Comment atteindre l’#immunité_collective qui nous protégerait de la #COVID-19 ?
▻https://www.pasteur.fr/fr/espace-presse/documents-presse/edito-comment-atteindre-immunite-collective-qui-nous-protegerait-covid-19
Qu’en-est-il des super-contaminateurs ? De la nature des contacts entre les différentes classes d’âge ? De la contagiosité des enfants ? Où en est-on de l’immunité collective en France à l’issue de la première vague épidémique ? Pourrions-nous être protégés par les coronavirus saisonniers responsables du rhume ? Et comment faire pour atteindre cette immunité collective avec le moins de conséquences sanitaires possibles ?
Lire l’intégralité du Commentaire « COVID-19, herd immunity : where are we ? » publié dans Nature Reviews Immunology
(en anglais)
COVID-19 herd immunity : where are we ?
▻https://www.nature.com/articles/s41577-020-00451-5.pdf
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Taking these considerations into account, there is little evidence to suggest that the spread of SARS-CoV-2 might stop naturally before at least 50% of the population has become immune.
Another question is what it would take to achieve 50% population immunity, given that we currently do not know how long naturally acquired immunity to SARS-CoV-2 lasts (immunity to seasonal coronaviruses is usually relatively short lived), particularly among those who had mild forms of disease, and whether it might take several rounds of re-infection before robust immunity is attained. Re-infection has only been conclusively documented in a very limited number of cases so far and it is unclear whether this is a rare phenomenon or may prove to become a common occurrence. Likewise, how a previous infection would affect the course of disease in a re-infection, and whether some level of pre-existing immunity would affect viral shedding and transmissibility, is unknown.
With flu pandemics, herd immunity is usually attained after two to three epidemic waves, each interrupted by the typical seasonality of influenza virus and more rarely by interventions, with the help of cross-protection through immunity to previously encountered influenza viruses, and vaccines when available10.
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For COVID-19, which has an estimated infection fatality ratio of 0.3–1.3%1,5, the cost of reaching herd immunity through natural infection would be very high, especially in the absence of improved patient management and without optimal shielding of indi- viduals at risk of severe complications.
Assuming an optimistic herd immunity threshold of 50%, for countries such as France and the USA, this would translate into 100,000–450,000 and 500,000–2,100,000 deaths, respectively.
Men, older individuals and those with comorbidities are disproportionally affected, with infection fatality ratios of 3.3% for those older than 60 years and increased mortality in individuals with diabetes, cardiac disease, chronic respiratory disease or obesity. The expected impact would be substantially smaller in younger populations.
An effective vaccine presents the safest way to reach herd immunity. As of August 2020, six anti-SARS-CoV-2 vaccines have reached phase III trials, so it is conceivable that some will become available by early 2021, although their safety and efficacy remain to be established. Given that the production and delivery of a vaccine will initially be limited, it will be important to prioritize highly exposed populations and those at risk of severe morbidity. Vaccines are particularly suited for creating herd immunity because their allocation can be specifically targeted to highly exposed populations, such as health-care workers or individuals with frequent contact with customers.
Moreover, deaths can be prevented by first targeting highly vulnerable populations, although it is expected that vaccines may not be as efficacious in older people. Vaccines may thus have a significantly greater impact on reducing viral circulation than naturally acquired immunity, especially if it turns out that naturally acquired protective immunity requires boosts through re-infections (if needed, vaccines can be routinely boosted). Also, given that there are increasing numbers of reports of long-term complications even after mild COVID-19, vaccines are likely to provide a safer option for individuals who are not classified at-risk.
For countries in the Northern hemisphere, the coming autumn and winter seasons will be challenging with the likely intensification of viral circulation, as has recently been observed with the return of the cold season in the Southern hemisphere.
At this stage, only non-pharmaceutical interventions, such as social dis- tancing, patient isolation, face masks and hand hygiene, have proven effective in controlling the circulation of the virus and should therefore be strictly enforced. Potential antiviral drugs that reduce viral loads and thereby decrease transmission, or therapeutics that prevent complications and deaths, may become significant for epidemic control in the coming months. This is until vaccines become available, which will allow us to reach herd immunity in the safest possible way.