Vaccines against Covid-19 trigger different periods of immunity and those that use messenger RNA (mRNA) technology are the ones that protect for the longest, according to a study conducted by scientists from Yale and North Carolina universities published last Friday (15) in the scientific journal PNAS.
The research sought to estimate the likelihood of future infections by Sars-Cov-2 among those who have already contracted the virus and in people immunized with vaccines from Pfizer, Moderna, AstraZeneca or Janssen. To this end, data from ten previous studies were compiled that evaluated the response induced by immunizers and by exposure to the virus and compared the level of anti-S IgG antibody over time.
Both naturally-infected and vaccine-induced antibodies gradually decrease, increasing the likelihood of infections, but the rates are different in each case.
According to the authors, while the median time to infection after vaccination with Moderna and Pfizer doses is 29.6 months, that observed with AstraZeneca and Janssen vaccines is 22.4 and 20.5 months, respectively. . In the case of natural infections, the median is 21.5 months.
From these numbers, the researchers suggest when to apply new doses. To allow no more than a 5% chance of future infection as a consequence of declining immunity, they indicate that persons fully vaccinated with Moderna and Pfizer receive the first post-vaccination booster within one year and that those fully immunized with AstraZeneca and Janssen take the booster dose at five months and 4.5 months, respectively.
For infectious disease specialist Alexandre Naime Barbosa, professor at Unesp and vice president of the Brazilian Society of Infectious Diseases, the article provides important information. The first is that the peak and duration of immunity with mRNA vaccines are much longer than those with natural infection, contradicting the idea that it is better to be exposed to the virus to acquire antibodies. The study also confirms that mRNA vaccines offer greater protection than viral vector vaccines.
“This, however, does not make viral vector immunizers unfeasible because they confer antibodies and reduce the chances of hospitalization and death”, emphasizes the professor. He also highlights that, with the advancement of the ômicron variant, the response by natural infection may now be shorter than indicated in the article, increasing the distance between the level of protection for contracting the disease and that offered by vaccines.
On the other hand, both Barbosa and the immunologist Cristina Bonorino, professor at UFCSPA (Federal University of Health Sciences of Porto Alegre) and member of the scientific committee of the Brazilian Society of Immunology, warn that the study did not consider the action of T cells, essential in antiviral responses.
“Antibodies prevent higher viral loads, but what really protects are T cells. It is common for research to only analyze antibodies because it is easier to measure them, but that is not the whole story, we cannot rely only on them”, says Cristina Bonorino.
She comments that the ideal would be to cross data on antibodies, T cells, number of cases and variants and that there is still no mathematical model for this analysis. “The problem in Brazil is that we do not have clear data from the Ministry of Health”, criticizes the professor.
It was because they couldn’t find enough data that the researchers didn’t include Coronavac in their analysis, says Jeffrey Townsend, a professor at the Yale School of Public Health and the paper’s lead author. He does mention, however, that the relative durability found in the research seems to match the relative efficacies reported in the clinical trials, so it would be possible to speculate that Coronavac’s protection time would be below that of AstraZeneca and Janssen.
“We are now working on the benefits of different booster schedules. The results have not yet passed peer review, but it stands to reason that boosters at substantially longer intervals than we have shown for the first post-vaccination booster are likely to be ineffective in suppressing infection. “, says Townsend.
Both the researcher and Bonorino believe that immunization schemes such as the one seen in Brazil, with vaccination with Coronavac and booster with Pfizer, for example, promote protection. The questions are which combination is best and when to take it.
The doctor also notes that, more than administering boosters, Brazil needs to prioritize the vaccination of its children at this time. “They were left without school, without vaccine and returned to school without immunization, subject to complications and long Covid. Children are the most neglected group in the pandemic”.
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