Most asymptomatic COVID-19 cases stay that way, small study says

There's been a lot of talk recently about the transmission dangers posed by people who are infected by the new coronavirus but show no symptoms.

Are many of these "asymptomatic" carriers simply in a stage of infection that later moves on to symptomatic COVID-19?

No, suggests a small, new study of cruise ship passengers and crew.

Japanese researchers were able to track outcomes for 96 coronavirus-infected passengers and crew aboard the cruise ship Diamond Princess, which made headlines recently.

"In this cohort, the majority of asymptomatic infected persons remained asymptomatic throughout the course of their infection," a team led by Dr. Yohei Doi, of Fujita Health University in Japan, reported June 12 in the New England Journal of Medicine.

The plight of the 700 passengers aboard the Diamond Princess grabbed the world's attention in February as coronavirus spread through the vessel while it was docked in Japan.

As Doi and his colleagues reported, 96 passengers who tested positive for coronavirus infection but had no symptoms, plus 32 of their cabin mates who tested negative, were transferred for observation and isolation to a hospital in central Japan.

This allowed doctors the unique opportunity of tracking changes in the patients' symptom status over time.

Among the 96 passengers who at first tested positive for coronavirus but showed no symptoms, only 11 -- about 11 percent -- went on to show any symptoms over the course of their infection, the Japanese team said.

That meant that those 11 patients "had been presymptomatic, rather than asymptomatic" for COVID-19, the researchers explained.

That's significant, due to controversy emerging earlier this week when the World Health Organization retracted a statement made by one of its officials that asymptomatic transmission of coronavirus "appears to be rare."

The WHO quickly walked back from that statement, saying there simply isn't enough data to know right now how infectious asymptomatic carriers are.

Speaking with ABC's "Good Morning America" on Wednesday, Dr. Anthony Fauci, head of the U.S. National Institute for Allergy and Infectious Diseases, said current evidence suggests that anywhere from 25 percent to 45 percent of people infected with coronavirus will never show symptoms.

"And we know from epidemiological studies they can transmit to someone who is uninfected even when they're without symptoms," he said.

Presymptomatic carriers are a bit different.

"People tend to be the most contagious before they develop symptoms, if they're going to develop symptoms," CNN Chief Medical Correspondent Dr. Sanjay Gupta told the news agency.

"They call that the presymptomatic period," he said. "So people tend to have more virus at that point seemingly in their nose, in their mouth. This is even before they get sick. And they can be shedding that virus into the environment."

The Diamond Princess also provided good data on which asymptomatic carriers might in reality be presymptomatic.

According to the study, patient age appears to be key: For every 1 year added to the patient's age, the odds that he or she would go on to develop symptoms rose by 8 percent.

The researchers also had data on 90 passengers who remained asymptomatic throughout the course of their infections. Among this group, their median age was 59, and 27 percent had chronic illnesses such as high blood pressure or diabetes, Doi and colleagues noted.

And by eight days after first testing positive for coronavirus infection, nearly half (48 percent) of those 90 patients had tested negative (meaning they had cleared the virus). By 15 days that number had risen to 90 percent.

More information

The U.S. Centers for Disease Control and Prevention has more on the new coronavirus.