Human-to-human transmission of the novel coronavirus was occurring since mid-December 2019 in China, with the outbreak initially doubling in size every week, researchers found.

Based on the first 425 cases with novel coronavirus-infected pneumonia, the epidemic appeared to double in size every 7.4 days in its earliest stages, with a basic reproductive number estimated at 2.2, reported Zijian Feng, M. Med, of the Chinese Center for Disease Control and Prevention in Beijing, and colleagues.

In addition, mean incubation period was about five days, with the 95th percentile at 12.5 days, the researchers wrote in the New England Journal of Medicine.

"Considerable efforts to reduce transmission will be required to control outbreaks if similar dynamics apply elsewhere," they noted.

Interestingly, the authors noted that while over half of those cases were linked to exposure to Wuhan's Huanan Seafood Market, the widely reported source of the outbreak in the earliest days of the virus, "it is now clear that human-to-human transmission is occurring and that the epidemic has been gradually growing in recent weeks."

Human-to-human transmission of novel coronavirus outside China is on the minds of the international community, particularly as the World Health Organization convenes its emergency committee on Thursday to decide whether the outbreak constitutes an international health emergency. Reuters reported a fifth case of novel coronavirus in France late Wednesday, in the daughter of a man, age 80, who was hospitalized with the disease.

But the reproductive number -- the average number of people who will contract the virus from one infected person -- is also of considerable interest, as it determines how efficiently the virus can transmit among the population. Based on their data, Feng and colleagues estimated a basic reproductive number for novel coronavirus of 2.2 (95% CI 1.4-3.9), which falls into the widely reported "between 1.5 and 3" basic reproductive number for the virus.

This would put novel coronavirus on par with the 2014 Ebola West Africa outbreak, whose basic reproductive number was 2, but far below measles, which ranges from 12-18.

"We're still learning a lot about the transmission of the virus, so the reproductive number is going to change rapidly," Amesh Adalja, MD, spokesperson, Infectious Diseases Society of America, told MedPage Today, speaking in general about the reproductive number of the virus.

Adalja also cautioned against putting a lot of emphasis into the reproductive number or using it as "a special magic number that applies to all situations," because each patient has his or her own reproductive number.

"If you isolate a patient and they don't transmit the virus to anybody, then their reproductive number is zero," he said.

Feng and colleagues examined data from the first 425 laboratory-confirmed cases of patients with novel coronavirus-infected pneumonia up through Jan. 22. Median age of patients was 59, though they ranged from age 15-89, and 56% were men. There were no cases in children younger than age 15, the authors said. Estimates of the epidemic growth rate were calculated from cases with symptom onset from Dec. 10 to Jan. 4.

Based on exposure data among ten confirmed cases, researchers estimated the mean incubation period was 5.2 days. Duration from illness onset until first medical visit for ill patients was about six days prior to Jan. 1, and down to about 4.5 days during Jan. 1-11.

On the U.S. front, CDC's daily briefing Wednesday afternoon revealed no new cases in the U.S. So far, there are 165 persons under investigation, and 68 of those have tested negative. CDC officials also said that they have evacuated 195 U.S. citizens from Wuhan, who are now on U.S. soil, as reported by the CDC and the Department of Health and Human Services earlier on Wednesday.

CDC officials did say they suspect they will find additional U.S. novel coronavirus cases, either from travelers or from close contacts of travelers from Hubei province in China.