Paris (AFP)

Mortality rate, level of transmission, time when a patient becomes contagious, incubation period: many unknowns still prevent us from determining the global impact of the epidemic that started in China and was caused by a new coronavirus.

- What mortality rate?

The severity of the epidemic will depend on the "interaction" of two factors: "the level of transmission of the virus and its dangerousness," said Michael Ryan, director of emergency programs at the WHO on Wednesday.

"A relatively mild virus can do great damage if many people get it," he added.

To date, 170 patients have died out of a total of nearly 7,700 cases in China. None died outside of China, while around sixty patients have been recorded in nearly twenty other countries.

"2% of confirmed cases are dead, which remains high when compared to the seasonal flu," said Ryan.

However, this rate is only indicative and decreases every day, since proportionally, there are more new confirmed cases than deaths.

Previously, only two deadly epidemics have been caused by a coronavirus, a large family to which the new virus belongs: SARS (severe acute respiratory syndrome) and Mers (Middle Eastern respiratory syndrome).

According to the WHO, the SARS epidemic had killed 774 people worldwide out of 8,096 cases in 2002/2003 before being brought under control, ie a mortality rate of 9.5%. Still ongoing, the Mers epidemic has killed 858 out of 2,494 cases since September 2012, representing a death rate of 34.5%.

For comparison, the WHO estimates that seasonal flu kills between 290,000 and 650,000 people a year worldwide.

- What level of contagion?

One of the important parameters is the number of people infected by an infected person, called "basic reproduction rate" (or R0).

In recent days, several estimates have been made by different research teams, ranging from 1.4 to 5.5.

The latest comes from Chinese researchers, authors of a study published in the American medical journal NEJM. They estimate that each patient infected an average of 2.2 people.

According to them, this figure is relatively low, closer to winter flu (of the order of 1.3) than to measles, very contagious (more than 12), and comparable to the SARS of 2002 (3).

- At what stage is a patient contagious?

This crucial question is still unanswered.

Chinese authorities said on Sunday that contagion was possible before symptoms even appeared (which is the case for influenza but was not for SARS).

However, this hypothesis is based on the observation of a few cases and is not confirmed with certainty.

"There is an urgent need to conduct research on this issue," insists Professor Mark Woolhouse of the University of Edinburgh (Scotland).

"Our main hope for controlling the epidemic is to quickly identify affected patients and isolate them to avoid contagion," he said. "If the transmission of the virus even before the onset of symptoms is confirmed on a large scale, the effectiveness of such measures would be compromised."

- What level of transmission between humans?

The main cases of direct contagion between humans have been observed in China. Three other cases have been reported in Vietnam, Germany and Japan.

The risk of such transmissions is "very low in developed countries," which have the means to contain the spread of the virus, according to J. Stephen Morrison, of the Center for International Strategic Studies (CSIS) in Washington.

However, if cases were exported "to certain African countries or other continents where the means of health security are limited, large epidemic foci could break out outside of China".

"This could be the prelude to a global pandemic," added Morrison, adding that for the time being, such a scenario is only theoretical.

- What incubation period?

It is the time between infection with the virus and the onset of the first symptoms.

WHO estimated Monday on average two to ten days. According to the Chinese study published in the NEJM, it is around 5.2 days on average and varies greatly depending on the patient. Previous work in the Netherlands has averaged 5.8 days.

The fact that the estimate is preliminary and "imprecise" justifies "a period of observation or quarantine of 14 days for those exposed," write the Chinese researchers in the NEJM.

- What symptoms?

The clinical picture of the respiratory disease caused by the new coronavirus becomes clearer after the analysis of the first 99 cases identified in China, published Wednesday in the medical journal The Lancet.

All of these patients had pneumonia (three-quarters of them affected both lungs), most had fever and cough, and a third suffered from shortness of breath.

The average age of these 99 patients is 55 years, two-thirds are men and half suffered from chronic diseases (cardiovascular problems, diabetes ...). As of January 25, 11 of these patients have died, 57 are still hospitalized and 31 have been discharged from hospital.

There is no vaccine or medication for the coronavirus, and medical care is taken to treat symptoms, including fever.

Another Chinese study published in The Lancet and based on 10 patients shows that the virus has changed little since its appearance in humans. "As it is transmitted to an increasing number of individuals, it is necessary to watch for the appearance of possible mutations," said one of the authors, however, Professor Weifeng Shi.

© 2020 AFP