Effectiveness of the CoronaVac vaccine in the elderly population during a Gamma variant-associated epidemic of COVID-19 in Brazil: A test-negative case-control study
Authors: Ranzani Otavio T. , Hitchings Matt D.T. , Dorion Murilo , D’Agostini Tatiana Lang , Cardoso de Paula Regiane , Pereira de Paula Olivia Ferreira , Faria de Moura Villela Edlaine , Scaramuzzini Torres Mario Sergio , Barbosa de Oliveira Silvano , Schulz Wade , Almiron Maria , Said Rodrigo , Dias de Oliveira Roberto , Vieira da Silva Patricia , Navegantes de Araújo Wildo , Gorinchteyn Jean Carlo , Andrews Jason R. , Cummings Derek A.T. , Ko Albert I. , Croda Julio 
Published: May 2021

Objective

To estimate the effectiveness of the inactivated whole-virus vaccine, CoronaVac, against symptomatic COVID-19 in the elderly population of São Paulo State, Brazil during widespread circulation of the Gamma variant.

Design

Test negative case-control study.

Setting

Health-care facilities in São Paulo State, Brazil.

Participants

43,774 adults aged 70 years or older who were residents of São Paulo State and underwent SARS-CoV-2 RT-PCR testing from January 17 to April 29, 2021. 26,433 cases with symptomatic COVID-19 and 17,622 symptomatic, test negative controls were selected into 7,950 matched pairs, according to age, sex, self-reported race, municipality of residence, prior COVID-19 status and date of RT-PCR testing.

Intervention

Vaccination with a two-dose regimen of CoronaVac.

Main outcome measures

RT-PCR confirmed symptomatic COVID-19 and COVID-19 associated hospitalizations and deaths.

Results

Adjusted vaccine effectiveness against symptomatic COVID-19 was 18.2% (95% CI, 0.0 to 33.2) in the period 0-13 days after the second dose and 41.6% (95% CI, 26.9 to 53.3) in the period ≥14 days after the second dose. Adjusted vaccine effectiveness against hospitalisations was 59.0% (95% CI, 44.2 to 69.8) and against deaths was 71.4% (95% CI, 53.7 to 82.3) in the period ≥14 days after the second dose. Vaccine effectiveness ≥14 days after the second dose declined with increasing age for the three outcomes, and among individuals aged 70-74 years it was 61.8% (95% CI, 34.8 to 77.7) against symptomatic disease, 80.1% (95% CI, 55.7 to 91.0) against hospitalisations and 86.0% (95% CI, 50.4 to 96.1) against deaths.

Conclusions

Vaccination with CoronaVac was associated with a reduction in symptomatic COVID-19, hospitalisations and deaths in adults aged 70 years or older in a setting with extensive Gamma variant transmission. However, significant protection was not observed until completion of the two-dose regimen, and vaccine effectiveness declined with increasing age amongst this elderly population.

Summary boxes

What is already known on this topic

Randomised controlled trials (RCT) have yielded varying estimates (51 to 84%) for the effectiveness of the inactivated whole-virus vaccine, CoronaVac, against symptomatic COVID-19.

Current evidence is limited on whether CoronaVac is effective against severe disease or death caused by the SARS-CoV-2 variant of concern, Gamma, or in the setting of extensive Gamma variant circulation.

More evidence is needed for the real-world effectiveness of CoronaVac and other inactivated vaccines among elderly individuals, a population that was underrepresented in RCTs of these vaccines.

What this study adds

A two-dose regimen of CoronaVac provides significant protection against symptomatic COVID-19, hospitalisations and deaths among adults ≥70 years of age in the setting of widespread Gamma variant transmission.

Significant protection did not occur until ≥14 days after administration of the second dose of CoronaVac.

The effectiveness of CoronaVac declines with increasing age in the elderly population.