Paris (AFP)

Mortality rate, level of transmission, time when a patient becomes contagious, incubation period: unknowns still prevent the global impact of the epidemic from China caused by a new coronavirus from being determined.

- What mortality rate?

More deadly than the flu, but less virulent than previous epidemics of coronavirus: this seems to be the danger of the new coronavirus baptized 2019-nCoV, even if its mortality rate is not yet known with precision.

To date, 361 patients have died out of 17,200 confirmed cases in China. The first death outside of this country was reported Sunday in the Philippines, a 44-year-old Chinese from the city of Wuhan, out of the 150 or so patients registered in 24 other countries.

"2% of confirmed cases are dead, which remains high when compared to seasonal flu," said Michael Ryan, director of emergency programs at WHO.

It is not known how many people are actually infected. The indicative mortality rate decreases every day since, proportionally, the number of new cases recorded increases faster than that of deaths.

A study published Friday in the medical journal The Lancet estimates at 76,000 (or more than ten times the official estimate) the number of people infected in Wuhan alone, the cradle of the epidemic, based on statistical projections.

The two previous deadly epidemics caused by a coronavirus, SARS (severe acute respiratory syndrome) and Mers (Middle East respiratory syndrome), had been much more virulent.

The SARS epidemic killed 774 people worldwide in 2002-2003 according to the WHO, including 349 in mainland China and 299 in Hong Kong. Out of 8,096 cases, this places his mortality rate at 9.5%. Still ongoing, the Mers epidemic has killed 858 out of 2,494 cases since September 2012, representing 34.5% mortality.

Seasonal flu is much more deadly in absolute numbers, since it kills between 290,000 and 650,000 people a year worldwide, according to the WHO.

In addition to the dangerousness of the virus, it is also its ability to be transmitted that will determine the severity of the epidemic. "A relatively mild virus can still do great damage if many people get it," said Ryan.

- What level of contagion?

One of the important parameters is the number of people infected with each infected person, called the "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.

It is higher than winter flu (around 1.3), significantly lower than measles, very contagious (more than 12), and comparable to Sras (3).

- Can we be contagious without symptoms?

The Chinese authorities have argued that contagion was possible before symptoms appeared (which is the case for the flu but was not for the SARS).

This hypothesis, which has not been confirmed with certainty, could complicate the control of the spread of the virus, as this would make it more difficult to locate infected persons.

Even if such a hypothesis were to prove, it would be necessary to "see what it weighs in the dynamics of the epidemic", underlines Pr Arnaud Fontanet, of the Pasteur Institute in Paris. Indeed, the cough of an infected patient is an important vector for the transmission of the virus, but a patient without symptoms does not cough.

- Does quarantine rule out the risks?

Apart from the containment measures in China, which target tens of millions of inhabitants, around Wuhan and Wenzhou, several countries have put in place a 14-day quarantine for their repatriated nationals.

This duration was decided on the basis of the probable incubation period of the new coronavirus: the WHO estimates the time between infection and the appearance of the first symptoms between two and ten days, while a Chinese study published in the NEJM estimates it at 5.2 days on average, with a large variation depending on the patient.

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.

Beyond that, a person who has not declared any symptoms and who has not been again exposed to a risk of contagion is considered to be no longer at risk.

- 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 in The Lancet.

All of these patients had pneumonia, most had fever and cough, and one third suffered from shortness of breath.

The average age of these 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 management involves treating symptoms, including fever. Some patients are still given antivirals, the effectiveness of which is being evaluated.

- What origin?

The path of a virus from bats, mentioned by researchers since the beginning of the epidemic, seems to be confirmed.

According to a study published this Monday in the journal Nature, the genome of the virus taken from five seriously affected patients, who worked on the market in Wuhan where the first cases appeared, is "identical to 96%" that of a coronavirus which circulates in bats.

However, we still do not know which animal transmitted it to humans. Identifying this intermediate host could help curb the epidemic.

In the case of SARS, where the animal in question had turned out to be the civet, the ban on the consumption of this mammal had made it possible to "prevent any reintroduction" of the virus, recalls Professor Arnaud Fontanet.

Conversely, one of the reasons why the Mers epidemic continues is the fact that the reservoir of the virus is the dromedary, a domestic animal.

© 2020 AFP