What is viral shedding?

A man sneezing shows how viral shedding can occur
Photo via JAMA Network/YouTube
  • It’s when an infected person releases virus particles into the environment
  • Viral shedding is highest when COVID-19 symptoms are getting worse
  • It’s possible for asymptomatic people to shed the virus, as well

Upon infection, a virus multiplies inside the body, and when that person sneezes, coughs, or (in extreme cases) speaks, it has the potential to be released into the person’s environment. According to WebMD, this occurrence is called viral shedding.

Knowing how viral shedding happens is beneficial in understanding how COVID-19 spreads, and it helps scientists figure out the point when a person with the coronavirus is the most infectious. Researchers reportedly use terms like “low” and “high” to describe levels of viral shedding. 

Based on available scientific data thus far, the novel coronavirus is most contagious when symptoms appear like they are getting worse—this means that viral shedding is high. What’s alarming is that some patients appear to already be contagious even before they start showing signs of the virus (otherwise known as being “asymptomatic”). A study in The Lancet showed that the virus is most contagious in the first five days after symptoms appear. It’s unclear at this point how long viral shedding occurs when a person is infected with COVID-19, but a study released in November showed that no live sample was found in patients nine days after their symptoms began.

To contrast that, viral shedding for viruses like SARS and MERS doesn’t begin until a week after symptoms appear.

In a Mayo Clinic Q&A podcast episode, infectious diseases expert Dr. Gregory Poland explained the difficulties in tracking viral shedding associated with COVID-19. “We don’t actually culture the [COVID-19] virus, so I can’t tell you that [patients] are shedding [it],” he said. “The genetic components of the virus is what we test for and that’s what we are actually detecting.”

Poland, who is also the head of Mayo Clinic’s Vaccine Research Group, said that most instances of viral shedding usually don’t occur for more than a couple of weeks, but for some COVID-19 patients, it seems to occur for a longer period of time.  

According to the Johns Hopkins Medicine point-of-care guide for COVID-19, “In ill hospitalized patients or those with health problems, [viral shedding] may not be so short, but 28 days is used by some hospitals to remove airborne precautions.” A recent paper noted that viral shedding from respiratory tract samples positive for the coronavirus has been found to continue up to 63 days after the first appearance of signs, and it appears to persist even after the symptoms have disappeared.

Another study done on a small group of patients with mild COVID-19 noted that the duration of viral shedding may depend on how severe the infection is; among 137 survivors of the virus, shedding ranged between 8-37 days, with a median of 20 days. The longest-known case of viral shedding in a person with a normal immune response, according to Newsweek, is 61 days. That was topped by a 71-year-old woman who apparently was shedding for at least 70 days.

Viral shedding questions were also in the news when President Trump tested positive for COVID-19 in early October and yet still wanted to debate Joe Biden and return to the campaign trail less than 14 days after his diagnosis.

(For what it’s worth, cats also apparently can shed the coronavirus to other cats, but dogs do not.)

To date, there is still no conclusive data when viral shedding begins for someone who has the coronavirus, but it is safe to assume for now that it may happen even when you’re not showing any symptoms. The Johns Hopkins Medicine guide stated that asymptomatic people may represent 30-60% of total infections (causing some governments to say that asymptomatic people shouldn’t even be tested for the virus). It also noted that viral shedding may occur one to two days before symptoms show.

“Just because you’re asymptomatic, it doesn’t mean it is a benign disease,” Poland warned. “It can be and does do cellular damage to you. It can affect your brain, your heart, your lungs … virtually every organ and you may not know that and you have the potential of spreading it to other people who may have more severe disease.”

Even with the FDA approving the emergency use of two coronavirus vaccines, people who get the vaccine are still being asked to wear masks afterward because it’s possible they could still shed the virus if it ever infects their body.

Poland’s recommendation, apart from the universal precaution of mask-wearing and good hand washing, is to get tested for COVID-19 if you think there’s a good chance you may have been exposed to it. Extensive contact tracing to pinpoint potential exposures may also be necessary to lessen the spread of COVID-19 in communities.

Sources: WebMD, Arkansas Center for Health Improvement, Mayo Clinic, Johns Hopkins Medicine, PubMed Central, Medscape

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