Can people spread the coronavirus if they don't have symptoms? 5 questions answered about asymptomatic COVID-19

Screening for symptoms of COVID-19 and self-quarantine can help prevent sick people from spreading the coronavirus. But more and more evidence suggests that people with no symptoms can also spread the virus. Monica Gandhi, an infectious disease doctor and researcher at the University of California at San Francisco, explains what is known about asymptomatic spread and why she believes it is a large part of what drives the pandemic.
What does it mean to be asymptomatic?
SARS-CoV-2 - the virus that causes COVID-19 - can produce a number of clinical manifestations.
Some people who are infected do not develop any symptoms at all. These patients are considered real asymptomatic cases.
When people get 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 pre-symptomatic phase.
Symptom screening can cover some cases of COVID-19, but about people who are infected but who show no symptoms? AP Photo / John Raoux
As a doctor of infectious diseases, when I hear of an asymptomatic spread of SARS-CoV-2, I think of a person who has no symptoms at the moment and is passing the virus on to someone else. It doesn't matter whether it's a real asymptomatic case or just a pre-symptomatic case. The risk to public health is the same.
How many people are asymptomatic?
The estimated proportion of cases of true asymptomatic cases - those that are infected and never develop symptoms - range from 18% to over 80%. The reasons for the enormous range 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 test people regardless of whether they have symptoms or not - an approach called universal mass testing - and to track them over time to determine whether they develop symptoms later. A recent mass test campaign in San Francisco found that 53% of infected patients were asymptomatic on the first test and 42% remained asymptomatic for the next two weeks.
Another recent article compared the results from 16 studies and estimated the overall rate of asymptomatic infections at 40 to 45%. This is in line with the San Francisco finding, but the studies examined were of different quality and size and are likely to include some pre-symptomatic cases.
Although none of these studies are perfect, much evidence supports a true asymptomatic rate of around 40% plus an additional proportion of patients who are pre-symptomatic.
Many places ask people with COVID-19 symptoms to stay away, but people who are infected and have no symptoms are unlikely to recognize that they have the virus.
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How can asymptomatic people spread the coronavirus?
Compared to most other viral infections, SARS-CoV-2 produces an unusually high proportion of virus particles in the upper respiratory tract - especially in the nose and mouth. If these virus particles escape into the environment, one speaks of virus excretion.
Researchers have found that pre-symptomatic people shed the virus at an extremely high rate, similar to seasonal flu. But people with the flu usually don't excrete viruses until they have symptoms.
The location of the shed is also important. SARS-CoV - the virus that caused the SARS epidemic in 2003 - does not excrete much from the nose and mouth. It replicates deep in the lungs. Because SARS-CoV-2 is present in large numbers in a person's nose and mouth, it is much easier for the virus to get into the environment.
When people cough or talk, they spray droplets of saliva and mucus into the air. Because SARS-CoV-2 is so heavily buried in the nose and mouth, these droplets are likely to spread like people without symptoms spread the virus.
How much asymptomatic spread takes place?
Public health experts are unsure of the extent to which asymptomatic or pre-symptomatic patients are spreading. However, there is some strong evidence that this is a major cause of this pandemic.
An early model estimate found that 80% of infections are due to the spread of undocumented cases. The undocumented patients were probably asymptomatic or had only extremely mild symptoms. Although interesting, the researchers have made many assumptions in this model, making it difficult to assess the accuracy of this prediction.
A study of outbreaks in Ningbo, China found that people without symptoms spread the virus as easily as people with symptoms. If at any point in time half of all infected people are without symptoms and these people can transmit SARS-CoV-2 as easily as symptomatic patients, it can be assumed that a large percentage of the spread comes from people without symptoms.
Even without knowing the exact numbers, the Centers for Disease Control and Prevention believe that transmitting people without symptoms makes a significant contribution to the rapid spread of SARS-CoV-2 around the world.
Preventive measures, especially wearing a universal mask, are the best ways to limit asymptomatic spread. David McNew / Stringer / Getty Images News about Getty Images
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What can we do to limit asymptomatic spread?
Every time a virus can spread to people without symptoms, you need to take preventive measures.
Social distancing measures and barriers work, but they have major economic and social impacts. These were necessary when epidemiologists didn't know how the virus spread, but now we know that it exits the upper respiratory tract in large quantities.
This means that wearing a universal mask is the best way to limit transmission, and there is evidence to support this idea.
On April 3, the CDC recommended that all members of the public wear face coverings outside and around the house. The World Health Organization finally followed suit and recommended universal public masking on June 5.
At this point, no one knows exactly how many cases of COVID-19 result from asymptomatic spread. But I and many other infectious disease researchers are convinced that it plays an important role in this pandemic. Wearing a mask and practicing social distancing can prevent asymptomatic spread and help reduce the damage from this dangerous virus until we get a vaccine.
[You need to understand the coronavirus pandemic and we can help you. Read The Conversation newsletter.]
This article has been republished by The Conversation, a nonprofit news site dedicated to the exchange of ideas from academic experts.

Continue reading:
Masks help stop the spread of the corona virus - science is simple and I am one of 100 experts who call on governors to require public masks to be worn
5 ways the world can now better deal with a pandemic than it did in 1918

Monica Gandhi receives funding from the National Institutes of Health for HIV-related research.

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