Asymptomatic spread of COVID-19 may be rare, new research finds

Symptomatic cases were far more likely to transmit the virus than asymptomatic ones, the authors found.

Published: December 22, 2020 11:47am

Updated: December 24, 2020 9:28am

A study published this month in the American Medical Association's JAMA Network Open journal has offered further evidence that asymptomatic spread of COVID-19 may be significantly lower than previously thought. 

Fears of COVID-19 spreading asymptomatically have persisted since shortly after the beginning of the pandemic. Many public health authorities in the United States and elsewhere initially argued that only those individuals with symptoms of COVID-19 should take precautions such as wearing masks and staying at home.

But fears that the virus may be spread to a significant degree by asymptomatic carriers soon led government leaders to issue broad and lengthy stay-at-home orders and mask mandates out of concerns that anyone could be a silent spreader.

"[A]symptomatic individuals may be carriers of the COVID-19 virus and may unknowingly spread the virus to other individuals in close proximity," read a characteristic mask mandate by Delaware Gov. John Carney in late April, one of many issued in the spring in response to fears of asymptomatic spread. 

Yet evidence in support of common asymptomatic spread has remained elusive, and multiple studies and surveys conducted since the spring have suggested its prevalence may have been overstated. 

Lack of asymptomatic transmission 'is notable'

In the new study published in the JAMA Network Open journal, a team of researchers from the University of Florida and the University of Washington conducted a "meta-analysis of 54 studies with 77,758 participants" to determine "the estimated overall household secondary attack rate" of COVID-19. (The "secondary attack rate" of a virus, according to the Centers for Disease Control and Prevention, quantifies "transmission of illness in a household, barracks, or other closed population" compared to transmission in the wider community.) 

The authors determined that symptomatic cases were far more likely to transmit the virus than asymptomatic ones. The "secondary attack rate" of symptomatic cases was 18%, they found, compared to 0.7% for asymptomatic ones, a 25-fold difference. 

"The lack of substantial transmission from observed asymptomatic index cases is notable," the authors argue. "These findings are consistent with other household studies reporting asymptomatic index cases as having limited role in household transmission."

The authors of the study did not respond to queries seeking more information about their findings, including what the apparently low rate of asymptomatic spread could mean for ongoing COVID-19 mitigation policies. 

Their findings, as they noted, have been echoed elsewhere, with numerous researchers across the globe arguing that the spread of COVID-19 from non-symptomatic individuals appears to be overall fairly rare. 

In an editorial in the BMJ this week, two British experts pointed to "the absence of strong evidence that asymptomatic people are a driver of transmission" as a reason to question such practices as "mass testing in schools, universities, and communities." 

"Searching for people who are asymptomatic yet infectious," they wrote, "is like searching for needles that appear and reappear transiently in haystacks, particularly when rates are falling. Mass testing risks the harmful diversion of scarce resources."

A study out of China last month determined that there were "no positive tests" out of 1,174 "close contacts" of 300 asymptomatic cases. "Compared with symptomatic patients," the authors wrote, "asymptomatic infected persons generally have low quantity of viral loads and a short duration of viral shedding, which decrease the transmission risk of SARS-CoV-2." 

Another study published in August found that, among several hundred index cases, asymptomatic individuals transmitted the virus at a rate of 0.3%, roughly equal to the rate found in this month's JAMA report. Asymptomatic cases "were least likely to infect their close contacts," the authors wrote. 

In June, meanwhile, World Health Organization epidemiologist Maria Van Kerkhove told news media that, at the time, it "appear[ed] to be rare that an asymptomatic individual actually transmits [COVID] onward."

"We have a number of reports," Kerkhove said, "from countries who are doing very detailed contact tracing — they are following asymptomatic cases, they are following contacts, and they are not finding secondary transmission onward, it's very rare." 

A flurry of negative responses to Kerkhove's remarks resulted in what a number of media outlets incorrectly characterized as a "walkback." Kerkhove the following day characterized the question of asymptomatic spread as still "a big open question," though she did not rescind her earlier characterization of it as "very rare." 

The World Health Organization in July told Just the News that "while someone who never develops symptoms can also pass the virus to others, it is still not clear to what extent this occurs, and more research is needed in this area." In an October update to FAQs about transmission of the virus, the WHO said it was "still not clear how frequently [asymptomatic spread] occurs."

The organization did not respond to queries from Just the News this week about its efforts to make that determination. 

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