But up to 25% of people infected may never have symptoms. And in others, their symptoms may not show up until 48 hours after being infected, according to new evidence. Yet researchers have said people in both groups may be “silent spreaders” of the virus.
Several recent studies have backed this up. One that studied transmission in Singapore and China found infection was transmitted between 2.55 and 2.89 days, respectively, before symptoms started.
Another that identified seven clusters of COVID-19 cases in which transmission likely occurred before symptoms found that 6.4% of cases were attributed to transmission before there were symptoms. Another study in China found that 12.6% of the transmissions could have occurred before symptoms began in the "source" patient.
"The initial epidemiologic [research] suggests that asymptomatic spreading is happening," says Timothy Schacker, MD, a vice dean for research and professor of medicine at the University of Minnesota Medical School.
So how exactly is the virus being spread?
"People think, 'If I don't feel bad, I don’t have it and can't give it to anyone,' and that is now misguided thinking," says Chad Petit, PhD, an assistant professor in the department of biochemistry and molecular genetics at the University of Alabama at Birmingham School of Medicine, who studies viruses.
Researchers don’t yet have all the answers. But an expert panel from the National Academy of Sciences told White House officials Wednesday night that the virus can possibly be spread through talking or just breathing.
A letter from the Academy reads in part: "Currently available research supports the possibility that SARS-CoV-2 could be spread via bioaerosols generated directly by patients' exhalation."
Robert Mason, MD, at National Jewish Hospital in Denver, says it's little things like touching your nose or touching your eyes, then touching a doorknob or other common surface, that can help spread it, or clearing your throat. "If you have a large number of people doing little things," that's a problem, he says.
Speech, Singing May Spread the Virus
After a person is exposed, "the virus is propagating in your body, but your immune system has not recognized that something is going on systemically," says Petit. "That's why you don't get a fever right away. Just because you don't have symptoms doesn't mean you don't have the virus."
The Singapore researchers say the spread of the virus before there are symptoms might happen "through generation of respiratory droplets or possibly through indirect transmission. Speech and other vocal activities such as singing have been shown to generate air particles, with the rate of emission corrsponding to voice loudness." They cite the report about singers in Washington state attending choir practice, with 40 of 60 later testing positive for the virus.
Basically, the infection is transmitted from the silent spreaders the same ways obviously sick people do, Petit says. "If somone sneezes or coughs and wipe their nose, and those droplets get on you or your hands and you touch your face -- that's thought to be the most common route of transmission at the moment," he says, although the virus shed on surfaces can also be infectious.
The CDC says that people are thought to be the most contagious when they are symptomatic, or sickest. But as more information emerges about how the virus spreads, more people are taking
to wearing masks. On Wednesday, Los Angeles Mayor Eric Garcetti said he was not waiting on revised guidance and advised residents to wear non-medical masks when out doing essential tasks such as grocery shopping.
Academy of Sciences member Harvey Fineberg, MD, told CNN he will start wearing a homemade mask when he goes food shopping, saving the surgical masks for health care providers.
Evidence of transmission by silent spreaders reinforces the importance of measures already in place, experts agree. That means continuing to use social distancing, frequent hand-washing, and disinfecting household surfaces.
WebMD senior writer Brenda Goodman contributed to this report.