Infected but Feeling Fine: The Unwitting Coronavirus Spreaders

Scientist Holding Petri Dish 3786247

The C.D.C. director says new data about people who are infected but symptom-free could lead the agency to recommend broadened use of masks.

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.