Those who get sick from COVID-19 may be capable of...

Those who get sick from COVID-19 may be capable of transmitting the virus for up to 48 hours before they have symptoms, according to the U.S. Centers for Disease Control and Prevention. Credit: Newsday / J. Conrad Williams Jr.

The image many might have of COVID-19 is an elderly person struggling to breathe, hooked up to a hospital ventilator. Or someone with a high fever, bad cough and shortness of breath.

Yet the face of COVID-19 also could be the teenager skateboarding down your street, or the middle-aged couple taking a stroll in the park.

They all may look healthy. They may feel healthy. But they also may be carrying the virus, unknowingly infecting those around them.

As many as 25% of people with the virus are asymptomatic and will remain so, Dr. Robert Redfield, the director of the U.S. Centers for Disease Control and Prevention, said in an interview broadcast Tuesday on National Public Radio. Those who get sick from COVID-19 may be capable of transmitting the virus for up to 48 hours before they have symptoms, he said. Both groups help explain why the virus has spread so quickly, he added.

It’s unknown how many asymptomatic people there are — or for that matter, how many people overall carry the virus. The COVID-19 case numbers that local and state health departments release daily only capture those who test positive. State guidelines prioritize testing for high-risk people, such as elderly people, health care workers and those with certain underlying illnesses. Others, even with symptoms, say they have been rejected for testing.

The public should assume anyone could be carrying COVID-19, said Danielle Ompad, an associate professor of epidemiology at the New York University School of Global Public Health in Manhattan.

”I don’t think people realize that just because they’re not sick doesn’t mean they can’t be infected,” she said. “It’s a little counterintuitive for people to think they could be spreading virus when they feel completely fine.”

Asymptomatic people are less likely to adhere to social-distancing guidelines, said Dr. Sharon Nachman, division chief for pediatric infectious diseases at Stony Brook Children’s Hospital.

People with a fever and cough typically are “being incredibly smart about staying home and not going near anyone or doing anything,” she said. But many asymptomatic people who unknowingly carry the virus might not understand the need to self-isolate, Nachman said.

People who don’t have the virus, in turn, are often hypervigilant for any signs of sickness. If someone coughs or sneezes, “Everybody steps back,” she said. They may let their guard down more with people they believe look healthy, she said.

That is why social distancing is so important, said Dr. Bruce Polsky, chairman of medicine at NYU Winthrop Hospital in Mineola and an infectious disease specialist.

“If you assume that a certain percentage of asymptomatic people will be shedding the virus, then it’s impossible to know in a real-life setting who those people are when you encounter them,” he said.

Redfield said the growing evidence that there is significant transmission of the virus by asymptomatic people led to a review of the CDC's guidance that masks should be worn only by the sick or those at high risk of COVID-19 complications. On Friday, President Donald Trump announced that the CDC is now recommending that everyone wear cloth or other nonmedical masks in public.

Polsky said before the announcement that he supports a revision of CDC guidelines. Surgical masks prevent the spread of droplets that contain the virus from people who are coughing, sneezing or even talking, he said. A mask gives uninfected people some protection from taking in the droplets, he said.

With COVID-19 so prevalent, “I would say it is prudent when you are going to be interacting with people, and this would include going grocery shopping and such, it is prudent to wear a mask,” he said.

Only a few weeks ago, even many medical offices were not assuming that some of their patients could be asymptomatic carriers of the virus, said Jeffrey Kwong, a professor of advanced nursing practice at the Rutgers School of Nursing in Newark, New Jersey.

Kwong also works in a Manhattan primary care practice, and he recalled that until mid-March, employees would only wear N95 respirator masks — which provide more protection than surgical masks — with someone with COVID-19 symptoms or other respiratory issues.

“As we were seeing more data and more cases, all of us came to the realization that this is much more prevalent and much more contagious than we initially thought,” he said.

The office then began requiring N95 masks for all patient interactions, and a few days later switched to only telehealth appointments, he said.

A study published March 16 in the journal Science estimated that in China, where the pandemic began, 86% of infections with the virus before Jan. 23 were not documented, and that those people were the infection source of 79% of cases that were documented.

Ompad said widespread testing would help indicate how many people are asymptomatic, and mass testing for the antibodies that the body creates to fight the virus would pick up people who had the virus but no longer do. If a vaccine emerges, and it is confirmed that people who have COVID-19 are immune from getting it again, data from testing would allow for prioritization of people who never carried COVID-19, she said.

Until then, Ompad said, "the best intervention is to change people’s behaviors” through social distancing.


Up to 25% of people with COVID-19 will not show symptoms.

People who get sick can spread the virus for up to 48 hours before they have symptoms.

Symptoms can appear between two and 14 days after exposure to COVID-19.

The virus can remain in the air for up to three hours.

It was detectable on plastic and stainless steel for up to two to three days.

It appears the fatality rate for COVID-19 is about 1% — ten times higher than the seasonal flu.

SOURCES: U.S. Centers for Disease Control and Prevention, study on the virus published in The New England Journal of Medicine, Harvard Health

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