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Abstract We conducted a matched retrospective cohort study of two cohorts to estimate inactivated vaccine effectiveness (VE) and its comparative effectiveness of booster dose among older people in Shanghai. Cohort 1 consisted of a vaccinated group (≥1 dose) and an unvaccinated group (3,317,475 pairs), and cohort 2 consisted of a booster vaccinated group and a fully vaccinated group (2,084,721 pairs). The Kaplan–Meier method and Cox regression models were used to estimate risk and hazard ratios (HRs) study outcomes. For cohort 1, the overall estimated VEs of ≥1 dose of inactivated vaccine against SARS-CoV-2 infection, severe/critical Covid-19, and Covid-19 related death were 24.7% (95%CI 23.7%−25.7%), 86.6% (83.1%−89.4%), and 93.2% (88.0%−96.1%), respectively. Subset analysis showed that the booster vaccination provided greatest protection. For cohort 2, compared with full vaccination, relative VEs of booster dose against corresponding outcome were 16.3% (14.4%−17.9%), 60.5% (37.8%−74.9%), and 81.7% (17.5%−95.9%). Here we show, although under the scenario of persistent dynamic zero-Covid policy and non-pharmaceutical interventions, promoting high uptake of the full vaccination series and booster dose among older adults is critically important. Timely vaccination with the booster dose provided effective protection against Covid-19 outcomes.

Abstract Background Limited data are available on the effectiveness of inactivated and Ad5-nCoV COVID-19 vaccines in real-world use—especially against Omicron variants in SARS-CoV-2 infection-naïve population. Methods A matched case-control study was conducted among people aged ≥ 3 years between 2 December 2021 and 13 May 2022. Cases were SARS-CoV-2-infected individuals, individuals with severe/critical COVID-19, or COVID-19-related deaths. Controls were selected from consecutively test-negative individuals at the same time as cases were diagnosed and were exact-matched on year-of-age, gender, birthplace, illness onset date, and residential district in ratios of 1:1 with infected individuals and 4:1 with severe/critical COVID-19 and COVID-19-related death. Additionally, two subsets were constructed to analyze separate vaccine effectiveness (VE) of inactivated vaccines (subset 1) and Ad5-vectored vaccine (subset 2) against each of the three outcomes. Results Our study included 612,597 documented SARS-CoV-2 infections, among which 1485 progressed to severe or critical illness and 568 died. Administering COVID-19 vaccines provided limited protection against SARS-CoV-2 infection across all age groups (overall VE: 16.0%, 95% CI: 15.1–17.0%) but high protection against severe/critical illness (88.6%, 85.8–90.8%) and COVID-19-related death (91.6%, 86.8–94.6%). In subset 1, inactivated vaccine showed 16.3% (15.4–17.2%) effective against infection, 88.6% (85.8–90.9%) effective against severe/critical COVIID-19, and 91.7% (86.9–94.7%) against COVID-19 death. Booster vaccination with inactivated vaccines enhanced protection against severe COVID-19 (92.7%, 90.1–94.6%) and COVID-19 death (95.9%, 91.4–98.1%). Inactivated VE against infection began to wane 12 weeks after the last dose, but two and three doses sustained high protection levels (> 80%) against severe/critical illness and death, while subset 2 showed Ad5-vectored vaccine was 13.2% (10.9–15.5%) effective against infection and 77.9% (15.6–94.2%) effective against severe/critical COVIID-19. Conclusions Our real-world study found high and durable two- and three-dose inactivated VE against Omicron-associated severe/critical illness and death across all age groups, but lower effectiveness against Omicron infection, which reinforces the critical importance of full-series vaccination and timely booster dose administration for all eligible individuals.