A new study published in June in the journal of the Tropical Medicine Institute of São Paulo (IMT), from the Faculdade de Medicina da USP (FMUSP), shows that levels of neutralizing antibodies against the Ômicron variant of the coronavirus, after adopting two doses of the CoronaVac vaccine and a Pfizer booster dose, decrease by about 25% when compared in the mean periods of 62.7 days and 151.5 days after the booster, respectively.

During the first period of testing with study participants, antibodies against Ômicron showed 85.3% positivity, while in the following analysis, this rate dropped to 60.5%. Both measurements were performed in vitro.

“Vaccination is essential because it induces neutralizing antibodies, but unfortunately these antibodies do not have a very long half-life, and it’s necessary that we are continuously strengthening our immune system”, says Prof. Maria Cassia Mendes Corrêa, coordinator of the study and professor at the Department of Infectious and Parasitic Diseases at FMUSP.

There is still limited knowledge about the duration of individual protection after a two-dose vaccination schedule with an inactivated vaccine (such as CoronaVac) followed by a booster dose of the mRNA type, as currently recommended by the Ministry of Health of Brazil.

Although the researchers conclude that this type of vaccination provides limited protection against the Ômicron variant as it declines significantly over five months, the findings also suggest that a second booster dose is beneficial and should be recommended.

“Our results reinforce the rationale of the Ministry of Health and other guidelines, which recommend vaccine boosters to induce the formation of neutralizing antibodies over time”, says Prof. Maria Cássia. The researcher also points out that, in addition to the immune system, other protective features such as social distancing and the use of masks continue to be important in order to contain the spread of the coronavirus.

Antibodies generated by old strains are not effective

In a more recent study published in the journal Clinics, researchers from the IMT evaluated whether the immune responses induced by infection with the dominant strain at the beginning of the pandemic or by the Gamma variant were effective against Ômicron.

Only 1.7% of samples collected from individuals after infection with the ancestral strain tested positive for neutralizing antibodies against Ômicron. In samples collected after infection with the Gamma variant, this rate was 4.1%. The scientists' conclusion is that antibodies generated after mild or moderate infection with these two strains do not protect against the more recent variant.