The biggest breakdown yet of novel coronavirus cases suggests that 80% are mild. Some patients never show symptoms.

Xiao Yijiu/Xinhua News Agency via APPatients infected with the coronavirus resting at a temporary hospital in the Wuhan Sports Centre in central China’s Hubei province on Monday.

The death toll from the novel coronavirus that originated in China crossed 2,000 this week, but new research suggests that most cases of the illness are mild.

A new report from the Chinese Centre for Disease Control and Prevention analysed records of all of China’s reported cases of the virus from December 8 to February 11 – a total of 72,314 patients. (For the latest official case total and death toll, see Business Insider’s live updates.)

The researchers found that 80.9% of the confirmed cases were mild. Those patients might experience a fever or dry cough but weren’t likely to have difficulty breathing or to develop a severe lung infection. The study also found that infected people could show no symptoms at all – that was the case for 1.2% of the patients.

A far higher portion of asymptomatic cases, however, has been found on the Diamond Princess cruise ship. The ship is host to the largest number of diagnosed COVID-19 cases outside China: at least 621. But 322 of those patients showed no symptoms, according to Japan’s Ministry of Health.

Diamond princess clothesline AP Photo/Eugene HoshikoPassengers on the balcony of the cruise ship Diamond Princess at the Yokohama Port near Tokyo on February 7.

It’s possible those people could develop symptoms later, but the situation also indicates that many coronavirus carriers in China may be going undetected, as they probably wouldn’t visit a doctor without experiencing symptoms. (Researchers still aren’t sure whether people can spread the virus when they have no symptoms.)

Health officials have suspected this for weeks.

“There’s another whole cohort that is either asymptomatic or minimally symptomatic,” Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said at a briefing in early February. “We’re going to see a diminution in the overall death rate.”

Severe coronavirus cases seem to be rare, and some cases go unreported

Of the patients in the Chinese CDC report, only 2,087 – 2.3% of all cases – developed symptoms the report defined as “critical”: respiratory failure, septic shock, or organ failure. All of the deaths came from that group.

“In severe cases, you basically flood your lungs and you can’t breathe,” Dr. Matthew Frieman, an associate professor who researchers coronaviruses at the University of Maryland School of Medicine, told National Geographic. “That’s how people are dying.”

Overall, however, the COVID-19 fatality rate is far lower than that of past coronavirus epidemics. Based on the ratio of deaths to confirmed cases so far, it appears to be about 2%. Severe acute respiratory syndrome (SARS), by contrast, killed 9.6% of those infected, and Middle East respiratory syndrome (MERS) killed 34.4% of cases.

Fauci and other experts, however, think the existing fatality rate will drop as more mild cases are counted. Researchers have also found evidence that some patients have less typical symptoms, like gastrointestinal issues, that can mislead medical workers.

“My guess is there’s a delay in a lot of the reporting,” Aaron Milstone, an epidemiologist at Johns Hopkins University, previously told Business Insider. “If people are sick and they’re not getting tested, then we don’t know about them.”

On Tuesday, officials in China’s Hubei province, where the virus originated, announced they would track and investigate people who purchased fever and cough medicine in an attempt to find unreported cases of the coronavirus.

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