Daily Briefing

How much is asymptomatic spread driving Covid-19? Here's what the evidence says.


Nearly 60% of all new coronavirus cases may come from people who carry the virus but who exhibit no symptoms of Covid-19, according to a new CDC model—findings experts say are even more concerning in light of a new, more contagious variant of the virus that's emerged.

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What the research says on asymptomatic spread

For the model, published in JAMA Network Open, researchers at CDC looked at data from a number of Covid-19 studies from last year, which they used to establish several baseline assumptions. For example, the researchers presumed that 30% of individuals who contract the new coronavirus never develop symptoms but remain 75% as infectious as those who do develop symptoms.

The researchers then modeled out different transmission scenarios based on people who were "pre-symptomatic" and "never-symptomatic," using a variety of incubation periods and infectious periods, as well as several different timeframes between infection and the onset of potential symptoms.

Overall, the model predicted that 59% of coronavirus transmission would come from people without symptoms, including 35% from people who were pre-symptomatic and 24% from those who never showed symptoms at all.

Jay Butler, CDC deputy director for infectious diseases and co-author of the study, admitted the researchers used "a fairly simple mathematical model" for such a complicated and continually evolving pandemic, but they were nonetheless "surprised how well it held up under a broad range of base assumptions."

Butler added, "I have no doubt that we'll still have people say, 'Well, what if you did X, Y, or Z?' But hopefully being able to adjust the variables will help address some of those concerns, such as what we did."

Why asymptomatic spread matters

According to Butler, CDC wanted to do a study on asymptomatic spread because "[u]nfortuantely, there continues to be some skepticism about the value of community-wide mitigation efforts for preventing transmission such as masking, distancing, and hand hygiene."

The modelling confirms that "controlling the [C]ovid-19 pandemic really is going to require controlling the silent pandemic of transmission from persons without symptoms," Butler said. "The community mitigation tools that we have need to be utilized broadly to be able to slow the spread of [the new coronavirus] from all infected persons, at least until we have those vaccines widely available."

The researchers added that the modelling has even more relevance in light of the new, more contagious variant of the new coronavirus—called B 1.1.7—that emerged in the United Kingdom and has been found in eight U.S. states as of Thursday.

Separately, Richard Menzies, an epidemiologist who directs the McGill International TB Centre, added that the findings confirm the necessity of continued testing, especially since it's still unknown whether people who receive either of the approved mRNA vaccines can still spread the pathogen. "If they were asymptomatic but equally contagious, then that's going to have quite an impact on the epidemic," he said.

Some researchers flag concerns

However, some researchers voiced skepticism with the model's baseline assumptions, the Washington Post reports.

For instance, Muge Cevik, an infectious disease expert at the University of St. Andrews in Scotland, said the model's 75% contagiousness baseline assumption—which Butler ascribed to CDC's literature review of peer-reviewed and pre-print studies—is flawed. Citing a different literature review published in September, Cevik held that the relative contagiousness of asymptomatic people compared to symptomatic people was 35%.

Cevik added the study also doesn't account for the type of environment in which coronavirus spread occurs. "Maybe asymptomatic transmission is important, but it may be much more important in long-term care facilities and households," Cevik said. "That might mean that we need to do much more targeted testing for high-risk populations," instead of mass screening.

On the flip side, Frank Esper, a virologist and pediatric infectious disease specialist at the Cleveland Clinic, pointed out that the literature review upon which the modeling was based on pulled data from early studies in China, and that the new, more transmissible strain in the United States and other countries may actually spur more infections and, perhaps, a longer pre-symptomatic period. "So, the CDC study may actually undershoot asymptomatic infections," he said.

But he still thought the modeling provided valuable insights—perhaps not so much that more testing of potentially exposed but asymptomatic people was necessary, but that merely testing and doing contact tracing for symptomatic people was insufficient.

"We need to continue to assume that everyone is potentially infected whether they know it or now," Esper said. "And even though we have ramped up our testing to a much greater capacity than in the first wave, we need to continue to wear masks and socially distance because just identifying people who are sick and isolating or quarantining them is not going to be enough to contain the pandemic" (Guarino, Washington Post, 1/7; Swift, Medscape, 1/7; Shammas/Guarino, Washington Post, 1/7).


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