Pourquoi plus de personnes vaccinées que de non vaccinées meurent de la Covid-19 en Angleterre
https://www.futura-sciences.com/san…
24 juillet 2021
Céline Duzarche
Le chiffre a fait le tour des réseaux sociaux, alimentant le discours sur l’inefficacité des vaccins. Pourtant, en y regardant de plus près, cette statistique n’a rien de surprenant et elle est même tout à fait logique. Futura explique pourquoi.
C’est une statistique assez perturbante dont se nourrissent les antivax : entre le 1er février et le 21 juin 2021 en Angleterre, il y a eu davantage de morts de la Covid-19 parmi les personnes vaccinées et testées positives au variant Delta que parmi les personnes non vaccinées, atteste un rapport du gouvernement. Plus exactement, sur les 257 décès, 45 personnes avaient reçu une dose, 118 avaient reçu deux doses, et 92 n’avaient reçu aucune injection. La conclusion hâtive qu’il pourrait être tiré de ces statistiques est que le vaccin n’est pas efficace, voire dangereux.
Les personnes âgées disproportionnellement touchées
Mais, c’est précisément là qu’il faut se méfier des chiffres. Car vacciné ou pas, les morts se comptent toujours principalement parmi les plus âgés : les plus de 50 ans représentent en effet 90 % des décès. « Une personne de 70 ans est 32 fois plus susceptible de mourir de la Covid-19 qu’un jeune de 35 ans », atteste Christian Yates, chercheur à l’université de Bath, au Royaume-Uni. Or, aucun vaccin n’est efficace à 100 %. Ce qui signifie qu’une personne âgée de 70 ans et vaccinée est toujours plus à risque de mourir de la Covid-19 qu’un jeune de 35 ans non vacciné. C’est relativement facile à comprendre : « Imaginez que tout le monde soit entièrement vacciné. Malgré l’excellente efficacité des vaccins, certaines personnes continueront malgré tout de mourir, explique Christian Yates. On aura alors 100 % de personnes vaccinées parmi les décès. Mais cela ne signifie pas que les vaccins ne sont pas efficaces ! »
Répartition des décès liés à la Covid-19 en France. / C.D, Futura, d’après Santé publique France
Vaccin : une réduction du risque d’hospitalisation de 87 %
Selon l’étude française Epi-Phare, les vaccins réduisent de 87 % le risque de forme grave de la Covid-19 chez les personnes de plus de 75 ans. En d’autres termes, le risque est réduit d’un facteur 9. Or, on voit que cette réduction est loin de compenser l’augmentation de 32 fois le risque de décès liée à l’âge. De plus, l’efficacité des vaccins contre le risque d’hospitalisation diminue un peu avec l’âge : elle est de 91 % chez les 75-84 ans et de 81 % chez les 85 ans et plus. « En supposant que le risque d’infection soit le même chez les personnes âgées et les personnes jeunes, il faut donc s’attendre à voir toujours plus de patients âgés de plus de 70 ans et doublement vaccinés mourir de la Covid-19 que de 35 ans non vaccinés », conclut Christian Yates.
Vacciner encore plus les publics à risque
Que conclure de tout cela ? Premièrement, la stratégie consistant à vacciner en priorité les personnes âgées a été la bonne : sans cela, le nombre de décès aurait été encore plus important. Deuxièmement, il est plus que jamais nécessaire d’augmenter encore la couverture vaccinale chez les personnes vulnérables. Au 15 juillet en France, 85 % des plus de 65 ans avaient reçu au moins une dose de vaccin et seuls 79 % étaient entièrement vaccinés. Enfin, les plus jeunes ne doivent pas non plus se sentir naturellement protégés. Même si leur risque de mourir de Covid-19 est moindre que chez leurs aînés, leur risque d’infection est bien plus élevé (du fait notamment des interactions sociales) et la Covid-19 peut entraîner des effets très handicapants à long terme.
Most COVID deaths in England now are in the vaccinated – here’s why that shouldn’t alarm you
https://theconversation.com/most-co…
14 juillet 2021, 12:24 CEST
Mis à jour le 15 juillet 2021, 16:30 CEST
Christian Yates,
Senior Lecturer in Mathematical Biology, University of Bath
Déclaration d’intérêts
Christian Yates ne travaille pas, ne conseille pas, ne possède pas de parts, ne reçoit pas de fonds d’une organisation qui pourrait tirer profit de cet article, et n’a déclaré aucune autre affiliation que son organisme de recherche.
