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People can transmit the virus that causes COVID-19 even if they don’t have symptoms. Getty Images
  • In a set of interim guidelines published on June 5, WHO estimated that 16 percent of people who contract SARS-CoV-2, the virus that causes COVID-19, never develop symptoms.
  • In an asymptomatic infection, someone contracts the virus and never develops symptoms.
  • In a pre-symptomatic infection, someone with the virus has not yet developed symptoms but eventually does so.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

During a press briefing on Monday, an official from the World Health Organization (WHO) told reporters that asymptomatic transmission of SARS-CoV-2 appears to be “very rare.”

These comments were made by Maria Van Kerkhove, PhD, an epidemiologist who’s acting as the technical lead of WHO’s COVID-19 response. SARS-CoV-2 is the novel strain of coronavirus that causes COVID-19.

Van Kerkhove’s comments triggered swift pushback from public health experts who want people to know that if someone contracts SARS-CoV-2, they can pass it on to other people — even if they don’t have symptoms.

“The bottom line is that we don’t have a good handle on the extent of asymptomatic transmission that is truly present in the community. Until we do, such individuals should be considered infectious and must wear a face mask or facial covering,” Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York, New York, told Healthline.

“We need messaging from public health leaders that is coherent, unified, and without equivocation in order to maintain the public trust,” he added.

After Monday’s press briefing, Van Kerkhove later clarified that people can transmit SARS-CoV-2 even if they never develop symptoms of COVID-19.

“I did not say that asymptomatic cases cannot transmit; they can,” Van Kerkhove reportedly said during an interview with TIME. “The question is, do they? And if they do, how often is that happening?” she continued.

In a set of interim guidelines published on June 5, WHO estimated that 16 percent of people who contract SARS-CoV-2 never develop symptoms.

They based that estimate on the results of a preprint review of research, which has been posted online but not yet peer-reviewed.

The authors of that preprint review found that asymptomatic people can transmit the virus, but some research suggests they do so at lower rates than people with symptoms.

In the wake of Van Kerkhove’s comments, many experts have emphasized that the true rates of asymptomatic infection and transmission of SARS-CoV-2 remain unknown.

“I think it’s really tough for us to know what the asymptomatic transmission rate is,” Dr. Eric Cioe-Pena, an emergency physician and Director of Global Health at Northwell Health in New Hyde Park, New York, told Healthline.

“Especially without 100 percent testing, you aren’t going to know who the asymptomatic people are,” he continued, “and the extent to which they’re able to tell who is transmitting [the virus] is still not well known because there’s no great epidemiological way of tracking it.”

Some researchers have reported higher estimates of asymptomatic infection, compared with the preprint review cited by WHO.

When scientists from Scripps Research Translational Institute assessed the available evidence, they found that up to 40 to 45 percent of SARS-CoV-2 infections may be asymptomatic. They also found that people with asymptomatic infections may be able to transmit the virus for weeks after contracting it.

Critics of Van Kerkhove’s comments have also drawn attention to the distinction between “asymptomatic” and “pre-symptomatic” infections.

In an asymptomatic infection, someone contracts the virus and never develops symptoms. In a pre-symptomatic infection, someone with the virus has not yet developed symptoms but eventually does so.

It can take up to 14 days for symptoms to appear after someone contracts SARS-CoV-2, and evidence suggests that it’s possible for them to pass the virus on to others before symptoms appear.

“Even if truly asymptomatic spread is very rare, pre-symptomatic transmission is likely to be important,” Carl Bergstrom, a biology professor at the University of Washington in Seattle, wrote on Twitter.

It’s also possible for people who have only mild symptoms to transmit the virus. In many cases, they may not even realize they’ve contracted it.

Although the true rates of asymptomatic and pre-symptomatic transmission remain unknown, the Centers for Disease Control and Prevention has reported cases in which people without symptoms appear to have passed SARS-CoV-2 on to others.

“Whether it’s a main form of transmission or not is still unclear,” Cioe-Pena said, “but there’s definitely data that show pre-symptomatic and asymptomatic transmission is occurring.”

To help limit the spread of the virus, Cioe-Pena encourages people to wash their hands, avoid touching their faces, and maintain 6 feet of distance from others when possible.

He also said that if everyone wore a face mask, that would help prevent transmission of the virus by people without symptoms.

“If I have the virus and I’m either pre-symptomatic or asymptomatic, then wearing a mask will mitigate the spread,” he said.

“The whole asymptomatic portion of this is why we’re asking everyone to wear masks,” he added.