As many as 25 percent of people infected with the new coronavirus may not show symptoms, the director of the Centers for Disease Control and Prevention warns — a startlingly high number that complicates efforts to predict the pandemic’s course and strategies to mitigate its spread.

In particular, the high level of symptom-free cases is leading the C.D.C. to consider broadening its guidelines on who should wear masks.

“This helps explain how rapidly this virus continues to spread across the country,” the director, Dr. Robert Redfield, told National Public Radio in an interview broadcast on Tuesday.

The agency has repeatedly said that ordinary citizens do not need to wear masks unless they are feeling sick. But with the new data on people who may be infected without ever feeling sick, or who are transmitting the virus for a couple of days before feeling ill, Mr. Redfield said that such guidance was “being critically re-reviewed.”

Researchers do not know precisely how many people are infected without feeling ill, or if some of them are simply presymptomatic. But since the new coronavirus surfaced in December, they have spotted unsettling anecdotes of apparently healthy people who were unwitting spreaders.

“Patient Z,” for example, a 26-year-old man in Guangdong, China, was a close contact of a Wuhan traveler infected with the coronavirus in February. But he felt no signs of anything amiss, not on Day 7 after the contact, nor on Day 10 or 11.

Already by Day 7, though, the virus had bloomed in his nose and throat, just as copiously as in those who did become ill. Patient Z might have felt fine, but he was infected just the same.

Researchers now say that people like Patient Z are not merely anecdotes. For example, as many as 18 percent of people infected with the virus on the Diamond Princess cruise ship never developed symptoms, according to one analysis. A team in Hong Kong suggests that from 20 to 40 percent of transmissions in China occurred before symptoms appeared.

The high level of covert spread may help explain why the novel coronavirus is the first virus that is not an influenza virus to set off a pandemic.

The new virus spreads about as easily as flu, “and when’s the last time anyone thought anything about stopping influenza transmission, short of the vaccine?” said Dr. Michael T. Osterholm, an infectious disease expert at the University of Minnesota.

With any vaccine still in early development, the best way to mitigate the pandemic is social distancing, he and other experts said. Because people may be passing the virus on to others even when they feel fine, asking only unwell people to stay home is unlikely to be enough. This is why many experts, going against recommendations by the C.D.C. and the World Health Organization, are now urging everyone to wear masks — to prevent those who are unaware they have the virus from spreading it.

Like influenza, some experts now say, this virus appears to spread both through large droplets and droplets smaller than five micrometers — termed aerosols — containing the virus that infected people might release especially while coughing, but also while merely exhaling. They emphasized that the level of virus in both types of particles is low, so simply jogging or walking by an infected person does not put people at risk.

“If you have a passing contact with an infectious person, you would have a very, very low chance of transmission occurring,” said Dr. Benjamin Cowling, an epidemiologist at the University of Hong Kong.

The risk goes up with sustained contact — during face-to-face conversation, for example, or by sharing the same air space for a prolonged time. In addition to its confusing stance on masks, “the W.H.O. has been saying aerosol transmission doesn’t occur, which is also perplexing,” Dr. Cowling said, adding, “I think both are actually wrong.”

Experts agreed that infections were being passed along by people who do not report symptoms — what they call asymptomatic transmissions — but they also noted some confusion around the term.

“There’s no standard definition for it, and you could say to yourself, Well, that’s kind of ridiculous: You either have symptoms or you don’t,” said Dr. Jeffrey Shaman, an infectious diseases expert at Columbia University. But studies by his team have shown, he said, that some people never notice their symptoms, others are unable to distinguish the infection from their smoker’s cough or allergies or other conditions, and still others may feel every pain acutely.

There is also a largely semantic debate about what proportion of people who appear to be perfectly fine but then become ill — as in the report in The New England Journal of Medicine of an apparently asymptomatic spreader who later acknowledged having felt mild symptoms.

Ultimately, Dr. Shaman said, these definitions are unimportant.

“The bottom line is that there are people out there shedding the virus who don’t know that they’re infected,” he said.

