What does science say about giving one or two doses of vaccine to those who have recovered from Covid-19

What does science say about giving one or two doses of vaccine to those who have recovered from Covid-19

The theme is at the center of the debate and there is no univocal position, also thanks to the diffusion of the delta variant. Here is what emerges from the studies and guidelines of different countries

Photo: Facebook Regione Lombardia While we continue to discuss how appropriate it is to proceed with a booster dose (the third dose) for people who have already completed their cycle vaccine and the first data arrive, others continue to arrive trying to clarify the appropriateness and methods of vaccination for those who have already had Covid-19. If in fact, at least initially, the general idea was that after having had the disease a single dose of vaccine could be enough, lately, also thanks to the arrival of the contagious delta variant for months now, the question appears less clear and the indications are not the same in different countries.

Certainly this is a complex question and involves several aspects: the duration, efficacy and intensity of the immune response, natural and induced by vaccines, and therefore the risk of infection and infection, even in light of viral variants. Apparently conflicting evidence also contributes to the complexity of the subject in question. Let's try to line up what we know so far, aware that knowledge when it comes to Covid-19 is still in the making and uncertain.

The indications of the institutions

At the moment, the latest update available on the website of the Italian Medicines Agency (Aifa) reiterates that for those who have had the infection it is possible to consider the hypothesis of a single dose of vaccine, preferably within 6 months and in any case no later than 12. The indications apply, with more force in reality, even in the event that the infection arises after the first dose of vaccine: "It is not recommended to give these people the second vaccine dose".

On the contrary, the American Centers for Disease Control and Prevention (CDC) recommend vaccination regardless of whether or not you have had Covid-19, by virtue of the fact that it is not known how long it lasts and how much it protects immunity natural acquired with infection. The World Health Organization (WHO) has the same opinion. Both do not specify the number of doses, but the clear reference to the greater protection offered by a complete vaccination compared to infection by the CDC alone suggests that the recommendations concern the complete vaccination cycle. All the more so with the circulation of the delta variant thus sustained.

Natural and "mixed" immunity

The doubt about the duration of natural immunity on the one hand and data available for the different vaccines that show greater efficacy for the complete vaccination cycle on the other, would lead to think that two doses of vaccine are the best choice to protect against Covid-19.

In the idea that the immunity developed and acquired with vaccines is better than the natural one. Certainly safer, not only because it avoids the risks associated with infection and the onset of post-infection complications (long-Covid) but also because it generally induces a less variable immune response, as recalled for example by the British Society for Immunology, less strong for example in those who have had Covid-19 in a mild form.

I’ve already had # COVID19, why do I need a #vaccine? 🤔

Our joint infographic with @UKCICstudy explores the difference between #immunity gained through natural infection vs #vaccination & answers this important question 🦠💉

Download it here 👉 https://t.co / 0RZNThPWlH pic.twitter.com/AL0kWTvED6

- British Society for Immunology (@britsocimm) August 4, 2021

But not all evidence points in the same direction, and alongside studies that affirm the superiority - in terms of protection from re-infections - of the double dose of vaccines over natural infection alone, others emerge that go in the opposite direction. Apparently, and in any case to be placed in context.

A controversial study

This is the case, for example, of a study from Israel (still in pre-print) which analyzed the incidence of coronavirus infections and complications in non-people vaccinated, vaccinated with two doses and with a single dose (of the Pfizer / BioNTech product), with extended follow-up during the spread of the highly contagious delta variant. And the results comparing so-called breakthrough infections (in the vaccinated population) or re-infections seem to show that natural immunity is greater and more protective than that offered by the vaccine. And how a dose of vaccine can further enhance the defense against infections. In particular, the paper states, "natural immunity confers a longer and stronger protection against infection, symptomatic illness and hospitalization caused by the Delta variant of Sars-CoV-2, compared to immunity induced by two doses of the BNT162b2 vaccine ".

But be careful (a lot), several experts interviewed by Science on the subject thundered: net of the limits of the study (retrospective, conducted for a single vaccine, and with cases analyzed in small numbers), the results of which will be deepened and validated by other investigations, vaccines remain the safest way to fight the virus. What appears interesting, and has also been seen elsewhere, is the effectiveness of the protection offered by a dose of vaccine in people who have already had the disease.

Several studies in fact have already suggested that complete vaccination in people already infected with the virus induces a greater immune response, both in terms of antibody levels and in terms of quantity and diversity of cellular responses. Among others, he remembered a piece that appeared in The Conversation a few weeks ago by Sunit K. Singh of Banaras Hindu University, suggesting that, despite the evidence in favor of "mixed" immunity, when in doubt it was preferable for the recovered to proceed with full vaccination. Even considering, as explained by Thomas McDade, the researcher in charge of a study just published in the Scientific Report, that if the first exposure to the virus, for example through infection, was weak, the antibody response induced by a single dose will not be much different than those who have not had Covid-19.

No to antibody tests before the vaccine

Certainly the general recommendations are those for vaccination, even for the recovered because the only stimulation given by the infection is too variable, especially for people who have had a mild form and because to date it is not yet clear what the quantity of neutralizing antibodies (or type of immune response as a whole) can offer protection. Soumya Swaminathan also reiterated this against the WHO, stressing, precisely for this reason, that antibody tests to understand whether or not you are immune are not recommended at the moment.

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