Herd immunity, that philosopher’s stone that would turn the pandemic into a memory, is a goal that is now far away. Perhaps unattainable. As new variants of the virus become more and more infectious, not only does it move away from that initially calculated 70% of the immunized population, it is virtually impossible to reach it in the short term. Although it is not known exactly what the new number may be, experts place it at around 90%, a figure that cannot be reached without vaccinating children under 12 years of age, for whom there is still no approved drug and which in Spain represent 11% of the population.
In São Paulo, the governor of São Paulo, João Doria (PSDB), announced last Saturday the desire to vaccinate children and adolescents aged 3 to 17 against covid-19. “Studies with this age group showed excellent results in terms of safety and efficacy”, he stated. On Friday, Anvisa confirmed having received an authorization request from the Butantan Institute to expand the age range for the application of vaccines against the coronavirus.
The idea of group protection is not just theoretical: it keeps diseases like measles and diphtheria under control and has managed to eliminate smallpox, the great infectious disease that humanity has eradicated. It is based on the fact that when enough of the population is immune to a virus, it is unable to spread. If a person becomes infected, but the vast majority of people around them are not susceptible to the infection, the virus will not be able to pass to another organism and will disappear in the patient, either killing it or being destroyed by its immune system.
The percentage of the population needed to achieve herd immunity depends on the infective capacity of the virus. And in SARS-CoV-2 this grew until reaching the delta variant, the most contagious to date. A report from the US Centers for Disease Control, to which the The Washington Post had access, indicates that each person can infect another nine, three to four times more than initially calculated, which makes it as contagious as chickenpox. And, in parallel with this greater transmission capacity, the estimates, which are always approximate, of the percentage of the vaccinated population needed to achieve herd immunity increase. If at the beginning there was talk of 70%, all the experts consulted consider that this limit has been exceeded and those who specify a figure increase it to around 90% or more.
For example, epidemiologist Javier del Águila says that the idea of herd immunity “does not seem very realistic in the current context”. “Many epidemiologists around the world have been dealing with the subject for a few months. It comes from more classic diseases such as chickenpox, measles, smallpox. covid-19 is very different; since it is a respiratory virus with such high transmissibility, several problems add up: coverage rates close to 95% would be needed. This is something very difficult, even in countries like Spain, where resistance to vaccination is very low”. This is associated, he adds, with the fact that variants such as delta trigger the curve when they encounter a group of susceptible people. “And when there are many infected people, as the vaccines are not perfect, it ends up also reaching those who have already received the two doses”, he points out.
José Jiménez, a researcher at the Department of Infectious Diseases at King’s College London, goes further and believes that it would be better not to establish percentages of group immunity, a goal that, he says, it is not known if it can be achieved and which, in any case, see how far away. “These are very theoretical estimates and can vary greatly depending on the effectiveness of vaccines and the emergence of new variants. The best message we can get across is to vaccinate as much as possible without setting any percentage as a goal,” he says. Vaccination will ensure that the vast majority of cases are mild or asymptomatic; also for the next Pandemic waves to be much less massive and for the coronavirus to stop being the social problem it has been until now. But probably not, at the moment, to stop the spread altogether.
A similar argument is used by Ignacio López Goñi, professor of microbiology at the University of Navarra: “Instead of obsessing about numbers, about group immunity, it might be more realistic to set the goal of reducing sanitary collapse. If this doesn’t happen, we could all go back to closer to normal. We are not going to eradicate the virus, we will probably have to live with it. For this, it is necessary to vaccinate the most vulnerable. But the virus will move where we leave it, now especially in unvaccinated ones.”
The most likely, in the opinion of Miguel Hernán, professor of epidemiology at Harvard University, is that the coronavirus will become endemic, as it happens with others. These types of pathogens are the ones that cause colds. “Possibly, in their time they were a pandemic and today there is no epidemiological surveillance of them because it is not necessary”, he says. The trend of covid-19 will be this if there are no mutations that make the virus escape the protection that vaccines provide against the most serious forms, as more and more people have some type of antibody, either because they have been vaccinated or because they have if infected. This is, at least, what this specialist considers most plausible, who points out that problems will come to people who for some reason do not generate defenses.
This is in addition to the fact that approved vaccines, although they are very good at preventing the most serious variants of the disease, do not completely prevent the spread. At present, there is no consensus on their ability to protect against delta variant infection. This same report points out that, although infections among vaccinated individuals continue to be infrequent, when they do occur they have the same transmission capacity as an unvaccinated person.
With all this, it would even be doubtful to achieve herd immunity by 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 the vaccine is 4%. To these must be added another portion of the population that cannot be vaccinated due to health problems, another that the system does not reach, another that, without being totally against it, does not bother to take it or prefers not to go. . Anyway, surpassing 80% without mandatory vaccination, something that is not being considered for the time being, will be really complicated.
The ethical debate about vaccinating children
To reach percentages of 95%, as described by Del Águila, it would be essential to vaccinate children as well. 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 disease severity decreases as well. Although the chances of a serious side effect from the vaccine are remote, in children under the age of 12 they are likely to be greater than those they get when they become infected. Countries such as the United Kingdom have already ruled out vaccinating teenagers, for whom there is an approved vaccine, for the same reason.
Federico de Montalvo, president of the Spanish Bioethics Committee, explains that with vaccination, individual and collective protection is sought. “Would it be ethical for children to take a risk to protect society while there are adults who don’t get the vaccine because they don’t want to?” he asks. De Montalvo believes that, when that time comes, the debate on mandatory vaccination of adults, which has not been on the agenda so far, should be resumed.
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Another derivative is that even if Spain, with its good rate of vaccination, achieves supposed herd immunity, the rest of the world will take a long time to do so. Other Western countries are facing serious obstacles to progress, as is the case of the United States, which is seeking all kinds of incentives for the population to get immunized. Even Israel, which started as world leader, has been stagnant for weeks at around 60% of the population with both doses, a number that Spain will reach in a few days. But these are problems of first world countries. For developing countries, where doses are barely enough and with very fragile health systems, group immunity is a chimera.
In Del Águila’s opinion, the biggest concern should be to provide vaccines to these countries instead of administering a third dose to the rich, as some countries are already doing, such as Israel itself. “As the virus circulates around the world, it will have more capacity to mutate and, the more this happens, the greater the chances of escaping from vaccines”, he says. This is the great fear of public health experts. As long as vaccination continues to prevent hospitalizations and deaths as it has done until now, a large number of vaccinees will keep the disease under control, even without group protection. But if a variant manages to break through that barrier, strong measures will again be needed to prevent sanitary systems from collapsing again.
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