A group of experts from the US drug agency, the FDA, and the World Health Organization (WHO) has just concluded that there is no solid evidence that a third dose of the vaccine is needed for the general population.
The researchers performed a comprehensive review of clinical trials on vaccine efficacy and observational analyzes of their performance in different countries. The work, published this Monday in the medical journal The Lancet, shows that the efficacy of these drugs remains very high against all variants of the coronavirus, including delta. “The body of evidence accumulated so far appears to show that there is no need for a third dose in the general population, as its protection against severe disease is still high,” the authors write.
Observational studies show that injections are on average 95% effective against severe covid-19 and 80% against infection, regardless of severity. And these data, remind the experts, are valid for all known variants. It is true, they add, that the effectiveness of vaccines is greater against severe covid-19 than against milder conditions.
“The studies published so far do not provide credible evidence that protection against severe covid-19, which is the main objective of vaccines, is decreasing”, highlight the authors of the analysis, led by WHO’s Head of Research, Ana María Henao. “Given the limited number of vaccines available, more lives will be saved if we give them to those who are most at risk of getting covid-19, which are those who have yet to receive a dose.”
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“While a third dose may provide some benefit, the advantages of immunizing the unvaccinated are much greater,” the study authors write. Following this advice “may accelerate the end of the pandemic, as it will prevent the evolution of more variants of the coronavirus,” they add.
The researchers recall an argument made by most immunity experts. The fact that some studies show that there is a drop in the level of antibodies — proteins in the immune system capable of blocking the entry of SARS-CoV-2 into cells — does not mean that the effectiveness of vaccines is decreasing. The fact that there is reduced protection of vaccines against moderate or asymptomatic covid-19 does not mean that there will be a failure in protection against severe covid-19. The possible explanation is that not only the antibodies act to protect against the severe form of the disease, but also the memory cells of the immune system, the study notes.
Those responsible for the article say it is possible that there are groups that need this third dose, especially the immunocompromised. They also reflect on the future. If new SARS-CoV-2 variants emerge, they will likely be descendants of existing ones. None of them have been shown to completely bypass the immune system. In his opinion, the most reasonable thing would be that, if a third dose is approved, it would be a new version of the vaccine specially designed against dominant versions of the pathogen, such as delta. It is the same, they emphasize, that is done with the flu vaccine every year. Both Pfizer and Moderna are conducting clinical trials of vaccines against the delta variant.
“We have to be very careful with the issue of the third dose”, warns Marcos López, president of the Spanish Society of Immunology. “One of the things that this work highlights is that more studies are needed on the need for the third dose. It is an issue that still needs to be seriously analyzed before being approved, as we have no evidence that the vaccine is failing”, he highlights.
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