Quotes from Experts

Asymptomatic transmission of COVID-19

SciLine reaches out to our network of scientific experts and poses commonly asked questions about newsworthy topics. Reporters can use these responses in news stories, with attribution to the expert.

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July 21, 2020


Why are some individuals who are COVID-19-positive asymptomatic?


“Nearly all respiratory viral diseases have a subset of infected persons who are asymptomatic. The only surprise with SARS-CoV-2 is that this proportion seems to be in the 40% range, suggesting the futility of relying on case finding and contract tracing as a primary disease control strategy. Rather, we need to complement testing and contact tracing with the ‘big five:’ masks, physical distancing, hand and surface hygiene, outdoor venues, and smaller groups.” (Posted July 21, 2020)

Sten H. Vermund, MD, PhD
Professor and Dean, Yale School of Public Health

“A significant proportion of those infected with SARS-CoV-2—the novel coronavirus that causes COVID-19—never develop symptoms. In reviewing data on asymptomatic SARS-CoV-2 infection from around the world, we concluded that a conservative estimate might be around 30%, and perhaps as high as 40 to 45%.

”Currently, we don’t know why some people who are infected with SARS-CoV-2 remain asymptomatic. Possible answers include age (some data have suggested that younger people are more likely to remain asymptomatic), previous exposure to other, less virulent coronaviruses (perhaps resulting in partial or full cross-immunity), or infection by lesser amounts of virus (conceivably provoking a commensurately lower immune response). But we should emphasize that these are only hypotheses, without much supporting evidence at this point.” (Posted July 21, 2020)

Joint statement by Eric J. Topol, MD, Professor of Molecular Medicine at Scripps Research and Founder and Director of the Scripps Research Translational Institute, La Jolla, California and Daniel P. Oran, AM, Digital Medicine Group, Scripps Research Translational Institute.

“Asymptomatic infections are common for respiratory infections and likely result from a combination of factors, including the person’s level of exposure to the virus, the extent of viral replication in the body, and the infected person’s immune response.” (Posted July 21, 2020)

Virginia Pitzer, ScD
Associate Professor of Epidemiology (Microbial Diseases), Yale School of Public Health

Do asymptomatic carriers of COVID-19 always develop symptoms eventually, or do some never display symptoms?


“It’s helpful to keep asymptomatic as a separate concept from presymptomatic. Everyone who gets ill will have a presymptomatic time when they are infectious but they are not yet aware of being ill. This can be less than a day or several days. When we refer to asymptomatic, we refer to persons with no substantial symptoms of disease at any time. (Even the WHO [World Health Organization] has been a bit muddled on this point in public pronouncements.)” (Posted July 21, 2020)

Sten H. Vermund, MD, PhD
Professor and Dean, Yale School of Public Health

“Asymptomatic carriers of SARS-CoV-2 never develop symptoms. In contrast, those who turn out to be presymptomatic eventually develop symptoms.

”Whether people are asymptomatic or presymptomatic, though, it now appears that around 45% of SARS-CoV-2 infections are transmitted by those who currently have no symptoms. So it’s important for all of us to assume that we’ve been infected, and to wear a mask and keep our distance from others outside our home.” (Posted July 21, 2020)

Joint statement by Eric J. Topol, MD, Professor of Molecular Medicine at Scripps Research and Founder and Director of the Scripps Research Translational Institute, La Jolla, California and Daniel P. Oran, AM, Digital Medicine Group, Scripps Research Translational Institute.

“While some individuals may be ‘presymptomatic’, meaning they are infectious but symptoms have not developed yet, others will never develop symptoms. It is estimated that between 20-40% of individuals infected with SARS-CoV-2 may never develop symptoms.” (Posted July 21, 2020)

Virginia Pitzer, ScD
Associate Professor of Epidemiology (Microbial Diseases), Yale School of Public Health

Are certain age groups more or less likely to be asymptomatic?


“It appears that children may be more likely to be asymptomatic than adults, but data are still coming in. Unfortunately, people jump to the conclusion that somehow this makes children less important for overall viral transmission, a very premature conclusion at present. Children are in inherent proximity to adults and, even if less infectious, they may make up for it by longer contact times and closer proximity. We just do not know yet, but I am dismayed that some pundits are already concluding the children are somehow less relevant for transmission of the virus. If so, that would make this virus unique among all communicable viruses!” (Posted July 21, 2020)

Sten H. Vermund, MD, PhD
Professor and Dean, Yale School of Public Health

“In general, younger people have appeared to be more likely to be asymptomatic, but the evidence is mixed.” (Posted July 21, 2020)

Joint statement by Eric J. Topol, MD, Professor of Molecular Medicine at Scripps Research and Founder and Director of the Scripps Research Translational Institute, La Jolla, California and Daniel P. Oran, AM, Digital Medicine Group, Scripps Research Translational Institute.

