Covid risk rises 6 months after Coronavac, even in young people, study shows

The two doses of Coronavac vaccine can protect against hospitalization and death from Covid, but six months after immunization the risk of contracting the disease increases considerably in younger individuals, aged between 18 and 39 years.

The chance of having symptomatic Covid is even greater among healthcare professionals.

In both cases, the risk is almost three times greater for those aged 18 to 39 years, regardless of whether they are health workers or not.

This risk starts to appear as early as three months after the primary vaccination with the immunizing agent, that is, after receiving the second dose.

These are the main results of a study conducted by Fiocruz researchers in partnership with several national and international research institutions, such as the Hospital das Clínicas of the USP Medical School (HC/FMUSP), the Federal University of Mato Grosso do Sul (UFMS) and the University of Florida. The article in pre-print format, that is, without peer review, was released last Friday (24) on the medRxiv platform.

The research analyzed 37,929 paired data from people vaccinated and who underwent RT-PCR exam in the state of São Paulo from January 17th (when the vaccination started) to September 30th. For each pair, one individual had a positive RT-PCR test within ten days of symptom onset and the other had a negative test (control).

With pairs of positive cases and controls formed, the researchers sought to understand the odds ratio of contracting symptomatic Covid over time in different age groups (18 to 39 years, 40 to 64, 65 at 79 and 80 years of age or more) and at risk of profession (health care professionals or not).

What was observed was that 40 days after the second dose, the risk for individuals aged 18 to 39 years or for healthcare workers increased with time, and especially after 182 days.

Initially, the risk was 1.45 for younger individuals, rising to 3.87 five to six months after the second dose (2.7-fold increase).

In the case of health care professionals, the vast majority of them trained by people under 65 years of age, the risk of contracting the disease for older workers was already reduced and, therefore, was not significant. In other age groups, the risk increased from 1.49, in workers aged between 18 and 39 years, to 4.48 six months after the second dose (or 3 times more); and from 1.26, in the case of those aged 40 to 64 years, to 3.53 after six months of the second dose (2.8 times higher).

For Fiocruz researcher Julio Croda, who coordinated the study, the research indicates that even young people who are not health workers have a high risk of illness at Covid over time. There are two hypotheses raised for this, explains the doctor. The first would be greater exposure of this group to events that cause crowding, such as parties in bars and nightclubs. The greater the exposure, the greater the risk of contracting the disease, even in vaccinated individuals.

“The second hypothesis for this increased risk in adults may be because the effectiveness of Coronavac, and not just Coronavac, as we showed in our first study, but of all vaccines, is lower with increasing age. Therefore, in the elderly, for now have less protection offered by vaccines, the observed risk of contracting the disease over time will not change that much, unlike younger individuals where this increase is more accentuated,” he explains.

The study is the first to provide scientific support for the need for a booster dose of the vaccine for all individuals over the age of 18 who have taken both doses of Coronavac. Recently, the WHO recommended an extra dose for all individuals over 60 years of age who received the immunizing agent.

“The strength of infection, as we call it, is conditioned by the lower effectiveness of vaccines for symptomatic disease, observed in several studies, including clinical trials [onde a Coronavac teve eficácia de 50,38%], so in the context of a new variant such as omicron, the booster dose becomes even more necessary,” he says.

In terms of the effectiveness of the vaccine, Croda explains that it was not possible to calculate the level of protection of the immunizing agent in different groups over time, because for this it would be necessary to include unvaccinated individuals.

“To calculate protection, it would be necessary to have people vaccinated with the two doses who are neither elderly nor immunosuppressed or health workers, six months after the second dose. But in Brazil the vaccination campaign with the booster dose has already started , first in these higher-risk groups, but now also for the entire population,” he says.

Despite having limitations, such as not being able to assess which variants caused the infections during the period observed, the study is important because it provides data on how Coronavac prevents hospitalization and death over time, but not symptomatic infection.

The new data with more than 39,000 patients indicate a need to apply a third dose of the immunizing agent or any other after four months of primary vaccination to reduce the spread of the disease, says the researcher.

“Coronavac is the most used vaccine in the world, with more than 2 billion doses applied. And despite the anticipation of the booster dose in Brazil [de 5 para 4 meses] having already been implemented as a public policy, even without sufficient evidence at the time, other countries have not yet adopted this measure, so it is a fact that can serve as a basis for this action in these places”, he says.