Alternatively, SARS-CoV-2 aerosolization may effectively increase the number of people with true viral exposures given the same contact network. Differing viral load kinetics between the two viruses might explain this distinction SARS-CoV-2 is often present intermittently in the upper airways for many weeks ( Qi et al., 2020 Cao et al., 2020), while influenza is rarely shed for more than a week ( Pawelek et al., 2012). This pattern is not evident for influenza which has more homogeneous individual transmission ( Cowling et al., 2009 Brugger and Althaus, 2020). SARS-CoV-2 super-spreader events, in which the duration of contact between a single transmitter and large number of secondarily infected people is often limited to hours, are well documented ( Hamner et al., 2020 Park et al., 2020). ![]() Overdispersion has been quantified: approximately 10–20% of infected people account for 80% of SARS-CoV-2 transmissions ( Endo et al., 2020 Bi et al., 2020). If the average population R 0 is greater than 1, then exponential growth of cases occurs in the absence of effective interventions ( Lloyd-Smith et al., 2005). An over-dispersed R 0 means that most infected people do not transmit at all (individual R 0 = 0) while a minority of infected people are super-spreaders (individual R 0 >5). Second, there is substantial over-dispersion of the secondary infection distribution (individual R 0) for an individual infected with SARS-CoV-2 ( Endo et al., 2020). Underlying this observation is a highly variable incubation period, defined as time between infection and symptom onset, which often extends beyond an infected person’s peak viral shedding ( Ganyani et al., 2020). First, most transmissions occur during the pre-symptomatic phase of infection ( He et al., 2020 Moghadas et al., 2020 Tindale et al., 2020). Two features have proven challenging in containing outbreaks. The pandemic is rapidly expanding in the United States and is re-emerging focally in many countries that had previous success in limiting its spread ( ). The SARS-CoV-2 pandemic is an ongoing tragedy that has caused nearly 2 million deaths and massively disrupted the global economy. Rather, a person infected with SARS-CoV-2 exposes more people within equivalent physical contact networks, likely due to aerosolization. The higher predisposition of SARS-CoV-2 toward super-spreading events cannot be attributed to additional weeks of shedding relative to influenza. ![]() SARS-CoV-2 super-spreader events occur when an infected person is shedding at a very high viral load and has a high number of exposed contacts. We identify that people infected with SARS-CoV-2 or influenza can be highly contagious for less than 1 day, congruent with peak viral load. ![]() We designed mathematical models which link observed viral loads with epidemiologic features of each virus, including distribution of transmissions attributed to each infected person and duration between symptom onset in the transmitter and secondarily infected person. Unlike influenza, most SARS-CoV-2-infected people do not transmit while a small percentage infect large numbers of people. SARS-CoV-2 is difficult to contain because many transmissions occur during pre-symptomatic infection.
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