Photo by Xiao Heyong/Xinhua

Fresh evidence is suggesting that the incubation period of the novel coronavirus in China could be longer than previously suggested 14 days, likely reaching up to 24 days in some cases. 

Experts believed that these extreme cases did not necessarily indicate an extended incubation period or adjustment of China's quarantine policy but admitted that the epidemic situation may evolve with more epidemiologic investigations and deeper understanding of the virus. 

In one case, a 60-year-old man living in Central China's Henan Province who has had close contact with a confirmed patient on January 25 showed no symptoms for 17 days but tested positive on Sunday, according to China Central Television. 

A recent article on the clinical characteristics of 2019 novel coronavirus infection in China, conducted by a group of authors including Zhong Nanshan, leading Chinese expert on the virus, revealed that the incubation period of the novel coronavirus varies from 0 to 24 days, which was longer than the previously suggested 14 day period by experts. 

The study was conducted based on a sample of 1,099 confirmed novel coronavirus pneumonia cases from 552 hospitals in 31 provinces and municipalities in the Chinese mainland. 

According to the prevention and control plan published by the Chinese National Health Commission, the incubation period of the virus can be as long as 14 days, and those who had close contact with confirmed or suspected novel coronavirus patients should be placed under a 14-day quarantine. 

Experts said that our understanding and research into the virus and the control of the outbreak was dynamic, so we could not rule out the possibilities of new circumstances in the future.  

The 14 day limit of the incubation period was based on clinical observation rather than laboratory experiments, thus it may not reflect all situations, Yang Zhanqiu, deputy director of the pathogen biology department at Wuhan University, told the Global Times. 

But even if some patients were found to have longer incubation periods, the number of them would be very low, Yang said.

Zeng Guang, chief epidemiologist at the Chinese Center for Disease Control and Prevention and one of the authors of the article, told the Global Times on Tuesday that the longest incubation period of 24 days was an extreme in the study, which did not necessarily indicate that the incubation period could be longer than 14 days. The average incubation period is about a week. 

"We will not change our current 14-day quarantine policies solely based on this study or some extreme individual cases, instead we may adjust the quarantine period based on fresh epidemiologic investigations," Zeng said. 

As for more patients testing positive for the novel coronavirus after the 14-day quarantine period, Zeng said that it's likely that the travel history and contact history of such patients were incorrect. They may have contacted other confirmed or suspected patients whom they were unaware of, Zeng said.