Efficacy of an inactivated SARS-CoV-2 vaccine in Chile

We provide estimates of the effectiveness of administering the CoronaVac vaccine in a nationwide mass vaccination campaign for the prevention of laboratory-confirmed Covid-19 and associated hospitalizations, intensive care admissions and deaths. Among fully immunized people, the adjusted vaccine efficacy was 65.9% for Covid-19 and 87.5% for hospitalization, 90.3% for admission to intensive care and 86.3% % for death. Vaccine efficacy results were maintained in both age subgroup analyzes, particularly in people 60 years of age or older, regardless of variation in testing and various factors regarding vaccine introduction to the patient. Chile.

The vaccine efficacy results in our study are similar to estimates that have been reported in Brazil for the prevention of Covid-19 (50.7%; 95% CI, 35.6 to 62.2), including estimates of cases leading to medical treatment (83.7%; 95% CI, 58.0 to 93.7) and composite endpoint estimates of hospitalized, severe or fatal cases (100 %; 95% CI, 56.4 to 100).27 The large confidence intervals for the trial in Brazil reflect the relatively small sample (9,823 participants) and the few cases detected (35 cases leading to medical treatment and 10 severe cases). However, our estimates are lower than the efficacy reported in Turkey (91.3%; 95% CI: 71.3 to 97.3),27 possibly due to the small sample size of this Phase 3 clinical trial (1,322 participants), differences in local transmission dynamics, and the predominance of elderly people among fully or partially immune participants in our study. Overall, our results suggest that the CoronaVac vaccine had high efficacy against serious illness, hospitalizations, and death, findings that underscore the potential of this vaccine to save lives and dramatically reduce demands on the healthcare system. .

Our study has at least three strengths. First, we used a rich health care administrative data set, combining data from an integrated immunization system for the total population and from the FONASA Ministry of Health, which covers about 80% of the population. Chilean. These data include information on laboratory tests, hospitalization, mortality, symptom onset and clinical history to identify risk factors for serious illness. Information on the region of residence also allowed us to control for differences in incidence across the country. We adjusted income and nationality, which correlate with socio-economic status in Chile and are therefore considered social determinants of health. The large population sample allowed us to estimate vaccine efficacy for both one dose and the full two-dose vaccination schedule. It also allowed for a subgroup analysis involving adults 60 years of age or older, a subgroup that has a higher risk of serious illness.3 and which is under-represented in clinical trials. Second, the data was collected during a rapid, high uptake vaccination campaign and during a period with one of the highest community transmission rates of the pandemic, which allowed for a follow-up period. relatively short and a prevention estimate of at least four outcomes: Covid-19 cases and associated hospitalizations, intensive care admissions and death. Finally, Chile has the highest Covid-19 testing rates in Latin America, universal access to health care, and a standardized public reporting system for vital statistics, which has limited the number undetected or undetected cases and deaths.14

Our study has several limitations. First, as an observational study, it is confusing. To account for known confounding factors, we adjusted analyzes for relevant variables that might affect vaccine efficacy, such as age, sex, underlying medical conditions, region of residence, and nationality. . The risk of misclassification bias that would be due to the temporal performance of the SARS-CoV-2 RT-PCR assay is relatively low, as the median time from symptom onset to testing in Chile is approximately 4 days ( 98.1% of the tests were RT-PCR tests). In this period of 4 days, the sensitivity and specificity of the molecular diagnosis of Covid-19 are high.37 However, there may be a risk of selection bias. Systematic differences between vaccinated and unvaccinated groups, such as care-seeking behavior or risk aversion, can affect the likelihood of vaccine exposure and the risk of Covid-19 and associated outcomes.38.39 However, we cannot be sure about the direction of the effect. People may be reluctant to get the vaccine for a variety of reasons, including fear of side effects, lack of trust in government or drug companies, or the belief that they don’t need it, and they may be more or less showers at risk. Vaccinated people can compensate by increasing their risky behavior (Peltzman effect).39 We addressed potential differences in access to healthcare by limiting the analysis to people who had undergone diagnostic tests, and found results consistent with those in our main analysis.

Second, due to the relatively short follow-up in this study, late results may not have developed yet in those infected towards the end of the study, as the time from symptom onset to hospitalization or death can vary widely.3.15 Therefore, efficacy estimates for serious illness and death, in particular, should be interpreted with caution. Third, during the study period, intensive care units in Chile were operating at 93.5% of their capacity on average (65.7% of patients had Covid-19).31 If fewer people were hospitalized than they would be in a regular ICU operation, our estimates of effectiveness for protection against ICU admission could be biased downward, and our estimates of Efficacy for protection against death could be skewed upward (for example, if patients received care at a lower level than would generally be received during normal functioning of the health system).

Fourth, although the national genomic surveillance of SARS-CoV-2 in Chile has reported the circulation of at least two viral lines considered to be variants of concern, P.1 and B.1.1.7 (or the gamma and alpha variants, respectively ),40 we lack representative data to estimate their effect on vaccine efficacy (Table S2). Results of a negative test design study of the efficacy of the CoronaVac vaccine in healthcare workers in Manaus, Brazil, where the gamma variant is now predominant, showed that the efficacy of at least one dose of the Covid-19 vaccine was 49.6% (95% CI, 11.3 to 71.4).29 Although estimates of vaccine efficacy in Brazil are not directly comparable with our estimates due to differences in the target population, the vaccination schedule (a window of 14 to 28 days between doses is recommended in Brazil41) and immunization status, they stress the importance of continuous monitoring of vaccine efficacy.

Overall, the results of our study suggest that the CoronaVac vaccine was very effective in protecting against serious illness and death, results which are consistent with the results of the Phase 2 trials.23.24 and with preliminary efficacy data.27

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