HONG KONG — The coronavirus that causes the Covid-19 illness had an unusually long incubation period in a minority of cases, one of China's top respiratory specialists said on Tuesday (Feb 18).

The unusual length has raised concerns about giving an accurate diagnosis, but Dr Zhong Nanshan said these cases were an exception.

He told a news briefing in Guangzhou that research suggested the incubation period could be as long as 24 days, but the median figure was three to seven days.

"It is a matter of considering the majority or the minority of cases. [Three to seven days] was the median figure and it applied for most cases," Dr Zhong said. "But I am not surprised that there are exceptions."

Dr Zhong and his team looked at data from 1,099 patients in 522 hospitals across the country. They had discovered only one patient with a 24-day incubation period and 13 cases with an incubation period of more than 14 days.

The 14-day mark is the World Health Organization's (WHO) advisory period for monitoring cases and is based on data from other coronavirus diseases.

"In our research paper, we must be truthful and hence we have stated that the incubation period ranged from zero to 24 days," Dr Zhong told the briefing on Tuesday.

"But there was only one out of all the 1,099 cases [with an incubation period of 24 days] and only 13 had incubation periods longer than 14 days."

The unusually long incubation period has raised alarm that the coronavirus could remain dormant for much longer than was previously thought, making an accurate diagnosis difficult.

WHO previously estimated that the incubation period of the coronavirus ranges between one to 12.5 days, with five to six days as the median.

On Sunday, China reported one case in Henan province where a patient only tested positive for the coronavirus 19 days after his admission to a hospital. SOUTH CHINA MORNING POST