Asymptomatic spread has been one of the most mysterious and haunting aspects of SARS-CoV-2, the virus that causes COVID-19.

Do people without any symptoms of COVID-19 help spread the virus? The alarming answer is yes.

Evidence continues to mount that a large percentage of people who test positive for COVID-19 don’t have any obvious symptoms.

Among the research related to asymptomatic spread of the coronavirus so far:

• Up to 50% of people who had COVID-19 in Iceland were asymptomatic after health officials did broad lab testing of the population there.
• Nearly 40% of children ages 6 to 13 tested positive for COVID-19, but were asymptomatic, according to just published research from the Duke University BRAVE Kids study. While the children had no symptoms of COVID-19, they had the same viral load of SARS-CoV-2 in their nasal areas, meaning that asymptomatic children had the same capacity to spread the virus compared to others who had symptoms of COVID-19.
• And, a study from Singapore early in the COVID-19 pandemic showed that people who were asymptomatic still were spreading SARS-CoV-2 to others.

“Asymptomatic spread definitely plays a role in community spread,” said Dr. David Beckham, an infectious disease specialist who studies viruses in a lab he runs at the University of Colorado School of Medicine.

Wearing masks helps prevent asymptomatic spread of COVID-19

That means it’s all the more critical for people to follow public health measures that clearly work, chief among them wearing masks, staying far apart from people and washing hands frequently.

“The closer we can get to 100% mask wearing, the quicker we can end this outbreak and get out of the current spike of the disease,” Beckham said.

Public health experts estimate that about 60-to-70% of people in the U.S. routinely are wearing masks when they’re out in public and are exposed to people outside of their homes. Boosting mask wearing to 80-to-85% would dramatically drive down infections and result in fewer illnesses and deaths from COVID-19.

“We need to remember to protect each other. Everybody has a grandparent or knows someone who is high-risk. Simply wearing a mask and maintaining six feet of distance from others works to reduce infections since we know that asymptomatic spread occurs,” Beckham said.

In general, masks protect other people. But, new research also shows that people who wear masks may not get as sick if they get exposed to people with COVID-19. The mask may reduce the viral load that the person wearing it receives.

Wearing masks clearly works as do other prevention measures, Beckham said.

“We all can significantly impact how much transmission is going on in the community. We all can protect grandparents and family members who may be at risk for severe disease.”

Avoid large Thanksgiving gatherings this year

Beckham and researchers at his lab study viruses similar to coronaviruses called flaviviruses. They include common viruses like West Nile, Dengue, tick-borne encephalitis and Zika virus. Throughout the pandemic, Beckham and researchers in his lab have been studying various aspects of SARS-CoV-2. He’s assisting with vaccine trials and is leading a clinical trial related to convalescent plasma. The results of that research are due out soon.

Like many medical experts, Beckham has canceled his plans to celebrate Thanksgiving with extended family. He and his wife will celebrate on their own with their children this year.

“It’s sad that we have to do this this year. But, we’re all working hard on vaccines and I’m hoping we can have a normal Thanksgiving next year,” Beckham said.

Until we have vaccines, he encouraged individuals to do as much as they can now to tamp down spread of the virus so we can all enjoy great gatherings and milestones in future years.

“It’s hugely important for people to understand that there are a lot of asymptomatic people and there’s a lot of asymptomatic spread,” Beckham said.

“But, we can protect each other if we just do simple things.”

Beckham’s advice for staying safe and preventing asymptomatic spread of COVID-19 includes the following basic tips:

• Wear masks in public.
• Keep gatherings as small as possible.
• Maintain at least six feet of distance from others.
• Comply with public health orders.
• Avoid gatherings with groups outside of your family. Be especially wary of indoor spaces with poor ventilation. Limit time indoors in public settings and always wear a mask.
• If you are outdoors and can stay at least six feet away from others and are not with anyone outside of your household, you can walk or exercise without wearing a mask. But, if you are in crowded areas like a parking lot or trailhead, Beckham urges people to wear masks.
• Wash hands frequently.

Beckham said researchers continue to learn more about how easily and how much asymptomatic people spread the virus.

“What’s the rate for asymptomatic people to spread the virus to someone else? That’s a separate question, which is even more difficult to get at,” Beckham said.

Over time, researchers will learn much more. For now, Beckham said there have been some small studies looking at testing and contact tracing.

“There’s no question that people who are infected but don’t have symptoms are transmitting the virus,” Beckham said. “It’s probably relatively common as a mechanism of spread.”

An analysis of multiple studies in the journal, PLOS Medicine, found that approximately 20-to-30% people infected with SARS-CoV-2 remained asymptomatic throughout the course of their infection. The remaining patients included in the studies went on to develop symptoms and the researchers defined them as “presymptomatic.” Both presymptomatic and asymptomatic people can transmit SARS-CoV2, and presymptomatic people transmit at higher rates than asymptomatic people. These data show that presymptomatic and asymptomatic infections contribute to SARS-CoV2 transmission, thus making prevention measures like hand hygiene, masks, testing, tracing, social distancing, and isolation strategies all the more essential to reduce and control the spread of the virus.

Is asymptomatic spread unusual?

While it’s confounding to many people that a virus can spread before the person who is infected with it even knows that they are sick or showing any symptoms, Beckham said it’s not unusual. SARS-CoV-2, the virus that causes COVID-19, is what’s known as an RNA virus.

“With RNA viruses and other respiratory viruses, it’s quite common for people to be asymptomatic or minimally symptomatic. That’s probably an important way for them to spread,” Beckham said.

West Nile virus is a good example, Beckham said.

“If you take everyone who gets infected (with West Nile), about 80% are asymptomatic. A lot of these viruses cause asymptomatic infections. That’s probably because our innate immune defenses fight off the virus before the infection gets going,” Beckham said.

Mosquitos rather than humans spread West Nile, so asymptomatic spread is a separate issue. But, with viruses like Zika and Dengue, a person can be infected and not have symptoms. Yet, that person may have enough of the virus in their body that a mosquito who bites them can become infected with the virus, and in turn, spread it to other people.

Is it common for viruses to affect people of varying ages differently?

Age also seems to affect the degree to which people are asymptomatic when they contract a virus. The Duke study of children with COVID-19 found that asymptomatic cases were highest among kids ages 6 to 13. Asymptomatic cases were less common — but still occurred 25% of the time — in children ages 0 to 5 and teens who were 14 to 20 years old. The study did not look at adults, but older people have fared worse when they get COVID-19.

Beckham said it’s quite common for different viruses to affect people of various ages in different ways. Some can be more severe in children or young adults. Other infectious illnesses like SARS-CoV-2 and the flu are more dangerous to older people. People with underlying health conditions and older people have been among those who have been most critically ill and who have died at higher rates from COVID-19.

“Kids seem to have lower overall rates of infection, but they clearly can get infected and they can be asymptomatic,” Beckham said. “There’s still a lot of work to be done to understand the epidemiology of these younger kids. I don’t think we know exactly what role they play in the spread of the virus.”

While researchers have much more to learn about how common asymptomatic cases of COVID-19 are and exactly how asymptomatic spread occurs, there’s plenty of evidence to warrant concern and careful behavior now.

Beckham’s take-home message to reduce asymptomatic spread boils down to this simple advice. “Wear your mask.”