People who carry the coronavirus but do not have symptoms likely are a key reason for the spread of COVID-19, despite widely publicized comments by a top World Health Organization official to the contrary, some experts say.
The official, Maria Van Kerkhove, technical lead for COVID-19 with WHO, said Monday that the spread of the virus by asymptomatic people is “very rare,” leading to an outcry by scientists who say evidence points to widespread transmission by those without symptoms.
On Tuesday, she backtracked, stating that she was referring only to “some two or three studies that have been published” and not the complete picture.
Concern about widespread transmission of the virus by people without symptoms is a key reason social distancing measures were enacted and why even those who feel healthy should wear masks in public, said Dr. David Hirschwerk, an infectious disease specialist at Northwell Health and vice chair of medicine at Long Island Jewish Medical Center and North Shore University Hospital.
“There’s been so much effort to try to get our communities to adhere to that that this would be a big step backward if this type of information was put out there and was felt to be fully valid,” he said.
One reason asymptomatic people — those who never develop symptoms and those who eventually get sick but are contagious before they show symptoms — are key to the virus’ spread is that they tend to be far more mobile than people who are sick, who typically stay at home, said Dr. Bettina Fries, chief of the infectious disease division at Stony Brook Medicine.
That’s one reason Fries and other experts are concerned that the large protests on Long Island and worldwide against police brutality and racism may lead to an uptick in coronavirus cases.
Many demonstrators wear masks, but many wear them incorrectly or pull them down when they talk or shout, she said.
“If you go somewhere and you scream and you shout, you spit, and that’s when you generate these aerosols” containing the virus that are expelled into the air, she said.
The effect of the protests on the spread of the virus may not be seen for weeks, because the young people who predominate at the protests are less likely to get seriously ill than, for example, grandmothers they could unwittingly infect days after contracting the virus, she said.
People who develop COVID-19 symptoms have a significant amount of virus in their noses and throats two to three days before becoming symptomatic, and some people’s viral loads are actually higher when they are asymptomatic than when they get sick, Hirschwerk said.
“Even though they might not be coughing and sneezing quite as much during that time when they’re pre-symptomatic or asymptomatic, they still have very high amounts of virus in their nose, and whether it’s by speaking, or whether it’s by occasionally coughing, or whether it’s by singing, laughing or talking loudly, these are all ways in which the virus can be transmitted into the environment,” he said.
The small studies that Van Kerkhove referred to include one in which researchers traced 91 contacts of nine asymptomatic carriers in China and found none to be infected, and another in which researchers found that 14% of 63 asymptomatic people infected others.
Other studies have found higher infection rates. One found that 23% of transmissions in Shenzhen, China, may have been through people who were asymptomatic but later got sick, and another found that 44% of people with COVID-19 in a hospital in Guangzhou, China, were infected by pre-symptomatic people.
Multiple studies have found a large percentage of people with the coronavirus do not have symptoms. A study published last week in the Annals of Internal Medicine analyzed 16 studies and concluded that asymptomatic people “seem to account” for 40% to 45% of those with the virus and will remain so, which “suggests that the virus might have greater potential than previously estimated to spread silently and deeply through human populations.”
Dr. Jessica Justman, an associate professor of medicine in epidemiology at Columbia University’s Mailman School of Public Health, said although it’s clear that asymptomatic people spread the virus, “the jury is still out” on how important a factor they are.
“Are they accounting for 5% of new infections or 50% of new infections?” she asked. “It’s not so clear.”
Studies have come to different conclusions because of variations in methodology, and more research is needed on the topic, she said.
Even so, Justman said, “It behooves us to assume it’s an important factor” and for people to continue to wear masks and practice social distancing.