Monitored, analyzed and fought by doctors around the world, the new coronavirus still keeps large gray areas more than three months after its appearance in China. Five key things we continue to ignore or misunderstand about the SARS-CoV-2 virus and the disease it causes, Covid-19.
The extreme amplitude of the severity of the symptoms never ceases to amaze: why here the Covid-19 produces no or few symptoms (80% of cases according to the WHO) while in some it induces a high fever, even a fatal pneumonia? “Research since February 2020 shows that the clinical spectrum of this disease can be very heterogeneous,” reports Leo Poon of the Hong Kong Faculty of Medicine. During the peak of the epidemic in China, he compared, with a Chinese team from the University of Nanchang (center), people with mild illnesses to the seriously ill. The results were published in the British medical journal The Lancet.
What do we learn? That severely affected people are “significantly older” than those with mild illness and that the concentration of virus in their samples is “about 60 times higher” than in those with mild illness.
Is it the result of a poorer immune response due in particular to age, or the consequence of exposure to higher doses of virus during contamination? Work on a different virus, measles, has shown that the severity of the disease is correlated with the dose of initial exposure to the virus. It is unclear whether the same is true for the Covid-19.
It is known that coronavirus is transmitted by physical contact and the respiratory tract. For example by the droplets of saliva expelled when a patient coughs. Can it circulate in suspension in the air like the seasonal flu? The question is not settled. An American study published in the New England Journal of Medicine ((NEJM) shows that the new coronavirus can survive in the laboratory for three hours in the form of particles in the air. But it is not known whether this ability is important for the transmission of the disease.
“Is the virus present in the environment, does it persist for a long time in the air or on inert surfaces. Well, we don’t know. We know we can find the virus, but we don’t know if this virus is infectious, ”comments Pr Karine Lacombe, head of the infectious diseases department at Saint-Antoine hospital (Paris).
How many have already caught the virus? Aside from the few countries that have rapidly implemented aggressive screening policies, such as South Korea and Germany, knowledge of the number of infected people is very approximate. Thus, the British government evaluated the cases in the country on March 17 at 55,000 when officially less than 2,000 had been tested positive.
Having a clear idea of the extent of the epidemic is crucial for isolating and caring for virus carriers. And, in a second step, it will become precious to know who has already caught the virus and is a priori already immune. This will only be possible with the arrival of a new generation of tests, called serological tests, which will target the detection in the blood of the immune signature left against SARS-CoV-2.
Will Covid-19 fade with the arrival of sunny days in the northern hemisphere? It is a possibility but not a certainty, say specialists. Respiratory viruses of the seasonal flu type are more stable in cold and dry weather, which favors their transmission in winter. A study by Hong Kong academics has shown that the SARS virus which struck Asia in 2002-2003 and which is a close cousin of the current coronavirus, resists better in low temperatures and low humidity.
But a recent American study, from Havard Medical School, stresses that “the only weather changes will not necessarily lead to the decline of Covid-19 cases without the implementation of major health interventions”.
Children are much less prone to Covid-19 than adults. And if they develop symptoms, they are usually mild. A Chinese study published in the journal Nature shows that out of ten children studied, affected by Covid-19, none developed a severe form, their symptoms being limited to sore throat, cough and mild fever.
According to this research, children living with sick people are two to three times less likely to get the virus than adults. Why? We do not know. The same characteristic was noted for the SARS virus in 2002-2003. “There are many things that we do not know, which calls for a lot of humility,” notes Pr Karine Lacombe.
(The essential / dpa)
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