Data from the first analysis of Projeto S, a study on the effectiveness of the CoronaVac vaccine that Butantan conducted in the São Paulo municipality of Serrana, show a direct vaccine effectiveness of 80.5% of the immunizing agent against symptomatic cases of Covid-19, and 95% against hospitalizations and 94.9% against deaths. The survey also indicates that with 52% of the population vaccinated with the two doses, the indirect effects begin to manifest themselves, protecting even those who did not take the immunizing agent. Furthermore, at the time of the study (between February and May 2021), most cases were caused by the gamma (P.1, Amazon) variant of SARS-CoV-2, which again shows that CoronaVac is effective against this strain – which predominated in Brazil throughout the first half of this year.
The results of the research, conducted by scientists from Instituto Butantan, the Serrana State Hospital, the University of São Paulo at the Ribeirão Preto School of Medicine and the Serrana Municipal Health Secretariat, are described in the article Projeto S: a stepped-wedge randomized trial to assess CoronaVac effectiveness in Serrana, Brazil, released this week on the SSRN preprint platform.
Projeto S is the first controlled clinical study to demonstrate the effectiveness of an immunizing agent in the real world and its indirect effect on the unvaccinated population, carried out during a pandemic and without a control group. The research pioneers in demonstrating that an inactivated virus vaccine used as a primary public health emergency measure can change the course of an epidemic. In addition, Projeto S – a stepped-wedge randomized clinical trial – shows that vaccines are the mainstay of containing the number of cases and viral transmission and of controlling the devastating effects of Covid-19.
Projeto S volunteers were vaccinated with CoronaVac, a vaccine from Butantan and Chinese pharmaceutical company Sinovac, in a two-dose schedule with a four-week interval. In total, 81.3% of the adult population and 60.9% of the urban population of Serrana completed the vaccination schedule, equivalent to about 27 thousand people. Of this number, 16% were elderly over 60 years old.
Overall vaccine efficacy was estimated by comparing the incidence of pre- and post-vaccination cases for the entire urban population. Direct efficacy was assessed in the relationship between the incidence of cases in fully vaccinated and unvaccinated individuals. Among those fully vaccinated, the direct effectiveness of the vaccine was 80.5% (95% CI, 75.1 to 84.7) in preventing symptomatic cases; 95% (95% CI, 86.9 to 98.1) against hospitalizations; and 94.9% (95% CI, 76.4 to 98.9) to prevent deaths. During the study period, 1,447 cases of Covid-19 were reported in Serrana; of these, 361 (24.9%) were sequenced, indicating a gamma variant incidence of 92% to 100% in the city.
When analyzing the impact of vaccination on the elderly population (over 60 years), the direct effectiveness of CoronaVac remains very high: 86.4% (95% CI, 74.5 to 93) in the prevention of symptomatic cases, 96.9% (95% CI, 86.1 to 99.3) against hospitalizations and 96.9% (95% CI, 73.9 to 99.6) to prevent deaths.
The researchers point out that it is not possible to set a minimum level of immunization to control Covid-19 in an area, but that the results of Projeto S demonstrate that when 52% of the population had received both doses of vaccine, the indirect protective effects began to be seen in the other groups that had not yet completed immunization – suggesting an immunization indicator to control the gamma variant of SARS-CoV-2. Furthermore, during the study period, the number of infections among children was also reduced, indicating the indirect effect of CoronaVac in this population, which was not immunized. However, they report that the direct effects of vaccination were greater than the indirect ones, reinforcing the need to vaccinate as many people as possible and quickly.