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.Funding: 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.