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When Italy, and also Europe and the whole world, are so globalized, they will be able to get theregroup immunity? What percentage of the population will need to be vaccinated against Sars-CoV2 so that the disease disappears and can return to a normal life? In these days when coronavirus vaccinations have also started in Europe, after the United Kingdom and the United States, many are wondering. At the beginning of the pandemic, many epidemiologists had hypothesized a coverage of 60-70% of the population, as also stated by the World Health Organization. In Italy, the race is now vaccinating 70-80% of the population to free itself from the threat of the virus and return to normal. Of course it is not easy to get an accurate estimate of the numbers needed to obtain herd immunity, but it is important to have an order of magnitude also to imagine when we will really be able to regain possession of our lives. A matter of statistics, but also of psychological help.
The ballet of percentages
As the New York Times has written for about a month, Dr. Anthony Fauci, a world-renowned American immunologist who is part of the task force to deal with the emergency, has begun to raise the bar in several television interviews, arguing that to obtain herd immunity, 70-75% of the population must be vaccinated and then pressed. in other interviews up to 80-85%. Fauci even went so far as to admit that even if it is somewhat difficult to digest, 90% of the population may need to be vaccinated to stop the circulation of the virus. Therefore, we are not far from the 95% of immunizations that are used to block measles, a highly contagious disease (it is believed to be the most contagious disease in the world because the virus can remain in the air for hours).
The mathematical formula
Today, many epidemiologists are convinced that the initial estimated range of vaccination coverage between 60 and 70% of the population was too low. As he explains Paolo Bonanni, an epidemiologist, professor of Hygiene and Preventive Medicine at the University of Florence, a mathematical formula is applied to calculate the immunity rate of the population necessary to achieve herd immunity. Herd immunity is obtained when the immunity coverage of the population is equal to 1-1 / R0 (one minus one over R0). The 66% value that has always been cited assumed that the coronavirus had a baseline transmissibility rate without containment measures. equal to 3 (Ro of 3). So 1-1 / 3 equals ⅔, that’s 66%. So it is obvious that R0 is not the same in all geographic areas and depends on the density of the population, social contacts as well as the intrinsic characteristics of the virus. A virus that is very transmissible everywhere per se, there is a higher population density and greater social interactions that the value rises, with a more dispersed population that value drops but always remains close to a value that biology has assigned to that virus.
The reason for the upward estimates
But what happened then? Why did the estimates go from 66% to over 80? In the course of the pandemic they took over variants clike the English and South Africans, they seem to have acquired greater transmission capacity from person to person, with a value greater than R0. Therefore it also increases the number of immune in the population needed to extinguish the epidemic trend and come up with the famous herd immunity. A more contagious variant takes over more easily because it has the biological advantage of spreading more. Let’s say that the value of R0 has become 5 instead of 3, applying the formula it would be 1-1 / 5 = 4/5 which is 80%. It all depends on how much the new variants have acquired in terms of ease of person-to-person transmission. We are not at the level of measles: Going from an R0 of 3 to an R0 of 15 seems very difficult to me, it would require a notable mutation that I think unlikely, but that the R0 has gone from 3 to 5 probable. Thinking that it takes more than 80% of the immune population (natural and vaccine immunity) to extinguish the epidemic trend, I think it is unlikely, explains Bonanni.
What does higher transmissibility have to do with this?
In theory, a highly infectious virus requires more time to achieve herd immunity than a non-diffusive virus: for this reason, although previously there was talk of vaccination coverage equivalent to 60-70 percent of the population, it may be necessary to reach at 75 -80 percent. It will take several months to do so. In a country where the virus had already affected 10 percent of subjects, vaccinating 5% of the population each month, immunity would be achieved in December 2021 instead of September, but these are only hypotheses. Certainly, the more the virus circulates, the more opportunities it has to mutate. That is why vaccination is very important: even if vaccines were slightly less effective with new mutations, they would still prevent most infections, the epidemiologist concludes.
The weak groups
Doctor Marc Lipsitc, an American epidemiologist and professor in the Department of Epidemiology at Harvard’s TH Chan School of Public Health in the New York Times admitted that Fauci is correct in arguing that it will take at least 85% immunity to completely block the virus, but it has given a glimmer of optimism: we don’t have to get zero transmission to get back to normal life. There are many illnesses like the flu that are spread all the time, but we don’t lock ourselves in for that. Already by vaccinating all those most at risk of developing a serious disease, Covid would become a milder disease. Ultimately, the first effects of the vaccine should be noticed with the vaccination of the highest risk groups: doctors and nurses who fight in the room every day and the elderly with chronic diseases.
December 28, 2020 (change December 28, 2020 | 15:15)
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