A 24-day incubation period sounds alarming for families already counting the days of a quarantine. The World Health Organization (WHO) said on 12 February 2020 that it had no plans to lengthen its 14-day guidance, and it called the figure an outlier.
The number came from a study led by Zhong Nanshan, an infectious diseases expert in China. Across 1,099 Chinese patients, the median incubation period was 3 days. The shortest was 0 and the longest was 24, according to The Straits Times. The study did not say how many patients had unusually long incubation periods.
The paper was posted to medRxiv, an online archive, as an unpublished manuscript. It had not been peer reviewed. The site describes the papers it hosts as preliminary and advises that clinical practice or treatment should not be guided by them.
Singapore’s Ministry of Health agreed with the WHO position, the Times reported. Its officials pointed out that the 24-day figure could stem from errors, faulty research or bias in the study’s methods.
Ryan on the “double exposure” problem
Dr Michael Ryan, executive director of the WHO’s Health Emergencies Programme, said extreme numbers like this often turn out to be wrong. He drew on earlier Ebola work. “We’ve seen very long incubation periods and then when we investigate, we find that there was a second exposure a week later, or 2 weeks later, and that’s when the actual infection occurred,” he said.
The idea is simple. Say a person meets an infected contact on a Monday and stays healthy. A week later, another infected contact passes on the virus. The days between the two meetings get counted as incubation, though they were not, and the stretch looks about 10 days longer than the real one.
Ryan urged skepticism toward extreme outliers. A figure like 24 days could mislead policy and cause needless alarm, he said, and the worry sparked by news of the unverified study showed as much.
According to The Straits Times, respiratory viruses usually incubate for 2 to 14 days. SARS and MERS-CoV, the two other coronaviruses of recent memory, fall in that range.
The wider news that day
The outbreak was also setting grim records. The virus killed over 1,000 people in China on Tuesday 11 February, its deadliest day so far.
There was some better news on testing. The WHO reported that 168 countries now have the technology to identify the virus. Singapore is among them, having produced its own test kit through A*STAR. On Saturday 8 February, Singapore also sent 10,000 test kits to affected regions in China.
