Protection against Covid verges on 100% with third dose of vaccine, study says

A study carried out with 1,310 employees at the Hospital das Clínicas in São Paulo showed that after the third dose of the immunizing agent against Covid-19, the production of antibodies rises to 99.7%, very close to the total.

The work had the support of Instituto Todos pela Saúde, from Itaú.

To expand protection against the omicron variant, on Saturday (18), the Minister of Health, Marcelo Queiroga, announced the reduction of the interval for applying the booster dose of the vaccine against Covid-19 from five to four months.

Research participants had been in follow-up since the beginning of the pandemic and received the first two doses of Coronavac and the booster of Pfizer.

Antibody dosage is one of the ways to measure the protection of a vaccine.

For Silvia Figueiredo Costa, an infectious disease specialist at Hospital das Clínicas and responsible for the study, probably, if the first two doses had been another immunizing agent, the response would be similar, which highlights the importance of boosting. However, it is not possible to confirm this hypothesis at the moment, as studies on the booster dose have started to come out recently.

“What makes us calmer, as part of the Brazilian population, Chile and other countries received the first and second doses of Coronavac, after boosting it with another manufacturer’s vaccine, there was this very high antibody production score”, evaluates Costa.

Reinforcement does not prevent mild forms of the disease, but protects from hospitalization. “We didn’t have any cases [no Hospital das Clínicas] with the third dose that he has been hospitalized,” he says.

For the analysis, the antibodies were measured four times and the collected serum samples were submitted to the IgG class antibody test (Immunoglobulin G) by the chemiluminescence method.

“It’s a state-of-the-art test, the same method as last year, but it’s now evaluating three parts of protein S [Spike] and it became more interesting to work on the vaccine response”, he explains.

In the same cohort, neutralizing antibodies — those capable of blocking the entry of the virus into cells — were analyzed, but the results will be known in 2022.

Before the availability of the vaccines, the antibody seroconversion rate in the studied group was 15.1% and was exclusively related to the person’s contact with the virus.

After the application of the first dose, in February, the rate rose to 28.9%. With the second dose, in April, the percentage reached 89.5% and this month, after the reinforcement of Pfizer, it reached 99.7%. “We wait two weeks after the third dose to allow time for antibody production.”

Of the group, only four people did not have antibodies against Covid-19.

Those who contracted the disease in the last two months were excluded from the survey. “We excluded so as not to have any bias in the serology result, because people who had Covid will have positive serology. So, one way of differentiating positive serology from the vaccine response from positive serology for those who had recent Covid is to exclude who caught the disease in the last two months, otherwise you cannot differentiate. This high positivity is because of the vaccine”, explains the doctor.

For the specialist, the study shows that Coronavac stimulated memory immunity. “When the reinforcement was done, the production of antibodies increased significantly, which would be one of the systems to protect the population from the disease, especially the severe form of Covid”, he says.

The research also shows that the second dose of Coronavac had already raised the production of antibodies to a high percentage, but with time the protection diminishes and, therefore, it is important to receive the booster.

In the opinion of the infectologist, it is not yet possible to establish the interval for revaccination. According to her, it will depend on the variants in circulation and the evolution of the pandemic from now on.

“It would be very important to vaccinate [a população] in Africa, which has a low rate of vaccination and in some countries in Asia so that we don’t have new variants circulating. The data with the micron show that the third dose, depending on the vaccine taken previously, has an antibody production of 70%”, says the doctor.

The work also sequenced samples from employees of the study group who tested positive for Covid-19 throughout the research and observed that, between the months of March and July, there was a predominance of infections caused by the gamma variant (P.1): March, 91 %; April, 98.5%; May, 98.9%; June, 100%; and July, 88.5%.

Delta began to be identified also in July among HC employees, when it was 6.5% of the cases, and in the following months it became predominant: August, 79.8%; September, 97.4%; October and November, 100%.

This sequencing did not detect the presence of the omicron.

“Even in Brazil there is a lower demand for the third dose, which worries us a lot. Individuals with some underlying disease and population over 60 years without the third dose may suffer a greater impact on the clinical manifestation of Covid-19 and the need for hospitalization, and we can avoid that,” says the doctor.

The doctor advises that the third dose is important, but it should not be seen as a passport to non-adherence to social distancing and the use of masks, as the population may have the asymptomatic form of Covid-19 and transmit it, even to children . “The pandemic is not over and we have to prevent the emergence of variants.”

The Government of São Paulo announced this Monday (20) the extension of the mandatory use of masks in collective spaces until January 31, 2022. The objective is to increase protection against Covid-19, from influenza, which causes the flu, and from other respiratory viruses.