Asymptomatic COVID-19 Cases: The Silent Spreaders

By the bioMérieux Connection Editors

Throughout the duration of the COVID-19 pandemic, there have been reports of “silent spreaders”—people who tested positive for the viral infection, but never experienced symptoms. These people are commonly referred to as asymptomatic.



So “symptomatic” COVID-19 means a person has reported a problem linked to this illness — in medicine, we sometimes call them “complaints.” Throw the prefix “a”—meaning “without”—in front of the word symptomatic, and you get “asymptomatic.” A person with no complaints, no problems. They feel healthy.

Dr. Paul Sax, Clinical Director of the Division of Infectious Diseases at Brigham and Women’s Hospital and Professor of Medicine at Harvard Medical School for wbur.org

Confusion often lies in the difference between asymptomatic and pre-symptomatic cases of COVID-19. As has been stated, those who are asymptomatic will never experience symptoms. Pre-symptomatic cases, however, go on to develop symptoms later in the course of the disease, then becoming symptomatic. When people do get sick from the coronavirus, it takes an average of five days and as many as two weeks to develop symptoms—the time between initial infection and the first symptoms is referred to as the pre-symptomatic phase.

Studies have shown that people become infectious during the pre-symptomatic period, thus, those who are asymptomatic or pre-symptomatic can contribute to the spread of the virus while not even knowing they are infected. Some studies have estimated that people without symptoms could be responsible for up to half the spread, making the virus extremely difficult to contain.



It doesn’t matter whether they are a true asymptomatic case or just pre-symptomatic; the public health risk is the same.

Dr. Monica Gandhi, Professor of Medicine, Division of HIV, Infectious Diseases and Global Medicine, University of California, San Francisco for sciencealert.com

How Many Cases of COVID-19 Are Asymptomatic?

Unfortunately, it is extremely difficult to tell how many cases are genuinely asymptomatic at this point in time. Studies have been undertaken to try to get a better idea, but these studies need to include sufficient follow-up with patients to exclude pre-symptomatic cases. For example, authors of a comprehensive literature review published at the beginning of June found that approximately 40% to 45% of those infected with SARS-CoV-2 will remain asymptomatic, but they acknowledged that only 5 of the 16 studies in their review included observations of an individual overtime, which would help rule out pre-symptomatic cases. 

Many experts state that the only way to get a definitive idea of the number of asymptomatic cases is to conduct universal mass testing. This entails testing people regardless of whether they have symptoms and tracking them over time to see if they develop symptoms later.



Only through “active surveillance” can we find people who have infections and are asymptomatic. Examples include testing all people admitted to the hospital for any reason, or testing groups who have had close contacts with an active case (household exposures, on cruise ships, in nursing homes), or doing tests as part of a population-based epidemiologic investigation.

Dr. Paul Sax for wbur.org

 A recent pre-print study detailed a mass testing campaign in San Francisco, in which community members were tested regardless of symptoms. The authors found that 53% of infected patients were asymptomatic when first tested and that 42% stayed asymptomatic over the following two weeks, which is in line with the percentage reported in the previously mentioned literature review (40% to 45%).

Asymptomatic Cases Demonstrate the Need for Masks & Physical Distancing 

Whether pre-symptomatic or asymptomatic, people not showing symptoms can spread the virus, which is why wearing masks and keeping distance are important to limiting transmission. Compared to most other viral infections, SARS-CoV-2 produces a high level of viral particles in the nose and mouth, which makes it that much easier for the virus to escape into the environment through talking, coughing, and even just breathing.

There are many studies and real-life examples that demonstrate that wearing a mask helps prevent the spread of COVID-19. A recent study in Health Affairs compared the growth rate of COVID-19 before and after mask mandates in 15 states and the District of Columbia and found that the mandates led to a slowdown in the daily growth rate. Another study found that countries with cultural norms or government policies favoring mask-wearing had lower coronavirus-related death rates.

As communities re-open around the world, masks may be the best protection against asymptomatic and pre-symptomatic spread.



Wearing a mask and practicing social distancing can prevent asymptomatic spread and help reduce the harm from this dangerous virus until we get a vaccine.

Dr. Monica Gandhi for sciencealert.com

Opinions in this article are not necessarily those of bioMérieux, Inc.

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