“There is evidence to suggest that younger individuals are more likely to be asymptomatic. For example, a study conducted among nursing home residents in Washington state found that most people who did not have symptoms but tested positive later went on to develop symptoms, whereas this proportion has been much lower in studies that are more representative of all the age groups in the general population.” (Posted July 21, 2020)

Virginia Pitzer, ScD
Associate Professor of Epidemiology (Microbial Diseases), Yale School of Public Health

What is known about the risks of COVID-19 transmission by asymptomatic individuals?


“There are clear cases of transmission from asymptomatic infected persons to others. We do not yet know whether it is as efficient as from symptomatic persons who may have higher viral loads. However, the symptomatic person may be in far less social circulation than the asymptomatic person, but at a population level, the asymptomatic persons who may be less infectious but are more likely to be socially linked may represent a very substantial public health threat.” (Posted July 21, 2020)

Sten H. Vermund, MD, PhD
Professor and Dean, Yale School of Public Health

“Based on the available evidence, we have concluded that asymptomatic individuals are capable of transmitting SARS-CoV-2, but we do not know how often this occurs.” (Posted July 21, 2020)

Joint statement by Eric J. Topol, MD, Professor of Molecular Medicine at Scripps Research and Founder and Director of the Scripps Research Translational Institute, La Jolla, California and Daniel P. Oran, AM, Digital Medicine Group, Scripps Research Translational Institute.

“Studies suggest that approximately half of all COVID-19 transmission may come from people with no symptoms (presymptomatic and asymptomatic cases).” (Posted July 21, 2020)

Virginia Pitzer, ScD
Associate Professor of Epidemiology (Microbial Diseases), Yale School of Public Health

What questions still remain about asymptomatic transmission, and what are researchers doing to answer them?


“How mobile and infectious are asymptomatic persons vs. symptomatic persons, and what are the aggregate relative contributions to overall transmission? (Hypothesis: maybe a symptomatic person is more infectious, but maybe they stay out of crowds, while the less infectious asymptomatic person is a greater public health threat because they do not know that they are a menace. Hence the need for distancing and mask use by us all!)

”Researchers are addressing this with quantitative viral studies, mathematical models, and outbreak investigations, among other strategies.” (Posted July 21, 2020)

Sten H. Vermund, MD, PhD
Professor and Dean, Yale School of Public Health

“The frequency of transmission of SARS-CoV-2 by asymptomatic individuals is currently unknown. To determine the frequency, researchers will need to do extensive contact tracing of infected individuals and perform genomic analyses to clarify the chains of viral transmission.” (Posted July 21, 2020)

Joint statement by Eric J. Topol, MD, Professor of Molecular Medicine at Scripps Research and Founder and Director of the Scripps Research Translational Institute, La Jolla, California and Daniel P. Oran, AM, Digital Medicine Group, Scripps Research Translational Institute.

“We still are not really sure how infectious asymptomatic people are relative to those with symptoms. While studies suggest viral shedding peaks on or before symptom onset, more studies are needed to identify and track viral shedding in asymptomatic individuals. While it is difficult to know who might become an asymptomatic case, this can be done by routinely sampling known contacts of other cases or others at high risk of infection. But at the same time, we also need to consider that asymptomatic individuals may be going out and coming into contact with more people while taking fewer precautions than those with symptoms.” (Posted July 21, 2020)

Virginia Pitzer, ScD
Associate Professor of Epidemiology (Microbial Diseases), Yale School of Public Health

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Sten H. Vermund, MD, PhD, Professor and Dean, Yale School of Public Health

None

Joint statement by Eric J. Topol, MD, Professor of Molecular Medicine at Scripps Research and Founder and Director of the Scripps Research Translational Institute, La Jolla, California and Daniel P. Oran, AM, Digital Medicine Group, Scripps Research Translational Institute.

None

Virginia Pitzer, ScD, Associate Professor of Epidemiology (Microbial Diseases), Yale School of Public Health

I am a member of the WHO Immunization and Vaccine-related Implementation Research Advisory Committee (IVIR-AC), have grant funding from NIH/NIAID and the Bill & Melinda Gates Foundation, and have received travel reimbursements from Pfizer and Merck for topics unrelated to COVID-19.