- People with asymptomatic COVID-19 can easily transmit the new coronavirus to others.
Researchsuggests that about 1 in 5 people who develop COVID-19 will have no symptoms, but they can still transmit the virus that causes the disease.
- A new study found that the virus spread easily in military barracks.
All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.
Researchers identified several cases of asymptomatic transmission among nearly 2,000 Marine recruits during basic training, even with strict quarantine measures, mask wearing, and physical distancing.
The study, which was published Nov. 11 in The New England Journal of Medicine, highlights the challenges of managing the transmission of SARS-CoV-2, the coronavirus that causes COVID-19, in group settings like military training, colleges, and schools.
“This study shows the power and the amazing prevalence of asymptomatic infection, and how efficiently the virus can be spread by asymptomatic people,” said Dr. Monica Gandhi, an infectious disease physician at the University of California, San Francisco.
Some research suggests that about 1 in 5 people who develop COVID-19 will have no symptoms. However, they can still transmit the virus to others, although it may be at lower levels than people who have noticeable symptoms.
“People have to get it out of their head that someone has to cough or have other symptoms in order to give coronavirus to you,” said Paul Tupper, PhD, a mathematics professor at Simon Fraser University.
“They could appear totally fine and still give it to you,” Tupper added.
Complicating matters, in the real world people who appear asymptomatic may actually be presymptomatic, meaning they will develop symptoms later.
Other people have such mild symptoms that they don’t notice them, or they may brush them off as something else like the stress of working from home or sleeping poorly.
Researchers from the Icahn School of Medicine at Mount Sinai and the Naval Medical Research Center chose Marine basic training because it’s largely a closed system. Once recruits arrive, they stay there for the duration of their training.
This makes it ideal for studying asymptomatic transmission of the coronavirus.
The study included over 1,800 Marine recruits from 9 different classes. The recruits underwent a 2-week quarantine at home before they arrived at basic training.
This was followed by another 2-week quarantine at the start of basic training. During this time, recruits stayed in 2-person rooms and were required to wear masks and physically distance from each other.
Each recruit class was housed in different buildings and didn’t interact. Most of the instruction was done outdoors, where better ventilation lowers the risk of the virus spreading.
Researchers monitored recruits daily for symptoms of COVID-19, including taking their temperature.
They also tested the recruits for the virus within 2 days of arriving, and 7 and 14 days after their arrival using a nasal swab PCR test.
Within 2 days of arrival, just under 1 percent of recruits tested positive for SARS-CoV-2. Another 1.9 percent of recruits tested positive on day 7 or 14.
Out of the 51 recruits who tested positive on one of those days, only 5 had any symptoms during the week leading up to the test.
Recruits who reported symptoms also had a PCR test done. This testing didn’t catch any of the positive cases identified through routine nasal swab screening.
Dr. Francesca Torriani, program director of infection prevention and clinical epidemiology at UC San Diego Health, said the study shows the limitations of quarantines and measures like mask wearing and physical distancing to control coronavirus transmission.
It also shows the limits of using symptoms to identify positive cases.
Tupper and his SFU colleague Caroline Colijn, PhD, recently published a study on the preprint server medRxiv modeling how coronavirus transmission could occur in public school classrooms.
They also looked at how well strategies such as isolation, contact tracing, and pooled testing could slow the spread of the virus.
Tupper and Colijn wrote a blog post describing their results.
“The results of the new study fit with what we found in our research, which was that there are limitations to only testing people once they have symptoms,” said Tupper. “Instead, the most effective thing you could do was screen everybody with coronavirus testing.”
Torriani said testing is especially important in group settings, where people have close and frequent contact with others. This starts right when they show up.
“If you are taking people from the outside into a congregate setting, you can’t assume they are negative,” she said. “You need to test them upon arrival before freeing them into the environment, so as to protect the population.”
In the study, new researchers only tested individuals to see how the virus spread, not as a way to flag people who should be isolated.
However, Torriani said many schools use testing to identify arriving students who have COVID-19.
“Students are segregated, and then they are tested before they are freed into their pod,” she said, but only if two test results show that they’re clear of the virus.
There are few group settings that have no contact with the outside world, which means there’s always a risk that the coronavirus will make its way in.
Torriani said the only true closed setting would be found during something like a scientific research mission on a boat. Once people are on the boat, they have no contact with the outside world until they return to port.
Most other group settings, though, are only partially closed. At universities, professors and staff come and go from the campus. Even students don’t stay on campus the entire time.
That’s why ongoing testing is needed.
“In an environment that is partially closed, you still have to continue testing because you may have transmissions that occur due to contacts that you can’t fully control,” said Torriani.
While regular testing can help us slow the spread of the coronavirus, there are other methods that can be equally effective.
This includes what Gandhi calls nonpharmaceutical interventions — things like wearing masks, physical distancing, hand hygiene, and good ventilation.
“There are many settings — like hospitals, grocery stores, and essential worker settings — where we’re not testing in a mass way,” said Gandhi. “But we’re relying on nonpharmaceutical interventions, and strict adherence to those, to stay safe. This has actually been relatively effective.”
Torriani said multiple layers of protection are needed, which includes testing, nonpharmaceutical interventions, contact tracing, and isolation.
This also includes smartphone apps that alert people that they’ve been potentially exposed to the virus.
“All of these things help,” said Torriani. “By trying collectively as a society to enforce them, we will be able to return to work and return to learning.”
“Without them, it’s going to be difficult to do that until we have a large swath of the population that is vaccinated,” Torriani added.