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Effectiveness of CoronaVac in the setting of high SARS-CoV-2 P.1 variant transmission in Brazil: A test-negative case-control study

Our take —

In a case-control study, available as a preprint and thus not yet peer-reviewed, of 528 matched pairs of healthcare workers in Manaus, Brazil, the estimated effectiveness of at least one dose of CoronaVac vaccine (Sinovac Life Sciences) against any SARS-CoV-2 infection (regardless of symptoms) was 35.1%, and was 49.6% against symptomatic infection. While the study suggests at least one dose of CoronaVac vaccine is not significantly protective against SARS-CoV-2 infection, these findings are subject to numerous limitations, including inattention to differences between healthcare workers who received one or two doses of CoronaVac.

Study design


Study population and setting

Between January 19 and March 25, 2021, investigators identified healthcare workers in Manaus, Brazil, with RT-PCR-confirmed SARS-CoV-2 infections (both with and without symptoms), who were matched to non-infected healthcare workers on age group, neighborhood of residence, and specimen collection dates. Manaus experienced two distinct waves of SARS-CoV-2 transmission in March and November 2020, respectively, with the latter wave attributed to the emergence of the P.1 variant. The authors conducted a matched analysis to identify the odds of SARS-CoV-2 vaccination by CoronaVac, comparing infected healthcare workers to non-infected controls.

Summary of Main Findings

Of the 67,718 healthcare workers registered in Manaus, 393 and 135 case-control pairs of symptomatic and asymptomatic illness, respectively, were formed. Controlling for age, sex, race, healthcare cadre, frequency of healthcare interactions, and history of SARS-CoV-2 infection before January 19, 2021, the odds of symptomatic COVID-19 infection were 50% lower among healthcare workers who received at least one dose of the CoronaVac vaccine in the 14 days prior to specimen collection. Receiving at least one dose of CoronaVac at least 14 days before specimen collection did not significantly protect against symptomatic or asymptomatic SARS-CoV-2 infection. The estimated effectiveness of at least one dose against any SARS-CoV-2 infection (regardless of symptoms) was 35.1%, and was 49.6% against symptomatic infection.

Study Strengths

In the absence of longitudinal, prospective data on individuals, investigators created matched pairs of SARS-CoV-2 infections and non-infected controls, which can provide valid proxies for SARS-CoV-2 risk measures required for estimating vaccine effectiveness. To examine CoronaVac vaccine effectiveness in the context of P.1 transmission, investigators restricted the study period to 2021, when P.1 accounted for >75% of sequenced SARS-CoV-2 specimens in Manaus.


Investigators did not enumerate or distinguish between individuals who had received one or two doses of CoronaVac, which could produce biased estimates of vaccine effectiveness. Additionally, healthcare workers were not matched on occupational cadre, nor was the setting in which participants worked (i.e., hospital intensive care unit, outpatient primary care center); because healthcare workers cadres may have differential SARS-CoV-2 exposure risks, inattention to occupational settings could further bias vaccine effectiveness estimates. Since recruitment into the study required availability of a SARS-CoV-2 test specimen, vaccine effectiveness measures could be inflated for healthcare workers with asymptomatic illness, as SARS-CoV-2 testing would likely be influenced by the presence of COVID-19 symptoms. Lastly, vaccine effectiveness was calculated over a two-month period (January to March, 2021), which limits inferences about the CoronaVac’s potential effectiveness over more prolonged periods (i.e., six months).

Value added

This is among the first studies to estimate the effectiveness of the CoronaVac vaccine in the context of P.1-dominant SARS-CoV-2 transmission.

This review was posted on: 14 May 2021