MADRID – The promise of herd immunity, the “philosopher’s stone” of collective protection that would leave the pandemic only in memory, is a goal that today seems distant, perhaps unattainable. As new variants of the virus are increasingly infectious, the portion of the immunized population needed for this is greater than the 70% initially estimated. Although the new number cannot be nailed down, experts place it at around 90%, a rate that cannot be achieved in most countries without vaccinating children under 12 years of age, for whom there is no approved vaccine.
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The idea of collective protection is not just theoretical: it is what keeps diseases like measles and diphtheria under control and managed to eliminate smallpox. Herd immunity is based on the fact that when enough of the population is immune to a virus, the pathogen loses its ability to spread. If a person becomes infected, but the vast majority of people around them are not susceptible to infection, they will not be able to jump to another individual and will disappear into the patient: either by killing them or by their own immune system.
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The percentage of the population needed to achieve herd immunity depends on the infective capacity of the virus. In Sars-CoV-2, this capability has grown with the Delta variant, the most contagious to date.
An internal report by the US Centers for Disease Control and Prevention (CDC), obtained by the Washington Post, indicates that each person with Delta can infect another nine: between three and four times more than initially estimated, which the makes it as contagious as chickenpox.
And, in parallel with this greater transmission capacity, estimates also increase, always approximate, of the percentage of the vaccinated population necessary to achieve collective immunity. In the beginning, 70% was thought, but all experts consulted by El País now consider that this limit has been surpassed. And those who point to new numbers suggest that you need to reach 90% or even more.
Epidemiologist Javier del Águila of the Autonomous University of Madrid, for example, believes that the idea of herd immunity “does not seem very realistic in the current context”:
— We have been poring over the subject for some time. Because it is a respiratory virus with such high transmissibility, several problems are added in the case of the coronavirus: in my opinion, vaccination coverage rates close to 95% would be necessary for herd immunity. This is something very difficult, even in countries like Spain, where the reluctance to vaccinate is very low – he says.
This is linked, he adds, to the fact that strains like Delta raise the infection curve when they encounter a susceptible group of people.
– And, when there are many infected, as the vaccines are not perfect, it ends up also reaching those who took the two doses – emphasizes the expert.
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José Jiménez, a researcher at the Department of Infectious Diseases at King’s College London, goes further. He believes it would be better not to set more group immunity percentages. According to him, it is not known if it is possible to achieve herd immunity goals and, if possible, we are far from ideal.
— These are very theoretical estimates and they can vary greatly depending on the effectiveness of vaccines and the emergence of new variants. The best message we can send is to get vaccinated as much as possible, without setting any percentage as a goal – he says.
For the specialist, vaccination, however, ensures that the vast majority of cases are mild or asymptomatic; it will also make the next epidemic waves less bulky and make the coronavirus no longer the social problem it has been posing so far. But it probably won’t be able to stop the propagation, at least not yet.
Ignacio López Goñi, professor of Microbiology at the University of Navarra has a similar opinion:
“Maybe instead of obsessing about numbers, about herd immunity, it’s more realistic to set the goal of reducing health collapse. If it’s avoided, we could all go back to being as close to normal as possible. We are not going to eradicate the virus now, we will have to live with it. For this, the most vulnerable need to be vaccinated. But the virus will continue to arrive where we let it enter, especially in the unvaccinated.
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The most likely, in the opinion of Miguel Hernán, professor of epidemiology at Harvard University, is that the coronavirus will become endemic, like the pathogens that cause colds.
— It is possible, for example, that they have even caused a pandemic in the past, but today there is no epidemiological surveillance of them because it is no longer necessary — he says.
The trend for Covid-19 will also be this if there are no mutations that make the virus escape the protection that vaccines approved by regulatory agencies provide against the most serious forms of the disease. More and more people have some type of antibody, either by vaccination or by a history of infection. In this scenario, the biggest problems will be for people who, for some reason, do not generate defenses against the virus.
This adds to the fact that approved vaccines, while very good at preventing the most severe variants of the disease, do not completely prevent transmission. At present, there is no consensus on the ability of immunizers to protect against Delta variant infection. The same report from the CDC, for example, indicates that, although infections among vaccinated individuals are still rare, when they do occur they demonstrate the same ability to retransmit as an unvaccinated person.
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With all this, it would even be doubtful to obtain the dreamed of collective immunity vaccinating all those over 12 years old. And achieving that goal is practically impossible. According to the latest survey by the Spanish Foundation for Science and Technology (Fecyt), the number of people who flatly refuse to receive vaccines in the European country is 4%. To these are added other sections of the population: those that cannot be immunized due to health problems, those that the health system does not reach, and those that do not bother to seek the vaccine, even though they are not against it. In short, surpassing 80% without mandatory vaccination (something that is not even being considered for now) is going to be very complicated.
To reach 95% vaccination percentages against Covid-19 as described by Del Águila, it would, of course, be essential to vaccinate children. But even when there is a vaccine for them, there will be an ethical debate that is difficult to resolve.
As age decreases, the benefit-risk balance of vaccines decreases, as the severity of the disease also decreases. Although the risk of a serious side effect from a vaccine is rare, in children under the age of 12 it is likely to be greater than the risk posed by the infection itself. Precisely for this reason, countries like the United Kingdom have already ruled out the vaccination of adolescents, for which there are approved immunization agents.
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Federico de Montalvo, president of the Spanish Bioethics Committee, explains that vaccination is aimed at individual and collective protection. “Would it be ethical for children to take risks to protect society while there are adults who don’t get the vaccine because they don’t want to?” De Montalvo believes that, when that time comes, the debate on mandatory vaccination of adults will have to be resumed, a topic that until now was not at stake in the Iberian country.
Another consequence is that even if Spain, with its good vaccination rate, achieves supposed herd immunity, the rest of the world will be slow to do so.
Other Western countries face serious obstacles to progress, such as the United States, which seeks all kinds of incentives for the population to get immunized. Even Israel, which started as a world leader, has been stagnant for weeks at about 60% of the population with full immunization, a number that Spain will reach in a few days.
But these are “First World problems”. For developing countries, where doses are barely enough, and with very weak health systems, group immunity is a chimera.
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In Del Águila’s opinion, we should be more concerned with giving vaccines to these countries than with a third dose in the rich, as Israel is already doing.
— As the virus circulates around the world, it will be more capable of mutating. And, the greater its occurrence, the greater the probability of escaping from vaccines – he says.
This is the great fear of sanitarists. As long as vaccines continue to prevent hospitalizations and deaths, a large number of vaccinees will keep the disease under control, even without herd immunization. But if a variant gets around that barrier, strong measures will be needed to prevent health systems from collapsing again.