Effectiveness of the CoronaVac vaccine in older adults during a gamma variant associated epidemic of covid-19 in Brazil: test negative case-control study

Abstract

Objective: To estimate the effectiveness of the inactivated whole virus vaccine, CoronaVac (Sinovac Biotech), 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: Community testing for covid-19 in São Paulo state, Brazil.

Participants: 43 774 adults aged ≥70 years who were residents of São Paulo state and underwent reverse transcription polymerase chain reaction (RT-PCR) testing for SARS-CoV-2 from 17 January to 29 April 2021. 26 433 cases with symptomatic covid-19 and 17 622 test negative controls with covid-19 symptoms were formed into 13 283 matched sets, one case with to up to five controls, according to age, sex, self-reported race, municipality of residence, previous covid-19 status, and date of RT-PCR test (±3 days).

Intervention: Vaccination with a two dose regimen of CoronaVac.

Main outcome measures: RT-PCR confirmed symptomatic covid-19 and associated hospital admissions and deaths.

Results: Adjusted vaccine effectiveness against symptomatic covid-19 was 24.7% (95% confidence interval 14.7% to 33.4%) at 0-13 days and 46.8% (38.7% to 53.8%) at ≥14 days after the second dose. Adjusted vaccine effectiveness against hospital admissions was 55.5% (46.5% to 62.9%) and against deaths was 61.2% (48.9% to 70.5%) at ≥14 days after the second dose. Vaccine effectiveness ≥14 days after the second dose was highest for the youngest age group (70-74 years)-59.0% (43.7% to 70.2%) against symptomatic disease, 77.6% (62.5% to 86.7%) against hospital admissions, and 83.9% (59.2% to 93.7%) against deaths-and declined with increasing age.

Conclusions: Vaccination with CoronaVac was associated with a reduction in symptomatic covid-19, hospital admissions, and deaths in adults aged ≥70 years in a setting with extensive transmission of the gamma variant. Vaccine protection was, however, low until completion of the two dose regimen, and vaccine effectiveness was observe to decline with increasing age among this elderly population.

Fig 1

Incidence of reported covid-19, vaccination coverage, and prevalence of SARS-CoV-2 variants of concern from 1 October 2020 to 29 April 2021 in São Paulo state, Brazil. Panels A-C show the 14 day rolling average of daily age group specific incidence of reported covid-19 cases, hospital admission rate, and mortality (events per 100 000 population). Panel D shows daily cumulative vaccination coverage in people aged ≥70 years. Population estimates for age groups were obtained from national projections for 2020. Panel E shows the monthly prevalence of SARS-CoV-2 variants among genotyped isolates in the GISAID (global initiative on sharing avian influenza data) database (extraction on 20 June 2021). Vertical bars show dates that adults aged ≥90, 80-89, and 70-79 years in the general population became eligible for vaccination

Fig 2

Flowchart of study population from surveillance databases, and selection of matched cases and controls. *Some participants contributed as controls and cases, and matching allowed for replacement of controls between cases. RT-PCR=reverse transcription polymerase chain reaction

Fig 3

Adjusted vaccine effectiveness ≥14 days after the second dose of CoronaVac (Sinovac Biotech) for subgroups of adults aged ≥70 years. Estimates of vaccine effectiveness were obtained from a conditional logistic regression model that included covariates of age and number of comorbidities and incorporated an interaction term between the category of interest and the period ≥14 days after the second dose