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Asymptomatic COVID-19: Five Questions Answered

Health + Wellness
Asymptomatic COVID-19: Five Questions Answered
Palm trees stand behind a street art piece by artist Pony Wave depicting two people kissing while wearing face masks on Venice Beach on March 21 in Venice, California. Mario Tama / Getty Images

By William Petri

Blood tests that check for exposure to the coronavirus are starting to come online, and preliminary findings suggest that many people have been infected without knowing it. Even people who do eventually experience the common symptoms of COVID-19 don't start coughing and spiking fevers the moment they're infected.


William Petri is a professor of medicine and microbiology at the University of Virginia who specializes in infectious diseases. Here, he runs through what's known and what isn't about asymptomatic cases of COVID-19.

How common is it for people to contract and fight off viruses without knowing it?

In general, having an infection without any symptoms is common. Perhaps the most infamous example was Typhoid Mary, who spread typhoid fever to other people without having any symptoms herself in the early 1900s.

My colleagues and I have found that many infections are fought off by the body without the person even knowing it. For example, when we carefully followed children for infection by the parasite Cryptosporidia, one of the major causes of diarrhea, almost half of those with infections showed no symptoms at all.

In the case of the flu, estimates are that anywhere from 5 percent to 25 percent of infections occur with no symptoms.

For the most part, symptoms are actually a side effect of fighting off an infection. It takes a little time for the immune system to rally that defense, so some cases are more aptly considered presymptomatic rather than asymptomatic.

How can someone spread coronavirus if they aren’t coughing and sneezing?

Everyone is on guard against the droplets that spray out from a coronavirus patient's cough or sneeze. They're a big reason public health officials have suggested everyone should wear masks.

But the virus also spreads through normal exhalations that can carry tiny droplets containing the virus. A regular breath may spread the virus several feet or more.

Spread could also come from fomites — surfaces, such as a doorknob or a grocery cart handle, that are contaminated with the coronavirus by an infected person's touch.

What’s known about how contagious an asymptomatic person might be?

No matter what, if you've been exposed to someone with COVID-19, you should self-quarantine for the entire 14-day incubation period. Even if you feel fine, you're still at risk of spreading the coronavirus to others.

Most recently it has been shown that high levels of the virus are present in respiratory secretions during the "presymptomatic" period that can last days to more than a week prior to the fever and cough characteristic of COVID-19. This ability of the virus to be transmitted by people without symptoms is a major reason for the pandemic.

After an asymptomatic infection, would someone still have antibodies against SARS-CoV-2 in their blood?

Most people are developing antibodies after recovery from COVID-19, likely even those without symptoms. It is a reasonable assumption, from what scientists know about other coronaviruses, that those antibodies will offer some measure of protection from reinfection. But nothing is known for sure yet.

Recent serosurveys in New York City that check people's blood for antibodies against SARS-CoV-2 indicate that as many as one in five residents may have been previously infected with COVID-19. Their immune systems had fought off the coronavirus, whether they'd known they were infected or not — and many apparently didn't.

How widespread is asymptomatic COVID-19 infection?

No one knows for sure, and for the moment lots of the evidence is anecdotal.

For a small example, consider the nursing home in Washington where many residents became infected. Twenty-three tested positive. Ten of them were already sick. Ten more eventually developed symptoms. But three people who tested positive never came down with the illness.

When doctors tested 397 people staying at a homeless shelter in Boston, 36 percent came up positive for COVID-19 — and none of them had complained of any symptoms.

In the case of Japanese citizens evacuated from Wuhan, China and tested for COVID-19, fully 30 percent of those infected were aymptomatic.

An Italian pre-print study that has not yet been peer-reviewed found that 43 percent of people who tested positive for COVID-19 showed no symptoms. Of concern: The researchers found no difference in how potentially contagious those with and without symptoms were, based on how much of the virus the test found in individuals' samples.

The antibody serosurveys getting underway in different parts of the country add further evidence that a good number – possibly anywhere from around 10 percent to 40 percent — of those infected might not experience symptoms.

Asymptomatic SARS-CoV-2 infection appears to be common — and will continue to complicate efforts to get the pandemic under control.

Reposted with permission from The Conversation.

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In early October, Britain's Prince William teamed up with conservationist David Attenborough to launch the Earthshot Prize, a new award for environmentalist innovation. The Earthshot brands itself the "most prestigious global environment prize in history."

The world-famous wildlife broadcaster and his royal sidekick appear to have played an active role in the prize's inception, and media coverage has focused largely on them as the faces of the campaign.

But the pair are only the frontmen of a much larger movement which has been in development for several years. In addition to a panel of experts who will decide on the winners, the prize's formation took advice from the World Wildlife Fund, Greenpeace and the Jack Ma Foundation.

With more and more global attention on the climate crisis, celebrity endorsement of environmental causes has become more common. But why do environmental causes recruit famous faces for their campaigns? And what difference can it make?

