CDC: Most Transmission of COVID-19 Likely From Asymptomatic or Presymptomatic Individuals

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Investigators from the CDC determined that the proportion of SARS-CoV-2 transmission in the community is likely to occur from individuals who are asymptomatic.

A study using a decision analytical model estimated that more than half of novel coronavirus disease 2019 (COVID-19) infections were likely transmitted by individuals not presenting with symptoms. These findings, from researchers at the United States Centers for Disease Control and Prevention, were published in JAMA Network Open.

Raw data from 8 studies in China were extracted and applied to the models in this analysis. Various projections were assessed for time-based transmission from presymptomatic, symptomatic, and asymptomatic individuals. The study assumed a median 5-day incubation period and symptom presentation by day 12 among 95% of symptomatic individuals.

The investigators estimated that 59% of transmissions came from individuals who did not present with symptoms (35% presymptomatic; 24% asymptomatic).

Assuming a transmission rate of 24% from asymptomatic individuals but altering the peak infectious time to 4 days, the transmission from presymptomatic individuals increased to 43%, making all non-symptomatic transmission 67%. By shifting the peak infectious time to 6 days, total non-symptomatic transmission was 51% (27% presymptomatic).

Broad uncertainty remained for transmission rates. Assuming transmission was high 2 days before and low 2 days after median symptom onset, transmission among non-symptomatic individuals ranged between 0% to 70%. With this broad definition of scenarios, the investigators observed a peak for situations in which at least 50% of new COVID-19 infections arose from individuals without symptoms; of these, 30% were likely from individuals who were asymptomatic.

Using the strictest model, which assumed 0% transmission from individuals who never developed symptoms, 25% of transmissions were caused by presymptomatic individuals.

This study was limited by only varying 2 values (peak infectious time and transmission among non-symptomatic individuals) and relying on assumptions derived from 5 studies in China for other input parameters.

The study authors concluded that using a wide range of scenarios about infectious and transmission rates, most models indicated more than half of COVID-19 transmissions arose from individuals who were not presenting with symptoms.


Johansson M A, Quandelacy T M, Kada S, et al. SARS-CoV-2 Transmission From People Without COVID-19 Symptoms. JAMA Netw Open. 2021;4(1):e2035057. doi:10.1001/jamanetworkopen.2020.35057.