Can people without symptoms spread COVID-19?
Experts say yes, although impact is being studied
A challenging aspect of COVID-19 is evidence it can be spread from people who have no sign of being sick, making it harder to know who to avoid and how to stay healthy.
It also makes the deadly virus harder to contain.
But it’s often been unclear what asymptomatic actually means, who can be asymptomatic and to what extent the disease can in fact spread from individuals who never show symptoms.
The World Health Organization didn’t help matters by recently making some confusing remarks about asymptomatic spread.
As more Americans start interacting with other people by returning to restaurants and stores and participating in other activities, here are answers to some commonly asked questions based on the current research.
What is an asymptomatic case?
Asymptomatic cases are individuals who are infected with COVID-19 but will never show symptoms such as a fever, cough, shortness of breath or fatigue.
The U.S. Centers for Disease Control and Prevention assumes that around 35% of all people carrying COVID-19 are asymptomatic. A study from broad testing in Iceland showed that up to 50% of people who have COVID-19 show no symptoms, Johns Hopkins University reported.
“There is no consensus. I think it’s a range between 30 and 60%,” said Dr. Farshad Fani Marvasti, a public health expert, physician and associate professor at the University of Arizona College of Medicine Phoenix.
While the exact percentage of asymptomatic cases is unknown, anyone can be asymptomatic, Marvasti said. The only way to know if someone is asymptomatic is if they test positive for the virus while never expressing symptoms.
More widespread community testing of those without symptoms would help individuals know if they’re carrying the virus and help them prevent further spread. It would also help researchers understand how many asymptomatic cases are out there and what percentage of people could be carrying the virus.
Can COVID-19 spread from people without symptoms?
Asymptomatic individuals can in fact spread the virus, researchers say, but the extent to which this happens is still being studied.
There was confusion earlier this week after a WHO official said it was “very rare” for an asymptomatic individual to transmit COVID-19 to another person.
The organization faced swift and strong backlash from scientists who said this was incorrect.
“The original statement that was made basically gives a completely false sense of security, where you feel like, ‘Well, if I don’t have any symptoms, then I’m not sick and I’m not going to make anyone else sick,’ and that’s just not true,” Marvasti said.
The following day, WHO backpedaled on the statement, saying it’s a misunderstanding to say asymptomatic transmission is rare. Instead, they clarified, it’s unclear how many individuals don’t have symptoms and how many of those spread the virus to others.
Current research shows that individuals who will never have symptoms can sometimes spread COVID-19 to others.
A group of researchers in China, for example, found in one study that an asymptomatic individual had the same amount of virus in their system as someone with symptoms, suggesting they could spread it to others.
Marvasti said data has shown the virus can be spread this way. The question is how often.
How often does COVID-19 spread from people without symptoms?
It’s too soon to know. Researchers are still trying to figure out to what extent asymptomatic individuals are driving the spread of disease.
“This is kind of science in the making,” Marvasti said.
Because COVID-19 is caused by a new virus, its patterns are being studied as it spreads, meaning more data is continually emerging and conclusions are changing.
It’s important to get a handle on asymptomatic spread as soon as possible because it could mean people who don’t know they are carrying the virus are spreading it to high-risk individuals such as elderly people or those with underlying health conditions.
“We still don’t have an understanding of how much of an impact asymptomatic spread can have, and given the risks of getting the virus, the hospitalizations, the risk of death, you want to err on the side of caution until all the data is collected and we have more consensus,” Marvasti said.
According to a Johns Hopkins University treatment guide updated June 3, viral spread from asymptomatic people could be responsible for 25% to 50% of total infections.
“Asymptomatically infected people who shed and spread is a likely explanation,” the guide reads. “People who are not ill will not as carefully take measures to avoid spread. This is in large part the rationale behind universal mask use.” There are challenges for researchers. It’s difficult to study an illness by focusing on people who don’t express the illness.
It’s difficult to determine who is asymptomatic, or the number of people without systems, because the person never knows unless they get tested. Given criteria for testing, especially early on, people without any symptoms rarely could even get a diagnostic test.
What’s the difference in spreads?
Two types of people with COVID-19 don’t display symptoms. One will never show symptoms and the other will develop symptoms later.
Both can infect others, but to degrees not yet certain.
Asymptomatic spread is infection of others by people who do not have symptoms and never will have symptoms.
Presymptomatic spread is infection of others by people who don’t yet have symptoms but will get symptoms later. It typically takes five or six days to show symptoms after exposure, but it can take up to two weeks.
For a time, both types of people will look and feel the same, with no outward evidence they’re carrying COVID-19.
The CDC has estimated that about 40% of all transmission happens before individuals show symptoms.
How does COVID-19 spread?
According to the CDC, COVID-19 is mainly spread by respiratory droplets from people who have symptoms. But it’s believed that asymptomatic people could infect others the same way.
COVID-19 most often spreads through small droplets from the nose or mouth of an infected individual. When that person coughs, sneezes or speaks, those droplets can be breathed in by a nearby person who can then catch COVID-19. That’s why public health experts encourage physical distancing and wearing a mask when close contact is necessary.
Droplets can also land on nearby surfaces. When others touch those surfaces then touch their eyes, nose or mouth, they could also get infected, also surface infection is likely less common.
Early data suggests COVID-19 may be able to spread through the air in closed spaces with ventilation systems, Marvasti said. That’s still being studied.
What do experts recommend?
Given the likelihood that people without symptoms — or with mild symptoms or oncoming symptoms — can spread COVID-19, health experts urge individuals take precautions.
This includes washing hands often with soap and water and staying six feet apart from others to reduce the likelihood of transmission from droplets.
Health officials recommend people avoid crowded places and restrain from touching their eyes, nose or mouth.
“Since some infected persons may not yet be exhibiting symptoms or their symptoms may be mild, maintaining a physical distance with everyone is a good idea if you are in an area where COVID-19 is circulating,” the WHO says.
These habits can help prevent infection from either symptomatic or asymptomatic individuals and can also prevent those who may not be showing symptoms from spreading the virus.
Until researchers know more about the extent of asymptomatic spread, Marvasti said, people should be taking extra precautions such as physical distancing and mask-wearing to ward against it.
“Don’t assume that if you’re asymptomatic you don’t have it or you can’t be a potential source of infection to others,” he said. “So take precautions.”
Marvasti added that given climbing numbers in Arizona suggesting community spread, there are likely many more individuals infected.
“If you’re in a place like Maricopa County, where we’re seeing a big explosion of the numbers, there’s a lot more people walking around with this than we may think, so we definitely want to be more cautious about it right now,” Marvasti said.