Majority of SARS-CoV-2 transmission due to asymptomatic individuals
Transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from asymptomatic individuals accounts for about half of all transmissions, according to a study published online Jan. 7 in JAMA Network Open.
Michael A. Johansson, Ph.D., from the U.S. Centers for Disease Control and Prevention in Atlanta, and colleagues examined the proportion of SARS-CoV-2 transmissions in the community that likely occurred from asymptomatic individuals. The relative amount of transmission was assessed from presymptomatic, never symptomatic, and symptomatic individuals across a range of scenarios.
The baseline assumptions for the model were that peak infectiousness was at the median of symptom onset and that 30 percent of individuals with infection remain asymptomatic and are 75 percent as infectious as those who develop symptoms. Based on these baseline assumptions, the researchers inferred that about 24 percent of all transmission was accounted for by persons with infection who never developed symptoms. In this base case, 59 percent of all transmission came from asymptomatic individuals: 35 and 24 percent from presymptomatic individuals and individuals who remain asymptomatic, respectively. At least 50 percent of new SARS-CoV-2 infections were estimated to have originated from exposure to infected, but asymptomatic, individuals under a broad range of values for each of these assumptions.
"These findings suggest that effective control also requires reducing the risk of transmission from people with infection who do not have symptoms," the authors write. "Multiple measures that effectively address transmission risk in the absence of symptoms are imperative to control SARS-CoV-2."
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This can't be good. The current crop of vaccines are very effective at reducing deaths but will not prevent individuals from getting infected with the virus. This leakiness of the vaccines allow pathogen populations to persist even at high levels of vaccination coverage, and reduced mortality of vaccinated individuals can lengthen their infectious period and hence promote the evolution of increased pathogen virulence.