After months of maintaining that healthy people didn't need to wear masks in public to prevent the coronavirus, the country's top public health officials reversed their position last week, recommending that everyone wear a face mask in public places where social distancing is difficult.
It's a stark change from the early weeks of the outbreak, when the federal Centers for Disease Control and Prevention said only two groups of people needed to wear them: people who were showing symptoms and people who were taking care of someone who was sick.
Q. But why did the recommendation change so dramatically?
A. Wearing a face mask largely protects those around you, experts say. The CDC now recommends wearing cloth face masks in public settings where social distancing is hard to maintain, such as grocery stores and pharmacies, and especially in areas where there is a significant level of community spread.
Citing several studies, the CDC said it changed its recommendation based on evidence showing that the virus can be transmitted by people who are asymptomatic, or who are not yet showing symptoms or feeling sick.
"We now know from recent studies that a significant portion of individuals with coronavirus lack symptoms ('asymptomatic') and that even those who eventually develop symptoms ('pre-symptomatic') can transmit the virus to others before showing symptoms," the CDC's webpage says. "This means that the virus can spread between people interacting in close proximity — for example, speaking, coughing, or sneezing — even if those people are not exhibiting symptoms."
Disposable surgical masks are typically designed to keep what's in a person's mouth away from other people, which is why the CDC didn't initially recommend them. Even with the new recommendation, the CDC emphasizes that cloth masks primarily protect those around you.
Homemade masks are recommended to help prevent further shortages of N95 respirators and surgical masks that are needed by health care workers. Some studies have found that homemade face masks aren't as effective as medical-grade ones, but health experts say some protection is better than nothing.
The main idea is that if everyone wears a mask, people in a community are all protecting each other, health experts say.
Q. How common is asymptomatic transmission?
A. Robert Redfield, director of the CDC, told NPR last week that as many as 25% of people infected with the coronavirus may be asymptomatic and that those who do eventually develop symptoms are able to spread the virus to others at least two days before getting sick.
But much remains unknown about the virus, including how many people are truly asymptomatic.
Dr. Maria Van Kerkhove, who leads the World Health Organization's emerging diseases and zoonoses unit, told ProPublica that the organization has found very few true asymptomatic cases.
But she said there are many cases where people test positive for the disease with no symptoms but develop symptoms later. That's known as being "presymptomatic."
"Most of the people who were thought to be asymptomatic aren't truly asymptomatic," Van Kerkhove said. "When we went back and interviewed them, most of them said, 'Actually I didn't feel well but I didn't think it was an important thing to mention. I had a low-grade temperature, or aches, but I didn't think that counted.'"
A study conducted by WHO in February found that in China as many as three-quarters of people who were initially asymptomatic eventually developed symptoms. Another study of transmission in China found that anywhere between 20% and 40% of infections might have happened before symptoms appeared, The New York Times reported.
Health experts say the important thing to note is that people can spread the disease without knowing they have it — which means everyone should consider wearing a mask to protect themselves and others.
Q. How can people who aren't coughing or sneezing get others sick?
A. Public health officials say the main way COVID-19 spreads is through droplets when people cough or sneeze. Those droplets have to leave a person's mouth or nose and make their way into another person's body to lead to an infection.
Health experts say respiratory droplets are produced in ways besides coughing and sneezing — people clearing their throat and spittle from talking are examples of ways respiratory droplets can form.
There's also some debate about whether the virus can be spread through airborne particles.
A study published in The New England Journal of Medicine in March found that the virus can live in air particles for as long as three hours, depending on the conditions.
And some health experts have pointed to an incident in Mount Vernon, Wash., in March as evidence of aerosol transmission, when about 60 healthy choir members gathered to sing and dozens later became ill with the virus. While some health experts say it's possible that droplets could have been spread through the singing, others have pointed to the incident as evidence of transmission through air particles.
"I think increasing evidence suggests the virus is spread not just through droplets but through aerosols," Dr. Gerardo Chowell, an epidemiologist at Georgia State University, told The New York Times. "It would make a lot of sense to encourage at the very least face mask use in enclosed spaces including supermarkets."
A mask shouldn't be used as a replacement for other disease prevention methods, the CDC says. Health experts say you should still stay at least 6 feet apart from others, avoid touching your face and frequently wash your hands for a cloth mask to be most effective.
It's also important to properly care for your face mask. The CDC says cloth masks should be washed frequently with soap and water, and it recommends disposing of surgical face masks whenever they get damp.