UPDATE: A World Health Organization expert sparked widespread confusion Monday when she said that asymptomatic spread of COVID-19 is "very rare."
The comment prompted massive pushback from scientists around the world, leading to an unusual backtracking from the organization the following day, clarifying that so-called asymptomatic transmission of the virus does occur.
During a media briefing Monday, one of the WHO's top epidemiology experts, Dr. Maria Van Kerkhove, was asked about coronavirus transmission. Van Kerkhove's response — that asymptomatic spread was "very rare" — resulted in confusion, particularly as governments and public health officials worldwide have instituted lockdowns and issued guidance to socially distance and wear face coverings in an effort, in part, to stop asymptomatic spread.
On Tuesday, Van Kerkhove said her assessment that asymptomatic spread was "very rare" was based on specific studies that may not have gone through peer review, and that it does not reflect a change in WHO guidance.
Van Kerkhove said there is a wide range of mathematical modeling of asymptomatic spread. The WHO estimates about 16 percent of people with the coronavirus never develop symptoms, but may indeed be able to spread the virus. But, she added, some estimates suggest "around 40 percent of transmission may be due to asymptomatic" spread.
The World Health Organization typically only schedules COVID-19 briefings on Mondays, Wednesdays and Fridays, but clearly arranged Tuesday's briefing to address the controversy.
The Centers for Disease Control and Prevention further clarified in a statement Tuesday, explaining there are two groups of people with the virus who may be contagious even though they aren't showing symptoms, such as fever, cough or shortness of breath. Some are asymptomatic, and never develop symptoms. Others are presymptomatic, meaning they go on to develop symptoms.
It's largely impossible for anyone to know which group they're in. "We are learning more about these issues every day," the CDC said in the statement.
"We know some people without symptoms may be able to spread the virus. That is why measures like cloth face coverings in public are so important," it added.
The Harvard Global Health Institute also issued a statement Tuesday, saying in part that "all of the best evidence suggests that people without symptoms can and do readily spread SARS-CoV2, the virus that causes COVID-19."
The group also said people may be most infectious in the days before they develop symptoms, when they're still feeling well, but may be quite contagious — referring to presymptomatic spread.
"You could be in the restaurant, feeling perfectly well and start to get a fever," Dr. Michael Ryan, executive director of the WHO's health emergencies program, said. "That's the moment your viral load could be actually quite high." Viral load refers to how much virus is in the body. A higher viral load usually means a person is more contagious.
COVID-19 is thought to spread through respiratory droplets, usually from sneezes or coughs. But even in the absence of those respiratory excretions, experts say tiny viral particles can spread through singing, speaking loudly or breathing heavily.
PREVIOUS STORY: Coronavirus patients without symptoms aren’t driving the spread of the virus, World Health Organization officials said Monday, casting doubt on concerns by some researchers that the disease could be difficult to contain due to asymptomatic infections.
Some people, particularly young and otherwise healthy individuals, who are infected by the coronavirus never develop symptoms or only develop mild symptoms. Others might not develop symptoms until days after they were actually infected.
Preliminary evidence from the earliest outbreaks indicated that the virus could spread from person-to-person contact, even if the carrier didn’t have symptoms. But WHO officials now say that while asymptomatic spread can occur, it is not the main way it’s being transmitted.
“From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual,” Dr. Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit, said at a news briefing from the United Nations agency’s Geneva headquarters. “It’s very rare.”
Government responses should focus on detecting and isolating infected people with symptoms, and tracking anyone who might have come into contact with them, Van Kerkhove said. She acknowledged that some studies have indicated asymptomatic or presymptomatic spread in nursing homes and in household settings.
More research and data are needed to “truly answer” the question of whether the coronavirus can spread widely through asymptomatic carriers, Van Kerkhove added.
“We have a number of reports from countries who are doing very detailed contact tracing,” she said. “They’re following asymptomatic cases. They’re following contacts. And they’re not finding secondary transmission onward. It’s very rare.”
If asymptomatic spread proves to not be a main driver of coronavirus transmission, the policy implications could be tremendous. A report from the U.S. Centers for Disease Control and Prevention published on April 1 cited the “potential for presymptomatic transmission” as a reason for the importance of social distancing.
“These findings also suggest that to control the pandemic, it might not be enough for only persons with symptoms to limit their contact with others because persons without symptoms might transmit infection,” the CDC study said.
To be sure, asymptomatic and presymptomatic spread of the virus appears to still be happening, Van Kerkhove said but remains rare. That finding has important implications for how to screen for the virus and limit its spread.
“What we really want to be focused on is following the symptomatic cases,” Van Kerkhove said. “If we actually followed all of the symptomatic cases, isolated those cases, followed the contacts and quarantined those contacts, we would drastically reduce” the outbreak.