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CDC modelling shows asymptomatic carriers are responsible for more than half of new COVID-19 infections


A model of SARS-COV-2 transmission published in JAMANetwork Open from the Centers for Disease Control (CDC) on January 7, 2021 shows that more than half of new COVID-19 cases are passed on by asymptomatic or presymptomatic carriers.

From the abstract:

Design, Setting, and Participants  This decision analytical model assessed the relative amount of transmission from presymptomatic, never symptomatic, and symptomatic individuals across a range of scenarios in which the proportion of transmission from people who never develop symptoms (ie, remain asymptomatic) and the infectious period were varied according to published best estimates. For all estimates, data from a meta-analysis was used to set the incubation period at a median of 5 days. The infectious period duration was maintained at 10 days, and peak infectiousness was varied between 3 and 7 days (−2 and +2 days relative to the median incubation period). The overall proportion of SARS-CoV-2 was varied between 0% and 70% to assess a wide range of possible proportions.

Main Outcomes and Measures  Level of transmission of SARS-CoV-2 from presymptomatic, never symptomatic, and symptomatic individuals.

Results  The baseline assumptions for the model were that peak infectiousness occurred at the median of symptom onset and that 30% of individuals with infection never develop symptoms and are 75% as infectious as those who do develop symptoms. Combined, these baseline assumptions imply that persons with infection who never develop symptoms may account for approximately 24% of all transmission. In this base case, 59% of all transmission came from asymptomatic transmission, comprising 35% from presymptomatic individuals and 24% from individuals who never develop symptoms. Under a broad range of values for each of these assumptions, at least 50% of new SARS-CoV-2 infections was estimated to have originated from exposure to individuals with infection but without symptoms.

Conclusions and Relevance  In this decision analytical model of multiple scenarios of proportions of asymptomatic individuals with COVID-19 and infectious periods, transmission from asymptomatic individuals was estimated to account for more than half of all transmissions. In addition to identification and isolation of persons with symptomatic COVID-19, effective control of spread will require reducing the risk of transmission from people with infection who do not have symptoms. These findings suggest that measures such as wearing masks, hand hygiene, social distancing, and strategic testing of people who are not ill will be foundational to slowing the spread of COVID-19 until safe and effective vaccines are available and widely used.

This study was reported in the Washington Post today with commentary. The article is freely accessible via the link– you don’t need a subscription to the Post to read it. Sorry I don’t have time to go over the study and comment in detail, but it basically confirms what scientists have been saying for months: the virus is spread by asymptomatic carriers in roughly half of cases.

Carriers don’t have as much virus in their breath as people who are sick (especially those who are gravely ill) but they spread the virus just as readily, and the higher contagiousness of the new UK strain B.1.1.7 makes this even more dangerous.

(Electron Micrograph of SARS-COV-2 from Groopman lab)

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