Effectiveness of BNT162b2 and ChAdOx1 vaccines against symptomatic COVID-19 among Healthcare Workers in Kuwait: A retrospective cohort study

Abstract

Background: The COVID–19 BNT162b2 vaccination rollout in Kuwait started on 24 December 2020 followed by ChAdOx1 on 3 February 2021. The study objectives were to assess the factors associated with vaccine coverage and determine vaccine effectiveness (VE) against SARSCoV2 infection in a healthcare worker (HCW) population. Methods: This retrospective cohort study was conducted among HCW working at a public secondary hospital in Kuwait. The follow–up period was from 24 December 2020 to 15 June 2021. The primary outcomes were vaccine coverage and PCR–confirmed SARS–CoV–2 infection for the VE analysis. Data on new SARS–CoV–2 infections (with or without symptoms) during study period in addition to HCWs characteristics (sex, age, nationality, and occupation) were extracted from the hospital records. The vaccine coverage and PCR confirmed SARS–CoV–2 infections were cross–tabulated by the HCWs characteristics. Furthermore, we used Cox regression to estimate time to infection hazard ratios in vaccinated (first and second dose) compared to unvaccinated HCWs. Only one ChAdOx1 dose was given during the study period. Results: There were 3246 HCWs included in the analysis. The median age was 38 years (IQR = 33 – 44), 63.4% were females, 46.8% aged 31 – 40, and 82.3% were non-Kuwaitis. Overall, 82.1% of HCWs received at least one vaccine dose (50.4% received only one dose of ChAdOx1, 3.3% received one dose of BNT162b2, and 28.3% received two doses of BNT162b2). 17.9% of HCWs remained unvaccinated by the end of the study. A significantly lower vaccination coverage percentage was amongst female HCWs, younger age group (20 – 30 years old), and administrative/executive staff. Symptomatic SARS–CoV–2 PCR confirmed infection prevalence was 7.3%. No asymptomatic infections were reported. The SARS–CoV–2 infection incidence rate was 126 per 100,000 person–days in the unvaccinated group; the incidence rates in partially vaccinated groups (≥ 28 days after ChAdOx1 first dose) and (≥ 14 days after receiving BNT162b2 through receipt of second dose) were 31.4 and 10.9 per 100,000 person–days, respectively. In the fully vaccinated group (≥ 14 days after BNT162b2 second dose), the incidence rate was 6.3 per 100,000 person–days. The estimated adjusted vaccine effectiveness of fully vaccinated was 94.5% (95% confidence interval [CI] = 89.4%–97.2%). The VE of partially vaccinated for ChAdOx1 and BNT162b2 was 75.4% (95% CI = 67.2%–81.6%) and 91.4% (95% CI = 65.1% – 97.9%), respectively. Conclusions: Both BNT162b2 and ChAdOx1 vaccines prevented most symptomatic infections in this population across age groups, nationalities, and occupations. A significant proportion (17.9%) of HCWs were unvaccinated despite the vaccine accessibility. The findings complement other VE studies and demonstrate the vaccine benefit for HCWs.

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