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Evidence of an incubation period twice as long as currently estimated is cause for concern among health authorities. A longer incubation period would have drastic consequences.
The incubation period for a new virus is normally between two and 14 days. The World Health Organisation WHO, the German Robert Koch Institute and the Federal Ministry of Health assume that's how long it takes between infection and the onset of symptoms for the coronavirus as well. Accordingly, suspected cases continue to be isolated for two weeks.
Zhong Nan Shan, head of China's national expert panel for the control of lung disease and a leading epidemiologist, also found that the first symptoms usually appear after three days. His team had examined 1,099 cases from 552 hospitals in China.
However, these preliminary results require further investigation, said Zhong Nan Shan, who discovered the SARS coronavirus in 2003. In rare cases, the incubation period for the new coronavirus could be up to 24 days, according to a study published on the medical research platform medRxiv earlier this month. The report put the median incubation period at three days with a range of zero to 24 days.
Individual cases or faulty control?
A few days ago, a report by the provincial government of Hubei, the epicenter of the virus, caused a sensation when a 70-year-old man showed the first symptoms 27 days after infection. However, according to the German coronavirus expert Christian Drosten from the Charité hospital in Berlin, such individual cases are of limited significance as exposure to a virus can be difficult to determine.
"A frequent source of error with apparently very long incubation periods is unnoticed interim exposure," Drosten said, adding that he does not see a reason to change existing incubation time estimates.
Different bodies react at different speeds
It is unsurprising that different patients react differently to pathogens and that incubation periods can sometimes vary considerably, according to Thomas Pietschmann, a molecular virologist at Twincore, the Centre for Experimental and Clinical Infection Research.
"Viruses have different characteristics when it comes to how they spread in a host and suppress the immune response at the same time. Such processes lead to a situation where it takes longer or the virus is detected earlier and the symptoms begin," Pietschmann told DW.
It is gradually becoming clearer that in some cases there are considerably longer incubation periods, said Jonas Schmidt-Chanasit from the German Centre for Infection Research. The vast majority of those infected show symptoms after just one week, but more cases mean more information about the disease that surfaced in late December 2019.
"We have more data showing that — and this must be emphasized again — in a few cases the incubation period can be longer, up to over a month," Schmidt-Chanasit told DW. "However, the majority of patients become ill within one week."
What does a significantly longer incubation period mean?
Should the virus' incubation period turn out to be significantly longer than the presumed 14 days, it could have a drastic impact on efforts to contain the epidemic. In this case, the usual 14-day quarantine period would be insufficient. If, for example, many people in China return to work following their two-week compulsory leave, it could possibly lead to a second wave of infections.
Crew and passengers of the Diamond Princess are to stay at home for another two weeks after two weeks of quarantine
Japan's Health Minister Katsunobu Kato has therefore called on all passengers and crew members of the cruise ship "Diamond Princess" to stay at home for an additional two weeks after the two-week quarantine on board. Two Australians had been diagnosed with the virus at home, although they had tested negative on the "Diamond Princess."