Many researchers believe that a larger than expected part of the population is protected against Covid-19.
- Many people are immune to Sars-CoV-2 through antibodies developed against other bacteria, this is cross-immunity.
- Several researchers estimate that the part of collective immunity necessary to stop the epidemic is between 10 and 20%.
- The next few weeks will be indicators as to whether a second wave is looming.
Containment efforts continue to be felt. The number of patients hospitalized for severe Covid-19 infection in intensive care units is 1,700, a figure “still decreasing since April 8”, noted the health agency Public Health France in its bulletin of May 21. At the height of the epidemic, 7,148 patients occupied intensive care units. The number of deaths also continues to decrease with 74 deaths in hospitals between May 21 and 22, notes France info.
cross-immunity
Herd immunity could be achieved faster than expected through cross-immunity. When a person recovers from an infection, they usually develop specific antibodies against that infectious agent. Sometimes these antibodies can fight bacteria or viruses from related species. This is called cross-immunity. In the case of the coronavirus, many people have thus developed protection against Sars-CoV-2 through past infection with other coronaviruses causing common colds. American researchers published a study in the journal Cellestimating between 40 and 60% the share of the population immunized against Covid-19 by this principle of cross-immunity.
For Yonathan Freud, emergency doctor at the Parisian Pitié-Salpêtrière hospital, there will be no second wave, precisely thanks to this principle of cross-immunity. “I understand that we are very cautious about the idea of making a new prophecy, because everyone was so wrong at the start, and me the first”, he advanced on his Twitter account. He relies on the drastic drop in the number of contaminations among his colleagues compared to the start of the epidemic to explain why there will be no second wave.
Why do I think there won’t be a 2nd wave, or at least the current epidemic is over. I will try to detail. It’s my belief, but if you don’t like it, you don’t have to read or comment, we won’t debate here.
— Yonathan Freund (@yonatman) May 21, 2020
10 to 20% herd immunity required
Some researchers estimate that the collective immunity needed to stop the epidemic would not be 60% of the population but would rather be between 10 and 20%. One of the consequences of cross-immunity is heterogeneity in the spread of the virus. This explains why some people are more vulnerable to Covid-19 and more vectors of transmission of the virus. “In an emergency, we often do the simplest (…). But the longer the epidemic lasts, the more time we have to refine our calculations and incorporate the different sources of heterogeneity into our models.”describes Gabriela Gomes, a mathematician from Liverpool (United Kingdom), to Mediapart. In a pre-publication in the journal BMJwhich has yet to be assessed by other researchers, she concludes that herd immunity is between 10 and 20%. “We obtain a similar result on our side, with a threshold around 14% when we take into account these strong heterogeneities”confirms to Laurent Hébert-Dufresne, researcher at the University of Vermont’s Complex Systems Center, in several posts.
Despite the possibility of having an exit from the epidemic earlier than expected, the time remains for caution. “All we can say is that today, we have no warning signal. But it is too early to draw from this observation that everything will be fine”, summarizes to AFP the epidemiologist Daniel Lévy-Bruhl, head of the respiratory infections unit within Public Health France. The figures on the declines in the number of deaths and people in intensive care are those resulting from the containment effort. “Maybe next week we will have elements”, he continues. Furthermore, he judges “premature to base any hope on cross-immunity” which is not unanimous, according to him.
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