Where the definitions may matter is in being able to understand the true scope of the pandemic.

Dr. Cowling’s team has analyzed data from China at various stages in the pandemic. The W.H.O.’s mission to China concluded that most people who were infected with the virus had significant symptoms. But in the early weeks of the epidemic, his analysis shows, China set a high bar for what constituted a confirmed case of infection — requiring respiratory symptoms, fever and a chest X-ray for pneumonia.

Their definition left out mild and asymptomatic cases and, as a result, the team vastly underestimated the scale and nature of the outbreak there.

“We’ve estimated in China that between 20 percent and 40 percent of transmission events occurred before symptoms appeared,” Dr. Cowling said.

A separate analysis of the hundreds of people cloistered aboard the Diamond Princess cruise ship bears out this scale. Once the ship docked in Japan on Feb. 5, researchers tested all of the passengers and reviewed those who tested positive for the virus on multiple occasions over a two-week period. They found that 18 percent of the infected passengers remained symptom-free throughout.

“The substantial asymptomatic proportion for Covid-19 is quite alarming,” said Dr. Gerardo Chowell, an epidemiologist at Georgia State University who worked on the analysis.

Dr. Chowell noted that the passengers on the ship tended to be older and therefore more likely to develop symptoms. He estimated that about 40 percent in the general population might be able to be infected without showing signs of it.

There have also been many hints, subtle and not, that the virus can be transmitted via aerosols. Sixty members of a choir in Mount Vernon, Wash., north of Seattle, gathered on March 10 for a practice session for over two and a half hours. None of them felt ill, and they made no contact with one another. But by this weekend, dozens of the members had fallen ill, and two had died.

Their experience points toward airborne transmission via aerosols, which can travel farther than the large droplets the W.H.O. and the C.D.C. have emphasized. The virus is still most likely to be expelled with a cough or a sneeze, as far as eight meters (about 26 feet), according to one study. But studies on influenza and other respiratory viruses, including other coronaviruses, have shown that people can release aerosols containing the virus simply by breathing or talking — or, presumably, by singing.

“I think increasing evidence suggests the virus is spread not just through droplets but through aerosols,” Dr. Chowell said. “It would make a lot of sense to encourage at the very least face mask use in enclosed spaces including supermarkets.”

Several studies have shown now that people infected with the new coronavirus are most contagious about one to three days before they begin to show symptoms. This presymptomatic transmission was not true of the coronaviruses that caused SARS and MERS.

“This is where we got very lucky with SARS, was that it really didn’t transmit until after people were showing symptoms, and that made it much easier to detect it and shut it down with aggressive public health measures,” said Dr. Carl Bergstrom, an expert in emerging infectious diseases at the University of Washington in Seattle.

With the new coronavirus, there is transmission by healthy-seeming people, and often severe symptoms and a high fatality rate. “That whole combination makes it very, very tough to fight using standard public health measures,” he said.

A separate analysis from the C.D.C. on Tuesday offered new evidence that a significant portion of people with severe coronavirus infections in the United States have underlying medical conditions. The agency looked at 7,162 cases, a small subset of the 122,000 cases in the U.S., but the findings provided a stark portrait. Of 457 people in that subset who were admitted to intensive care units, 32 percent suffered from diabetes; 29 percent had heart disease; and 21 percent had lung disease. Overall, 78 percent of people with Covid-19 admitted to the I.C.U. had at least one pre-existing condition. The study did not look at deaths.

Rapid tests for infection might help detect people, especially health care workers, who are infected yet feel normal. Masks may help. But experts kept returning to social distancing as the single best tool for stopping the chain of transmission in the long term — not lockdowns, necessarily, but canceling mass events, working from home when possible and closing schools.

“We can’t assume that any of us are not potential vectors at any time,” Dr. Bergstrom said. “This is why even though I’m feeling great, and have felt great and haven’t been exposed to anybody with any symptoms of anything, that’s why it would be irresponsible of me to go out and about today.”

Matt Richtel contributed reporting.

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