Partenaires
University of Bath
University of Bath apporte un financement en tant que membre adhérent de The Conversation UK.
Voir les partenaires de The Conversation France
More vaccinated people are dying of COVID than unvaccinated people, according to a recent report from Public Health England (PHE). The report shows that 163 of the 257 people (63.4%) who died of the delta variant within 28 days of a positive COVID test between February 1 and June 21, had received at least one dose of the vaccine. At first glance, this may seem alarming, but it is exactly as would be expected.
Here’s a simple thought experiment : imagine everyone is now fully vaccinated with COVID vaccines – which are excellent but can’t save all lives. Some people who get infected with COVID will still die. All of these people will be fully vaccinated – 100%. That doesn’t mean vaccines aren’t effective at reducing death.
The risk of dying from COVID doubles roughly every seven years older a patient is. The 35-year difference between a 35-year-old and a 70-year-old means the risk of death between the two patients has doubled five times – equivalently it has increased by a factor of 32. An unvaccinated 70-year-old might be 32 times more likely to die of COVID than an unvaccinated 35-year-old. This dramatic variation of the risk profile with age means that even excellent vaccines don’t reduce the risk of death for older people to below the risk for some younger demographics.
PHE data suggests that being double vaccinated reduces the risk of being hospitalised with the now-dominant delta variant by around 96%. Even conservatively assuming the vaccines are no more effective at preventing death than hospitalisation (actually they are likely to be more effective at preventing death) this means the risk of death for double vaccinated people has been cut to less than one-twentieth of the value for unvaccinated people with the same underlying risk profile.
However, the 20-fold decrease in risk afforded by the vaccine isn’t enough to offset the 32-fold increase in underlying risk of death of an 70-year-old over a 35-year-old. Given the same risk of infection, we would still expect to see more double-vaccinated 70-year-olds die from COVID than unvaccinated 35-year-olds. There are caveats to that simple calculation. The risk of infection is not the same for all age groups. Currently, infections are highest in the youngest and lower in older age groups.
COVID infections are currently highest in young people. Andy Rain/EPA
Think of it as ball-bearing rain
One way to imagine the risk is as a rain of differently sized ball bearings falling from the sky, where the ball bearings are the people that get infected with COVID. For simplicity’s sake, let’s assume there are roughly equal numbers of ball bearings in each age group. In each age category, there is also a variation in the size of the balls. The balls representing the older groups are smaller, representing a higher risk of death.
Now imagine there’s a sieve that catches many of the balls. Most people who get COVID will not die (most balls get caught in the sieve). But some of the smaller balls fall through. The older you are, the more likely you are to fall through the holes. The balls that make it through the first sieve are hugely skewed towards older age ranges, represented by the smaller ball bearings. Before COVID vaccines came along, the people that fell through the holes represented the people who would die of COVID. The risk was massively skewed towards older people.
Vaccination provides a second sieve underneath the first, to prevent people from dying. This time, because we haven’t vaccinated everyone, it’s the holes in the sieve that are of different sizes. For older people who’ve had both doses, the holes are smaller, so many ball-bearings are stopped. The vaccines will save many of those who would previously have died.
For younger people the holes in the vaccine sieve are currently bigger as they are less likely to have received both doses and so more likely to fall through the sieve.
If all the filtering were just done by the second sieve (with no skew in risk of death by age, represented by the first sieve), then we might expect younger unvaccinated people to account for a larger proportion of the deaths. But it isn’t. The first sieve is so hugely biased towards older people that even with vaccination, more of them slip through the second sieve than the younger unvaccinated people. Given the UK’s vaccination strategy (vaccinate older, more vulnerable people first), you would expect high proportions of the people who die from COVID to have been vaccinated. And that is exactly what we see in the data.
The fact that more vaccinated people are dying than unvaccinated people does nothing to undermine vaccine safety or effectiveness. In fact, it’s exactly what we’d expect from the excellent vaccines, which have already saved tens of thousands of lives.
Correction : An earlier version of this article stated : “The report shows that 163 of the 257 people (63.4%) who died within 28 days of a positive COVID test between February 1 and June 21, had received at least one dose of the vaccine.” It should have read : “The report shows that 163 of the 257 people (63.4%) who died of the delta variant within 28 days of a positive COVID test between February 1 and June 21, had received at least one dose of the vaccine”.
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