Background: Which virological factors mediate overdispersion in the transmissibility of emerging viruses remains a longstanding question in infectious disease epidemiology. Methods: Here, we use systematic review to develop a comprehensive dataset of respiratory viral loads (rVLs) of SARS-CoV-2, SARS-CoV-1 and influenza A(H1N1)pdm09. We then comparatively meta-analyze the data and model individual infectiousness by shedding viable virus via respiratory droplets and aerosols. Results: The analyses indicate heterogeneity in rVL as an intrinsic virological factor facilitating greater overdispersion for SARS-CoV-2 in the COVID-19 pandemic than A(H1N1)pdm09 in the 2009 influenza pandemic. For COVID-19, case heterogeneity remains broad throughout the infectious period, including for pediatric and asymptomatic infections. Hence, many COVID-19 cases inherently present minimal transmission risk, whereas highly infectious individuals shed tens to thousands of SARS-CoV-2 virions/min via droplets and aerosols while breathing, talking and singing. Coughing increases the contagiousness, especially in close contact, of symptomatic cases relative to asymptomatic ones. Infectiousness tends to be elevated between 1-5 days post-symptom onset. Conclusions: Intrinsic case variation in rVL facilitates overdispersion in the transmissibility of emerging respiratory viruses. Our findings present considerations for disease control in the COVID-19 pandemic as well as future outbreaks of novel viruses.
Bibliographical noteFunding Information:
Funding: Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery
supported by the Canadian Institutes of Health Research (Canadian COVID-19 Rapid Research
Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant program, NSERC Senior Industrial Research Chair program and the Toronto COVID-19 Action Fund.. We thank T. Alba (Toronto) for discussion on statistical methods. We thank J. Jimenez (Colorado) for discussion on the characteristics of aerosols and droplets. We thank E. Lavezzo and A. Chrisanti (Padova) and A. Wyllie, A. Ko and N. Grubaugh (Yale) for responses to data inquiries. P.Z.C. was supported by the NSERC Vanier Scholarship (608544). D.N.F. was supported by the Canadian Institutes of Health Research (Canadian COVID-19 Rapid Research Fund, OV4-170360). F.X.G. was supported by the NSERC Senior Industrial Research Chair program, NSERC Discovery Grant program and the Toronto COVID-19 Action Fund.
inquiries. P.Z.C. was supported by the NSERC Vanier Scholarship (608544). D.N.F. was
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