A real goal or a chimera? When it comes to herd immunity, under the fourth wave of the Covid pandemic, the debate among scientists is heated. Can it ever be reached? And what does it take to get it? What role do vaccines play in this interlocking game that has so far failed?
“In May 2020, we and other scientists predicted that many regions of the world may never reach the herd immunity threshold for Covid-19, which is the point where enough people are immune to infections and transmission begins to occur. slow down – they explained Erin Mordecai e Mallory Harris, biologists at Stanford University, in an analysis published in the British newspaper The Guardian -. This remains true today, even as vaccines have become accessible in wealthy nations and many people have developed immunity through vaccinations, boosters and previous infections. “Furthermore, according to the two researchers,” the herd immunity threshold has commonly been misunderstood as a universal goal to be achieved at the beginning of the pandemic “.” But the threshold has always been changing – they add -. It depends on how transmissible the pathogen is and on the behavioral and immunological characteristics of the population in which it is spreading: how much they mix and how easily they become infected “.
Where does immunity arise
First of all, it is necessary to clarify what is meant by the concept of herd immunity. It is a phenomenon of indirect protection from an infectious disease that occurs in the population when the antibody coverage (number of vaccinated, or sick and then cured) exceeds a critical threshold, such that even those who are not vaccinated can be reasonably considered “safe. “.
The mechanism underlying this concept is reminiscent of that of a flock: the sheep move in groups, huddled together and protecting each other; as a metaphor we can compare the central sheep to non-immunized (vulnerable) individuals, and the more peripheral ones to individuals who instead have antibodies high enough to be considered protected: if a large number of individuals are immune to a pathogen, its overall circulation is limited .
With Omicron 90% immune target
But what variables affect the mechanism that should lead to herd immunity? He explains it Giovanni Maga, director of Cnr Pavia: “When we talk about population immunity, the percentage depends on how contagious the virus is in circulation, how it is transmitted and the structure of the population itself. In this regard, the Omicron variant would take at least a year to obtain the immunization of over 90% of people. By this I mean that individuals, because they are cured or vaccinated, are able to prevent an infectious agent from infecting them or, if they are infected, from accusing serious consequences – continues Maga -. the mechanism is triggered: if a significant portion of the population acquired the ability to resist infection, the infection would be limited to a few sporadic cases, which would lead to an endemic virus, such as the flu: the infected would be few and in any case they would not represent a if anything, it would be a characteristic of the population “.
The obstacles in the way
But Covid changes its face frequently, the variants are many and above all always new. A reality that stands in the way, as an obstacle, to the possibility of achieving the desired immunity. “The fact is that the Covid virus and the Omicron variant, the most powerful of the latest arrivals, are new – recalls Maga -. And among other things, variants are constantly emerging, new viral strains that may have different ability to infect. For example. , the immunity acquired with the first Covid version may not be sufficient to protect against new strains. And this is a factor that prevents the immunity of the population “.
“A second hindering factor is precisely the population – adds the virologist -. In the absence of an external flow of people, at a certain point the population will become resistant to the virus. But if the situation is” open “, with the continuous encounter of individuals from one country to another, as is the case today, it is difficult to achieve immunity. Trivially, it has already happened with herpes simplex type 1, which has infected 70% of the world population, but has been present since the appearance of man it has adapted to such an extent that it does not lead to serious effects. “
“Better the middle way”
So could you choose to aim for a reasonable middle ground? According to the virologist, yes. “Given that this ideal population immunity is difficult to achieve, we can arrive at an intermediate solution: reduce the impact of the virus, that is to ensure that even if it infects many people, it sends few to the hospital – he speculates -. This goal? We need to create immunity, then use vaccines. Because this use has the purpose of guaranteeing coverage to a very high share of the population, making the infection manageable. In the end, in this way, there will only be an outbreak , but no longer a large number of people in need of hospital treatment “.
At this point we return to the Omicron variant, which is very contagious, therefore able to strike on the large scale. “If we achieve immunity thanks to vaccinations, the impact could be slight – assures Maga -. If we manage, with the use of new concept vaccines, to induce immunity on the one hand and limit infections on the other, even if the virus infects it will not be able to induce serious illness. We could imagine, at that point, to have so many people who become infected, recover and acquire immunity “.
In search of vaccines that anticipate variants
Vaccines capable of anticipating the appearance of new variants could provide a good gear in the aim of achieving this result. “These are vaccines that identify the anti-signs, that is the parts that do not change – concludes Maga -. Now they are in the development phase. We have studies underway, but not yet ready for clinical trials. to clinical trials on vaccines tailored to Omicron (pan-coronavirus, which not only induce a very broad spectrum of neutralizing antibodies, but also a robust cell-mediated response with B and T lymphocytes). Some exploit a mixture of Spike proteins from different Existing coronaviruses, others are studying more stable proteins, in addition to Spike, able to guarantee immunization, combining several. How long will it take? No less than a year “.
Bassetti: “We will reach you in the spring”
The virologist, on the other hand, hypothesizes shorter times Matteo Bassetti, director of the Infectious Diseases Clinic at the San Martino hospital in Genoa. Who explained: “If the rate of infections from Covid continues to be so high (we will soon reach 400 thousand cases a day, according to physicist Giorgio Sestili), in spring we will achieve herd immunity. The example, for the virologist, comes from neighboring Great Britain which, with the widespread use of the Omicron variant, “now has 98% of the general population that is somehow protected from the virus”, he stresses. Adding: “Why? Either she had the vaccination, or she came into contact with the virus, and therefore she is protected at least from the most demanding forms. Let’s say that the British have achieved herd immunity, which we hope to achieve soon too “.
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