'Count Me In'

"We need celebrities to reach those people who we cannot reach ourselves," says Sarah Marchildon from the United Nations Climate Change secretariat (UNFCCC) in Bonn, Germany.

Marchildon is a proponent of the use of celebrities to raise awareness of environmental causes. In addition to promoting a selection of climate ambassadors who represent the UN on sustainability issues, Marchildon's team has produced videos with well-known narrators from the entertainment world: among them, Morgan Freeman and Mark Ruffalo.

"We choose celebrities who have a lifestyle where they are already talking about these issues," Marchildon explains.

"Sometimes they reach out to us themselves, as David Attenborough did recently. And then they can promote the videos on their own social channels which reach more people than we do — for example, if they have 20 million followers and we have 750,000."

Environmental groups focused on their own domestic markets are also taking this approach. One Germany-based organization that uses celebrities in campaigns is the German Zero NGO. Set up in 2019, it advocates for a climate-neutral Germany by 2035.

German Zero produced a video in March 2020 introducing the campaign with "66 celebrities" that supported the campaign, among them Deutschland 83 actor Jonas Nay and former professional footballer Andre Schürrle. They solicit support as well as financial contributions from viewers.

"Count me in," they say, pointing toward the camera. "You too?"

"We are incredibly grateful for the VIPs in our videos," says German Zero spokeswoman Eva-Maria McCormack.

Assessing Success Is Complex

But quantifying the effectiveness of celebrity endorsement of campaigns is not a straightforward process.

"In order to measure effectiveness, first of all you need to define what is meant by success," says Alegria Olmedo, a researcher at the Zoology Department at the University of Oxford.

Olmedo is the author of a study looking at a range of campaigns concerning pangolin consumption, fronted by local and Western celebrities, in Vietnam and China. But she says her biggest stumbling block was knowing how to measure a campaign's success.

"You need a clear theory of change," explains Olmedo. "Have the celebrities actually helped in achieving the campaign's goals? And how do you quantify these goals? Maybe it is increased donations or higher engagement with a cause."

A popular campaign in China in recent years saw famous chefs Zhao Danian and Shu Yi pledge to abstain from cooking endangered wildlife. While the pledge achieved widespread recognition, both Olmedo and Marchildon say it's difficult to know whether it made any difference to people's actions.

"In life we see a thousand messages every day, and it is very hard to pinpoint whether one campaign has actually made a difference in people's behavior," she explains.

Awareness Is Not Enough

Many campaigns that feature celebrities focus on raising awareness rather than on concrete action — which, for researcher Olmedo, raises a further problem in identifying effectiveness.

"Reach should never be a success outcome," she says. "Many campaigns say they reached a certain number of people on social media. But there has been a lot of research that shows that simply giving people information does not mean they are actually going to remember it or act upon it."

But anecdotal evidence from campaigns may suggest reach can make an active difference.

"Our VIP video is by far the most watched on our social media channels," McCormack from German Zero says. "People respond to it very directly. A lot of volunteers of all ages heard about us through that video."

However, some marketing studies have shown that celebrity endorsement of a cause or product can distract from the issue itself, as people only remember the person, not the content of what they were saying.

Choosing the Right Celebrity

Celebrity choice is also very important. Campaigns that use famous faces are often aiming to appeal to members of the public who do not necessarily follow green issues.

For certain campaigns with clear target audiences, choosing a climate scientist or well-known environmentalist rather than a celebrity could be more appealing — Attenborough is a classic example. For others, images and videos involving cute animals may be more likely to get a message heard than attaching a famous face.

"We choose celebrities who have a lifestyle where they are already talking about these issues," says Marchildon from the UN. "You need figures with credibility."

McCormack cites the example of Katharine Hayhoe, an environmental scientist who is also an evangelical Christian. In the southern United States, Hayhoe has become a celebrity in her own right, appealing to an audience that might not normally be interested in the messages of climate scientists.

But as soon as you get a celebrity involved, campaigns also put themselves at risk of the whims of that celebrity. Prince William and younger members of the royal family have come under fire in recent years for alleged hypocrisy for their backing of environmental campaigns while simultaneously using private jets to fly around the world.

But Does It Really Work?

While environmental campaigns hope that endorsement from well-known figures can boost a campaign, there is little research to back this up.

"The biggest finding [from my study] was that we were unable to produce any evidence that shows that celebrity endorsement of environmental causes makes any difference," says Olmedo.

This will come as a blow to many campaigns that have invested time and effort into relationships with celebrity ambassadors. But for many, the personal message that many celebrities offer in videos like that produced by German Zero and campaigns like the Earthshot Prize are what counts.

The research may not prove this conclusively — but if the public believes a person they respect deeply personally cares about an important issue, they are perhaps more likely to care too.

"I personally believe in the power this can have," says Marchildon. "And if having a celebrity involved can get a single 16-year-old future leader thinking about environmentalist issues — that is enough."

Reposted with permission from DW.

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