At least one of every two instances of human-to-human transmission of the new coronavirus is believed to occur while the first patient is not yet showing symptoms, according to an estimate by a group of Japanese university researchers.

Based on its determination, the team, headed by Hokkaido University professor Hiroshi Nishiura, has called for preventive measures as well as reinforcing the medical care system against a potential sharp rise in coronavirus patients, rather than focusing exclusively on isolation as a way to contain the disease.

According to the estimate based on 26 human-to-human infection cases released by six countries such as China, Thailand and the United States, the timing of the secondary infection was shorter than previously thought.

Although the average period of incubation for the coronavirus originating from central China is believed to be about five days, the researchers found that second patients in more than half of the 26 human-to-human infection cases under study developed pneumonia symptoms within five days.

The team concluded that these second patients were infected with the virus during its incubation period for the first patients.

"The findings suggest that it is difficult to contain the illness simply by isolation," Nishiura said. "We need to put more emphasis on prevention of infection for people with high risks such as the elderly."

"It is also important to prepare the medical care system so that we will not experience confusion even in the event of a greater number of pneumonia patients," he added.

The number of people with the new coronavirus in mainland China has topped 31,000, with the death toll in the country rising to 636, according to the Chinese government.

Many countries including Japan are struggling to contain the spread of the disease by isolating infected patients.