available any conclusive evidence on the individual effect of restriction of mass gatherings. However there was no new evidence to support hypotheses that restrictions can be effective at reducing influenza transmission in these settings. Surveillance reports from multi-location sports championships (including World Cup tournaments, the Winter Olympiad and the Asian Youth Games) provided no evidence of significant influenza transmission at such events, and one new study supported this view.
[...]
Conclusion: The evidence to help address important public health questions around mass gatherings and influenza transmission is sparse, especially in the context of an influenza pandemic. This remains the case following the recent update. However, it is possible to draw three principal conclusions from the review.
Firstly, mass gatherings are very varied and the type, size, duration and setting of such events may play a role in the risk of influenza transmission. Secondly, there is some evidence that influenza may be transmitted at certain kinds of mass gatherings. Thirdly, limited - and mainly historical - evidence indicates that restrictions of mass gatherings can reduce transmission when part of a package of other public health interventions including isolation and school closures.
In terms of policy implications, it is clear that mass gatherings should not be considered as a homogenous group of events; the adoption of a common terminology for describing mass gatherings and the development of a hierarchy based on factors such as event size, duration, and crowd density may be helpful towards risk assessment and policy decision making. The inclusion of updated evidence to support a new type of setting, ships strengthens this argument that certain type of settings should be more carefully analysed.
While there is little, if any direct evidence, to support banning mass gatherings, voluntary rather than legislated restrictions may, if implemented as part of a package of other public health measures, be a pragmatic and beneficial approach should the severity of a future pandemic warrant extraordinary measures. In addition, the practical implications of potential policies aimed at mass gathering restrictions should be carefully assessed, including economic and logistical ramifications. [split up into paragraphs for easier reading. ks]
Firstly, mass gatherings are very varied and the type, size, duration and setting of such events may play a role in the risk of influenza transmission. Secondly, there is some evidence that influenza may be transmitted at certain kinds of mass gatherings. Thirdly, limited - and mainly historical - evidence indicates that restrictions of mass gatherings can reduce transmission when part of a package of other public health interventions including isolation and school closures.
In terms of policy implications, it is clear that mass gatherings should not be considered as a homogenous group of events; the adoption of a common terminology for describing mass gatherings and the development of a hierarchy based on factors such as event size, duration, and crowd density may be helpful towards risk assessment and policy decision making. The inclusion of updated evidence to support a new type of setting, ships strengthens this argument that certain type of settings should be more carefully analysed.
While there is little, if any direct evidence, to support banning mass gatherings, voluntary rather than legislated restrictions may, if implemented as part of a package of other public health measures, be a pragmatic and beneficial approach should the severity of a future pandemic warrant extraordinary measures. In addition, the practical implications of potential policies aimed at mass gathering restrictions should be carefully assessed, including economic and logistical ramifications. [split up into paragraphs for easier reading. ks]
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