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No, a recent study didn’t find that there was “no asymptomatic” spread of COVID-19
If Your Time is short
• The journal article in question found people with symptoms were more contagious than those without, but asymptomatic people did transmit the virus.
• The paper looks only at household transmission, not community spread. An asymptomatic individual who went out in the community could still infect a sizable number of people.
• The paper itself warns that the number of studies looking at asymptomatic transmission is small, making it hard to draw sweeping conclusions.
Critics of policies to contain community spread of the coronavirus have taken to social media to tout a study they said showed no risk of asymptomatic individuals spreading the virus.
This argument was shared on Twitter by Jenny Beth Martin, a co-founder of the Tea Party Patriots, and was later amplified by Tomi Lahren, the conservative commentator and Fox Nation host, on both Twitter and Facebook.
Martin tweeted, "Report: University of Florida researchers have found no asymptomatic or presymptomatic spread of Covid. The study was published online by the Journal of the American Medical Association. This could change everything."
Lahren shared Martin’s tweet, adding the comment, "So ya mean locking down the country, destroying businesses, robbing students of real learning and shaming people into giving up our rights was not necessary?! Ya don’t say...."
The post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)
Neither social media post provided a link to the study in question, but it appears to be a paper published on Dec. 14 in JAMA Network Open, an open-access medical journal published by the American Medical Association. The article was co-authored by four researchers from the University of Florida and two affiliated with the University of Washington.
We reached out to several medical experts, including one of the co-authors, and they agreed that the portrayal of the journal article on social media ignored several caveats included in the article and made unjustified logical leaps. (Attempts to reach out to Martin and Lahren were unsuccessful.)
The study was a "meta-analysis" — a paper that pools existing studies on a topic and tries to draw broader conclusions using a larger data set than any of the studies had individually.
Specifically, the study looked at how much the virus spread within households. The researchers found that among the studies that met their parameters, the "secondary attack rates" — that is, the likelihood of spread from an infected person to a previously uninfected person within a household — were higher among people who had symptoms than among those who didn’t: 18% for symptomatic individuals vs. 0.7% for asymptomatic individuals.
The notion that people with symptoms are more likely to infect others is not a new one. Coughing, for example, is a symptom of COVID-19 and an especially effective way to spread the virus. One meta-analysis study published in September by researchers from Harvard University, Canada, Italy, Switzerland and the United Kingdom found that "asymptomatic patients can transmit" the virus to others, but that "such individuals are responsible for fewer secondary infections than people with symptoms in the same studies." Other studies have also shown reduced rates of transmission among asymptomatic individuals.
In other words, there’s consistent evidence that asymptomatic individuals are less likely to spread the virus than symptomatic individuals are. But the paper didn’t find the rate of spread to be zero. Moreover, Martin’s original tweet mischaracterized the paper’s conclusions, and Lahren’s comment exacerbated the misunderstanding.
Here are the two major fallacies, according to experts.
Only four of the 54 studies analyzed in the paper even tracked asymptomatic spread, making it hard to draw detailed conclusions. The paper noted this twice, cautioning that "less data were available" on asymptomatic cases and that "there were few studies" involving asymptomatic individuals.
The difference between symptomatic and asymptomatic spread was "notable, but it was not meant to upend" the broader understanding of the subject, said Natalie E. Dean, an assistant professor of biostatistics at the University of Florida, and one of the co-authors of the paper.
The paucity of data on asymptomatic spread is not surprising, experts said, since it requires a more extensive testing regimen, raising the cost and difficulty for researchers.
Such studies "would require repeated, complete-household testing to ascertain causality and the origin and timing of infections in the household," said Brooke Nichols, an infectious-disease mathematical modeler at the Boston University School of Public Health.
In fact, Nichols said, we simply don’t know enough yet to determine how commonly asymptomatic spread occurs. "The four studies that the JAMA Open Network meta-analysis has reviewed had remarkably small sample sizes, and the studies were not designed to answer this specific question," Nichols said.
The study in question looked only at spread within households. A household is a more limited environment that is easier to control and study than a full community.
The study’s focus on households, rather than on community spread, is relevant because Lahren’s critique largely addresses policies that govern community interactions, such as restrictions on businesses, public gatherings and schools.
The course of the spread in the United States makes clear that most transmission has been occurring at the community level, said William Schaffner, a professor of preventive medicine, health policy and infectious diseases at Vanderbilt University.
"It is self-evident that here in the U.S. and in other countries, when we have let down our guard, that's been followed by an explosion of COVID transmission that results in people being put into hospitals and ICUs," Schaffner said. "Countries such as Australia and New Zealand that have really instituted these interventions, including masking, social distancing, and controlling the size of groups, have controlled the spread better than any other countries."
In addition, the papers that have addressed community spread suggest that asymptomatic spread does occur.
Experts added that while one might think that transmission of the virus is more likely to occur in the enclosed spaces of a household, this ignores a key factor: Asymptomatic people will encounter many more people if they’re out and about while infected.
"If their viral loads are lower, but their contact rates are higher — the number of people a person sees in one day — then the likelihood of transmission could become similar," Nichols said.
Facebook posts said a study published in the Journal of the American Medical Association by University of Florida researchers has found no asymptomatic or presymptomatic spread of COVID.
The journal article in question did find lower transmission rates from asymptomatic individuals compared with symptomatic individuals, though the rate was not zero, as the post said.
The posts ignore that the paper itself warns that the number of studies looking at asymptomatic transmission is small. And the paper looks only at household transmission, not community spread. Asymptomatic people going about their lives in the community would encounter many more people than someone stuck at home, meaning they could still infect a sizable number of people.
We rate the statement Mostly False.
Jenny Beth Martin, tweet, Dec. 27, 2020
Tomi Lahren, tweet and Facebook post, Dec. 27, 2020
JAMA Network Open, "Household Transmission of SARS-CoV-2: A Systematic Review and Meta-analysis," Dec. 14, 2020
MedRxiv, "Defining the role of asymptomatic and pre-symptomatic SARS-CoV-2 transmission – a living systematic review," Sept. 1, 2020
International Journal of Infectious Diseases, "Delivery of infection from asymptomatic carriers of COVID-19 in a familial cluster," May 2020
PLOS Medicine, "Occurrence and transmission potential of asymptomatic and presymptomatic SARS-CoV-2 infections: A living systematic review and meta-analysis," Sept. 22, 2020
Nature, "What the data say about asymptomatic COVID infections," Nov. 18, 2020
Email interview with Brooke Nichols, infectious- disease mathematical modeler at the Boston University School of Public Health, Dec. 28, 2020
Email interview with Tara C. Smith, Kent State University professor of epidemiology, Dec. 28, 2020
Interview with Natalie E. Dean, assistant professor of biostatistics at the University of Florida, Dec. 28, 2020
Interview with William Schaffner, professor of preventive medicine, health policy and infectious diseases at Vanderbilt University, Dec. 28, 2020
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No, a recent study didn’t find that there was “no asymptomatic” spread of COVID-19
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