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why we have not yet achieved herd immunity despite vaccines

The coronavirus pandemic Together, it has forced us to become familiar with a number of terms and concepts related to health and epidemiology that we may not have used before.

One of the most important, perhaps, is ‘herd immunity’. From the initial stages of the outbreak, it was sold by many political representatives as one of the possible endings of the pandemic, and there are even not a few governments that at various times have assured to direct their actions towards that goal. And yet it has often seemed that its precise definition was unclear or varied over time, as did the measures to achieve it; something that contributes to generating uncertainties regarding the future of the pandemic.

What exactly is herd immunity?

Herd immunity, explains 20Minutos Joan Caylá, member of the Spanish Epidemiology Society and president of the UITB Foundation (Unitat d’investigació en tuberculosis a Barcelona), occurs “when a population group is immunized in such a way that makes it difficult for people to get infected. So the few that are not immunized, or vaccinated, benefit from the fact that the rest of the population is. “

Caylá clarifies that this adequate immunization in a person can be achieved through two different routes: “vaccination or the virus having passed and stay with antibodies. “

As an example of a herd immunity that exists in Spain, the expert refers to the measles case: “Measles affected Spain a lot 40 years ago, which was when an effective vaccine was available. It is a disease with a very high ‘R’ (reproduction rate), that is, it is very contagious. to this, doing some statistical and epidemiological calculations it is deduced that more or less 95% of the population must be vaccinated to achieve herd immunity “.

“They began to be vaccinated and, in the first years, as the percentage of immunized was still very low, there were still a very high number of cases. But, as vaccination coverage increased, there were fewer and fewer cases, “he continues.

“Today there are still some small cases, but they are always imported. For example, if a person comes from the Philippines, say a place where there is still measles, and comes into contact with people who are not immune because they did not get vaccinated or did not pass the infection. Therefore, in Spain we have achieved group immunity to measles “.

The way to the disappearance of a disease?

Herd immunity does not therefore imply the definitive end of the disease. “It would be a first step. Following the example of measles, in Spain it would be eliminated but not eradicated. We speak of eradication when the pathogen no longer exists on a global scale; for this reason, no matter how well an infection is controlled in one country, if there is one in the neighboring country, it can always re-enter, “Caylá clarifies.

And it is that neither eradication nor herd immunity cannot be achieved in all cases: “In some diseases yes and in others no“This epidemiologist details:” There are some for which there is no vaccine, or for which the vaccine is not very effective. For example, for tuberculosis there is a vaccine, but it is not very effective and those who have passed the infection do not have immunity, so they can have episodes again “.

The Deputy Minister of Public Health of the Community of Madrid, Antonio Zapatero, has reacted this Monday to the images of the denialist demonstration that was held yesterday in the Plaza de Colón in Madrid and has pointed out that the fact that acts like this take place with everything what we are living seems to him

With respect to SARS-CoV-2, the situation is complicated. “In theory, herd immunity could be achieved,” says the expert. “But what happens is that at the beginning we started with a variant that had a basic reproduction number (R) of 2 or 3 (the basic reproduction number is the average number of new cases generated by a given case over the course of an infectious period; that is, the number of people a patient infects on average during the course of the disease) “.

“Doing the calculations that I mentioned before, which are actually quite simple,” he continues, ” a vaccination coverage of 70% to achieve theoretical herd immunity. “

Why are we still not immune to covid?

However, there are already several countries that have achieved this coverage and despite this they have not achieved the long-awaited herd immunity. “What has happened,” explains Caylá, “is that the delta variant that is more contagious arrived: one case no longer infected two or three people, but between five and eight. Therefore, what was initially 70% it became more than 90%, probably up to 95% “.

Epidemiological data of covid-19 in Spain in December 2020 and 2021.

“Now in Spain and other countries it is said that there is vaccination coverage of between 80 and 90%, but be careful; when we talk about herd immunity we are talking about the entire population, and when those vaccination percentages are given it is usually higher twelve years, despite the fact that the child population is also part of the ‘herd’ and must also be considered“, he argues.

“With which, if by vaccinating children we reach 90% of the total population, it would be a great advance. Even so, the coronavirus is very complicated: now the omicron variant is being introduced, which seems to require that those of us who are vaccinated with two doses we have to get vaccinated with a third “, concludes. “That is to say, compared to the measles vaccine, which gives a very good immunity for life, what we have now for covid is much more relative.”

As we can see, the requirements to achieve herd immunity are not only complex to calculate (even more so in the early stages of a pandemic, when limited information is still available) but also vary as the situation develops. Thus, political strategies aimed at obtaining herd immunity through contagion, such as the one announced by the United Kingdom Government before the development of the first vaccines (and which, fortunately, was corrected shortly after the criticism of the scientific community ), not only show a miserable understanding of epidemiology, but also seem frivolous that can entail a very high cost in human lives (If 70% of a population of 67.2 million people are infected with a disease that, as calculated at the time, has a mortality of 1%, it is not difficult to calculate that about 470,000 people may die, without taking into account factors such as population aging).

Is covid an endemic disease?

On the other hand, it has often been said that the coronavirus will become an endemic disease, which, explains Caylá, means that “it is a disease that is among us and that each year has a certain incidence among the population. That is, that it is not a very rare disease, but rather has a relative frequency. “

This might seem the opposite of what happens with herd immunity, but in reality, the disease reaching this status could be a step prior to obtaining the desired herd immunity. “Covid, at first, is an epidemic disease. It is a new disease, which initially affects many people, and then it starts to stay with us in a stable way, but with less incidence “.

Stock image of a PCR test.

The expert refers, again, to the case of measles: “For many years, there were cases in Spain. It was an endemic disease, but herd immunity was achieved “, aim.

“With the covid the same thing can happen a little. The problems of new variants and that the vaccines are not effective enough can make covid endemic, such as tuberculosis or sexually transmitted diseases such as AIDS, syphilis, gonorrhea … “.

What can we expect?

Regarding what we can ultimately expect to happen with the covid, Caylá continues with the same comparison: “I see it likely that it will follow the measles scheme. Vaccines they will help a lotBut in that case, from the discovery of the vaccine it took between 25 and 30 years to achieve herd immunity. “

Two people look at a poster with the guidelines for receiving the third dose of Pfizer in Valladolid.

“The covid is going to stay with us for a while. What it is about is minimize this endemic and that, if it can be, each year affects fewer cases “, adds the epidemiologist.

And he concludes by launching an appeal to the responsibility of each citizen: “To do this, we must assume that, although the majority of the population follows the indications, there are minority groups that do not follow the recommendations and this complicates a lot. It would try is that together we can control the disease so that affects us less and less. “

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