Medscape Editorial Team
The overall efficacy of CoronaVac , the SARS-CoV-2 vaccine developed by the Chinese pharmaceutical company Sinovac Biotech , is 50.38% in trials conducted by the Butantan Institute in São Paulo, Brazil. This calculation was based on an analysis of data from 9,242 volunteers divided into two groups. One group received the vaccine (4,653 participants) and the other a placebo (4,599 participants).
There were 252 confirmed COVID-19 diagnoses, 85 in the vaccinated group and 167 in the placebo group. The most frequent side effect was pain at the injection site, and 0.3% of participants had some type of allergic reaction, according to the study researchers. The announced efficacy is above the 50% threshold established by the World Health Organization for vaccine approval and also adopted by the Brazilian Health Regulatory Agency (ANVISA).
The final data from the CoronaVac trials were presented on Tuesday, January 12, at a press conference at the Butantan Institute headquarters, due to intense pressure from the scientific community and the media. [ 1 ] On Thursday, January 7, the São Paulo state government and the Butantan Institute released preliminary data after several postponements. According to the newspaper Folha de S. Paulo , information on overall efficacy was already included in the data submitted to ANVISA for evaluation.
The justification given by the director of the Butantan Institute , Dr. Dimas Covas, for withholding preliminary data was that it would be disrespectful to publicly announce the final data before submitting it to ANVISA (the Brazilian Health Regulatory Agency). He also mentioned contractual issues with their Chinese partner, Sinovac Biotech . Indeed, it is understandable that there would be internal matters involved in an international study conducted during the largest and most serious global health crisis of the century.
Part of the presentation was dedicated to explaining the data announced the previous week, which indicated a 78% efficacy rate for this vaccine in preventing mild cases, and a 100% efficacy rate for moderate and severe cases. The Butantan Institute 's own Instagram profile displayed the following message: "100% decrease in severe cases, 100% decrease in moderate cases, 100% decrease in hospital admissions, 78% decrease in visits to health centers, and 78% decrease in mild cases." This format caused considerable confusion.
"The main data point shows the result obtained in a specific part of the study," stated Dr. Ricardo Palácios, medical director of clinical research at the Butantan Institute , to explain the difference between the overall efficacy of 50.38% presented on Tuesday the 12th and the 78% announced on Thursday the 7th. In this context, it is easier to understand the meaning of the figures that circulated during the week.
Dr. Marcelo Gomes, Ph.D., a public health researcher at the Oswaldo Cruz Foundation (Fiocruz) and coordinator of the Infogripe system, a surveillance service that constantly analyzes large volumes of data, told Medscape in Portuguese that “22% of vaccinated individuals presented mild symptoms. This explains a 78% decrease in the number of mild cases requiring assistance at a health center. No vaccinated person who contracted the virus required hospitalization, representing a 100% decrease in potential hospital admissions.” In numerical terms, 50 out of every 100 vaccinated individuals contracted COVID-19, and only 22% of these 50 people presented mild symptoms and required any type of assistance at a health center.
"This does not mean that it prevented 100% of moderate and severe cases because there is still no statistically significant data to support this claim," Dr. Gomes pointed out.
Dr. Ricardo Palácios, who led the study design, addressed this issue at the press conference. He stated that it remains necessary to confirm the trend of 100% prevention of severe cases. The number of severe cases in the control group is very low (seven severe cases requiring hospitalization) and lacks statistical significance. Therefore, it is necessary to wait for new cases to appear.
Dr. Marcelo Gomez pointed out that "in this study protocol, the estimated timeframe for severe cases is one year, with the goal of having a high or sufficient number of severe cases in both study groups to allow for an assessment of the study's effectiveness." Specialists consulted by Medscape in Portuguese stated that, based on the available evidence, a considerable decrease in the number of hospitalizations for severe cases is expected.
Encouraging results
Although CoronaVac 's protection rate is lower than that of other vaccines, such as those using messenger RNA technology with overall efficacy exceeding 90% , the results were welcomed by the scientific and medical communities. The expectation is that the vaccine will have the optimal capacity to control the pandemic.
Regarding the 50.38% efficacy rate, Dr. Dimas Cova noted that CoronaVac faced a "very difficult" challenge in Brazil because it was tested on healthcare professionals. "In these professionals with very high exposure, the infection rate exceeded 20%, the highest among all at-risk populations. This is the most demanding test a vaccine can face."
According to Dr. Ricardo Palácios, the inclusive definition of COVID-19 cases adopted by the study likely influenced the results.
"We broadened the scope to capture even the mildest possible cases. This led to discrepancies and compromises the comparison of this study with others. We knew it was a delicate balance, and we were sacrificing efficacy to obtain a higher number of cases and faster results; nothing will change, except that it will result in a protected human being; I repeat: we have a vaccine," the expert pointed out.
Many details of CoronaVac remain to be investigated. Studies will be conducted to evaluate the vaccine's effectiveness in 1,400 elderly people with underlying health conditions, pregnant women in their third trimester, and children and adolescents aged 3 to 17.
Dr. Marcelo Aurélio Sáfadi, from the Department of Infectious Diseases at the Brazilian Society of Pediatrics and adjunct professor at Santa Casa de Misericórdia in São Paulo, Brazil, invited to the press conference, noted: “There are still gaps in our research, because we don't yet know what the long-term protection will be like or the impact on transmission, but I want to emphasize that no vaccine has solid data that allows us to anticipate its impact on transmission.” Researchers are also studying the time interval between doses. Dr. Dimas Covas pointed out that “studies show good results with CoronaVac even with an interval of up to 4 weeks, but a single dose is not enough.”
Dr. Natália Pasternak, Ph.D., a microbiologist at the Instituto Questão de Ciência and a leading professional in the field of science communication who attended the press conference, stated that this is probably not the vaccine of our dreams, but it is the one that is closest to our reality.
He added that “a vaccine with 50% efficacy is acceptable. It’s probably not the best in the world, but nobody said it was going to solve all our problems or that it would be perfect, with 95% efficacy, easy to distribute, cheap, and in single doses. That vaccine doesn’t exist. CoronaVac does exist, it reduces the risk of contracting COVID-19 by 50%, and it has very good potential to decrease the risk of severe illness, which starts counting from the moment a patient needs medical assistance.”
“I want this vaccine, and I want my parents to receive it. It is feasible and suitable for Brazil, compatible with local production, storage, and cold chain. A vaccine is only as good as the country's vaccination coverage,” the expert emphasized, adding that a broad public awareness campaign is needed to convince the population to get vaccinated, demonstrating that it is safe and effective, in order to achieve the necessary vaccination coverage. Otherwise, the expected effect of herd immunity will not be achieved.
Dr. Rosana Richtmann, an infectious disease specialist, president of the Comitê de Imunização da Sociedade Brasileira de Imunizações and consultant to the Ministry of Health , thanked the organizers for the presentation of the final data and warned that vigilance should not be relaxed even after receiving the vaccine.
"At first, we won't be safe yet, and we can't let our guard down. No vaccine has yet demonstrated a decrease in contagiousness, but science will bring us these answers. I believe in vaccines, but having a vaccine without vaccination is pointless. The best vaccine is the one that's available."
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