Monitored, analyzed and combated by doctors around the world since its appearance in China at the end of December, the new coronavirus still keeps large gray areas. Five key elements around the virus are, for example, still little known or even overlooked.
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Why benign for some, extremely important for others?
“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. In fact, Covid-19 produces no or few symptoms in 80% of cases, while in some it induces a high fever, even fatal pneumonia.
During the peak of the epidemic in China, Leo Poon compared, with a Chinese team from the University of Nanchang (center of the country), people weakly reached with severe patients. The results were published in the British medical journal The Lancet. We learn that severely ill 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.
It is not known, however, whether this worsening of the disease in some people is due to a poorer immune response due to age or whether it is the result of exposure to higher doses of the virus during contamination. The appearance of serious cases in younger patients and without comorbidity is particularly questioning.
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An airborne virus?
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, as explained by Public Health France. But we don’t know if it can circulate in suspension in the air, like the seasonal flu.
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 virus, but we don’t know if this virus is infectious “, comments Pr Karine Lacombe, head of the infectious diseases department at the Saint-Antoine hospital (Paris).
Aside from the few countries that have rapidly implemented aggressive screening policies, such as South Korea and Germany, knowledge of the number of infected remains very approximate. Thus, the British government evaluated on March 17 the cases in the country at 55,000, while 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.
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Is the virus sensitive to the weather?
Respiratory viruses of the seasonal flu type are more stable in cold and dry weather, which favors their transmission in winter. Covid-19 may therefore fade with the arrival of sunny weather in the northern hemisphere, but there is no certainty.
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 Harvard 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”.
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Why are children spared?
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, but it is not yet known why. 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.
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