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Detecting COVID-19 symptoms and self-quarantining are good for preventing sick people from spreading the coronavirus. But mounting evidence suggests that symptom-free people are also spreading the virus. Monica Gandhi, an infectious disease doctor and researcher at the University of California, San Francisco, explains what is known about the asymptomatic spread and why she believes it may be a large part of what is driving the pandemic.
What does it mean to be asymptomatic?
SARS-CoV-2, the virus that causes COVID-19, can produce a variety of clinical manifestations.
Some infected people never develop any symptoms. These patients are considered true asymptomatic cases.
When people get sick from the coronavirus, it takes an average of five days and up to two weeks to develop symptoms that can range from very mild to extremely dangerous. The time between the initial infection and the first symptoms is called the presymptomatic phase.
As an infectious disease doctor, when I hear about the asymptomatic spread of SARS-CoV-2, I think of a person who has no symptoms the moment they give the virus to someone else. It does not matter if it is a true asymptomatic or simply pre-symptomatic case; The risk to public health is the same.
How many people are asymptomatic?
Estimates of the proportion of true asymptomatic cases, those who are infected and never develop symptoms, range from 18% to over 80%. The reasons for the wide variety of estimates are still unclear, but some studies are better than others.
The most accurate way to determine the rate of asymptomatic cases is to evaluate people regardless of whether or not they have symptoms, an approach called universal mass testing, and track them over time to see if they develop symptoms later. A recent mass testing campaign in San Francisco found that 53% of infected patients were asymptomatic when they were first tested and 42% remained asymptomatic for the next two weeks.
Another recent article compared the evidence from 16 studies and estimated that the overall rate of asymptomatic infection is 40% to 45%. This is in line with the San Francisco finding, but the studies sampled were of varying quality and size and probably included some pre-symptomatic cases.
Although none of these studies is perfect, much evidence supports a true asymptomatic rate of around 40%, plus some fraction of patient additions that are pre-symptomatic.
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How can asymptomatic people spread the coronavirus?
Compared to most other viral infections, SARS-CoV-2 produces an unusually high level of viral particles in the upper respiratory tract, specifically in the nose and mouth. When these viral particles escape into the environment, this is called viral shedding.
Researchers have found that pre-symptomatic people clear the virus at an extremely high rate, similar to seasonal flu. But people with the flu usually don’t clear the virus until they have symptoms.
The location of the detachment is also important. SARS-CoV, the virus that caused the SARS epidemic in 2003, does not release much from the nose and mouth. It replicates deep in the lungs. Since SARS-CoV-2 is present in large quantities in a person’s nose and mouth, it is much easier for the virus to escape into the environment.
When people cough or speak, they spray droplets of saliva and mucus into the air. Since SARS-CoV-2 sheds both in the nose and mouth, these droplets are likely to transmit the virus to people without symptoms.
How much asymptomatic spread is occurring?
Public health experts don’t know exactly how much spread is caused by asymptomatic or pre-symptomatic patients. But there are some revealing clues that it is a major driver of this pandemic.
An initial modeling estimate suggested that 80% of infections could be attributed to the spread of undocumented cases. Presumably, the undocumented patients were asymptomatic or had only extremely mild symptoms. Although interesting, the researchers made many assumptions in that model, making it difficult to judge the accuracy of that prediction.
A study looking at outbreaks in Ningbo, China found that people without symptoms transmit the virus as easily as people with symptoms. If half of all infected people have no symptoms at some point, and those people can transmit SARS-CoV-2 as easily as symptomatic patients, it’s safe to assume that a large percentage of spread comes from people without symptoms.
Even without knowing the exact numbers, the Centers for Disease Control and Prevention believes that transmission of symptom-free people is a major contributor to the rapid spread of SARS-CoV-2 worldwide.
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What can we do to limit asymptomatic spread?
Whenever a virus can be transmitted by people without symptoms, they must resort to preventive measures.
Social distancing measures and blockades work, but they have great economic and social repercussions. This was necessary when epidemiologists did not know how the virus was spreading, but we now know that it is shed in large amounts from the upper respiratory tract.
This means that the use of universal masks is the best tool to limit transmission, and there is evidence to support that idea.
On April 3, the CDC recommended that all members of the public wear face covers when outside the home and near others. The World Health Organization finally did the same and recommended universal public masking on June 5.
At this point, no one knows exactly how many cases of COVID-19 are from asymptomatic spread. But I and many other infectious disease researchers are convinced that it is playing an important role in this pandemic. Wearing a mask and practicing social distancing can prevent asymptomatic spread and help reduce damage from this dangerous virus until we get a vaccine.
Monica Gandhi, Professor of Medicine, Division of HIV, Infectious Diseases and Global Medicine, University of California, San Francisco.
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