COVID-19 Resource Center

COVID-19 Resource Center


WHO. COVID-19 Virtual Press conference, 8 June 2020. Accessed June 10, 2020.

Disclosures: Glatt reports no relevant financial disclosures. Healio Primary Care was unable to confirm Van Kerkhove and Ryan’s relevant financial disclosures prior to publication.
June 12, 2020
4 min read

Is asymptomatic spread common in COVID-19?


WHO. COVID-19 Virtual Press conference, 8 June 2020. Accessed June 10, 2020.

Disclosures: Glatt reports no relevant financial disclosures. Healio Primary Care was unable to confirm Van Kerkhove and Ryan’s relevant financial disclosures prior to publication.
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Although the idea that asymptomatic transmission is common in COVID-19 has been accepted by many experts and institutions — including the CDC — a recent statement by an employee of WHO has led many to question its prevalence.

During a virtual press conference, Maria D. Van Kerkhove, PhD, an infectious disease epidemiologist and COVID-19 technical lead with WHO, was asked about asymptomatic spread in light of a report from Reuters that stated half of new cases in Singapore were asymptomatic. Van Kerkhove said that in unpublished data the organization is seeing from countries with strong contact tracing programs, “they’re not finding secondary transmission onward. It’s very rare and much of that is not published in the literature.”

Quote from Glatt on asymptomatic transmission

The following day, WHO held a Q&A session on social media to clarify its stance on asymptomatic spread of COVID-19.

During the session, Van Kerkhove said that her statement was based on data from a small subset of studies that included asymptomatic cases in their contact tracing efforts as well as some unpublished data, and did not include modeling done by researchers “because there are so many unknowns” about asymptomatic spread.

She noted that “there’s a big range from the different models depending on how the models are done and where they’re done ... but some estimates of around 40% of transmission may be due to asymptomatic [people].”

What we know

Aaron E. Glatt, MD, professor and chair of medicine at Mount Sinai South Nassau and the hospital’s epidemiologist, told Healio Primary Care that in terms of how frequently asymptomatic transmission occurs in COVID-19, “we really don’t know the correct answer to that.”

A review published in the Annals of Internal Medicine by Daniel P. Oran, AM, and Eric J. Topol, MD, of the Scripps Research Translational Institute in La Jolla, California, estimated that asymptomatic patients could make up 40% to 45% of all SARS-CoV-2 infections, and that these patients could potentially transmit the virus for longer than 14 days.

In some asymptomatic cases, Oral and Topol wrote that, “the viral load of such asymptomatic persons has been equal to that of symptomatic persons, suggesting similar potential for viral transmission.”

Recently, WHO’s guidance on when to wear face masks referenced a study that has not yet been peer reviewed, which estimated that highest rate of asymptomatic transmission was 2.2%.

Glatt, who is also a spokesperson for the Infectious Disease Society of America, explained that “the real answer is probably somewhere in between 2.2% and 40%.”


Oran and Topol’s review included information from studies, unpublished manuscripts, news reports and information shared on Twitter from April 19, 2020, through May 26, 2020.

There were 16 cohorts in the review, including the passengers on the Diamond Princess cruise ship; the U.S.S. Theodore Roosevelt naval vessel; Vo’, Italy; and nursing facility residents in King County, Washington.

Oran and Topol found that only a small fraction of asymptomatic patients from cohorts with longitudinal reporting may develop symptoms eventually. In the cohort of nursing facility residents in King County, Washington, 88.9% of asymptomatic patients developed symptoms, but the authors noted that these patients were older, had more comorbid conditions and may have had repeated exposure to infection.

They also noted that of the 4,954 crew members tested on the U.S.S. Theodore Roosevelt, 17.3% were positive, and 60% of those cases were asymptomatic. As many of those cases continued to test positive for the virus after 14 days of quarantine, this cohort suggests that these patients may be able to transmit the virus to others past the typical quarantine period.

“The early data that we have assembled on the prevalence of asymptomatic SARS-CoV-2 infection suggest that this is a signicant factor in the rapid progression of the COVID-19 pandemic,” Oran and Topol wrote. “Medical practice and public health measures should be modied to address this challenge.”

Asymptomatic vs. presymptomatic

“The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets,” Van Kerkhove said during the WHO session.

Glatt explained that while the exact prevalence of asymptomatic spread is unclear, “what we can say, however, is that people who have symptoms are probably more contagious than people who don’t have symptoms at all.”

In addition, he noted that “many people who will develop symptoms in a couple of days but are now asymptomatic are probably highly contagious, even in the presymptomatic phase.”

Therefore, Glatt stressed that it is important to differentiate presymptomatic patients from asymptomatic patients who will never develop symptoms when monitoring risk transmission.

Presymptomatic transmission is the main driver behind decisions for the general public to wear masks, Glatt explained.

“One of the major concerns that we’ve had — and still have — is that there are people walking around who are healthy. They’re not restricting where they go,” he said, noting that people who were presymptomatic are probably highly contagious, particularly in the day or 2 before they became ill.


“In the past when we didn’t wear masks, that was a disaster,” Glatt said.

When everyone in the public wears masks, it helps to prevent transmission of the virus from presymptomatic patients and even from those who are in the early stages of infection and have started to experience symptoms, Glatt explained, noting that “you’re wearing the mask to protect everybody else” from the virus.

“People should remember that the reason some places are in the relatively good position they are in is because of masks and social distancing,” Glatt said. “It’s not because we have a vaccine. It’s not because we have a great treatment — it’s what we’ve done as a public that’s been more important than what the doctors have done.”

Surveillance efforts

To determine the prevalence of asymptomatic spread, “we need to really, really, have good contact tracing,” according to Glatt.

Michael Ryan, MD, executive director of WHO’s Health Emergencies Program, noted that the world health agency has been “focusing on identifying suspect cases, testing those cases and ensuring that their contacts are quarantined,” when offering advice on strategies to combat the spread of the novel coronavirus, “and a lot of studies will suggest that that’s been a very important intervention in this outbreak.”

He continued that while he is “absolutely convinced” that asymptomatic transmission is occurring, focusing efforts on detecting suspected cases of COVID-19 with clinical symptoms and identifying their contacts and quarantining them can help to lower the R0 — the basic reproduction number — to below one.

“We won’t stop all transmission, but what we do is suppress transmission,” he said.


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Reuters. Exclusive: Half of Singapore's new COVID-19 cases are symptomless, taskforce head says. Accessed June 12, 2020.

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WHO. Advice on the use of masks in the context of COVID-19. Accessed June 11, 2020.

WHO. COVID-19 Virtual Press conference, 8 June 2020. Accessed June 10, 2020.