ABSTRACT
BACKGROUND: China has been using inactivated COVID-19 vaccines as primary series and booster doses to protect the population from severe to fatal COVID-19. We evaluated primary and booster vaccine effectiveness (VE) against Omicron BA.2 infection outcomes. METHODS: This was a 13-province retrospective cohort study of quarantined close contacts of BA.2-infected individuals. Outcomes were BA.2 infection, COVID-19 pneumonia or worse, and severe/critical COVID-19. Absolute VE was estimated by comparison with an unvaccinated group. RESULTS: There were 289,427 close-contacts ≥3 years old exposed to Omicron BA.2 cases; 31,831 turned nucleic-acid amplification test (NAAT)-positive during quarantine, 97.2% with mild or asymptomatic infection, 2.6% had COVID-19 pneumonia, and 0.15% had severe/critical COVID-19. None died. Adjusted VE against any infection was 17% for primary series and 22% when boosted. Primary series aVE in adults >18 years was 66% against pneumonia or worse infection and 91% against severe/critical COVID-19. Booster dose aVE was 74% against pneumonia or worse, and 93% against severe/critical COVID-19. CONCLUSIONS: Inactivated COVID-19 vaccines provided modest protection from infection, very good protection against pneumonia, and excellent protection against severe/critical COVID-19. Booster doses are necessary to provide strongest protection.
ABSTRACT
Phase 3 clinical trials and real-world effectiveness studies showed that China's two main inactivated COVID-19 vaccines are very effective against serious illness. In November 2021, an outbreak occurred in the Inner Mongolia Autonomous Region that provided an opportunity to assess the vaccine effectiveness (VE) of these inactivated vaccines against COVID-19 caused by the delta variant. We evaluated VE with a retrospective cohort study of close contacts of infected individuals, using a generalized linear model with binomial distribution and log-link function to estimate risk ratios (RR) and VE. A total of 8842 close contacts were studied. Compared with no vaccination and adjusted for age, presence of comorbidity, and time since last vaccination, full vaccination reduced symptomatic infection by 62%, pneumonia by 64% and severe COVID-19 by 90%; reductions associated with homologous booster doses were 83% for symptomatic infection, 92% for pneumonia and 100% for severe COVID-19. There was no significant decline in two-dose VE for any outcome for up to 325 days following the last dose. There were no differences by vaccine brand. Inactivated vaccines were effective against delta-variant illness, and were highly effective against pneumonia and severe COVID-19; VE was increased by booster doses.
ABSTRACT
BACKGROUND: In partial response to the coronavirus disease 2019 (COVID-19) pandemic, countries around the world are conducting large-scale vaccination campaigns. Real-world estimates of vaccine effectiveness (VE) against the B.1.617.2 (Delta) variant are still limited. An outbreak in Ruili city of Chinaprovided an opportunity to evaluate VE against the Delta variant of two types of COVID-19 vaccines in use in China and globally - inactivated (CoronaVac and BBIBP-CorV) and adenovirus type 5 vectored (Convidecia) vaccines. METHODS: We estimated VE using a retrospective cohort study two months after the Ruili vaccination campaign (median: 63 days). Close contacts of infected people (Chinese nationality, 18 years and above) were included to assess VE against symptomatic Covid-19, COVID-19 pneumonia, and severe COVID-19. We calculated the relative risks (RR) of the outcomes for unvaccinated compared with fully vaccinated individuals. We used logistic regression analyses to estimate adjusted VEs, controlling for gender and age group (18-59 years and 60 years and over).We compared unvaccinated and fully vaccinated individuals on duration of RT-PCR positivity and Ct value. FINDINGS: There were 686 close contacts eligible for VE estimates. Adjusted VE ofad5-vectored vaccine was 61.5% (95% CI, 9.5-83.6) against symptomatic COVID-19, 67.9% (95%CI: 1.7-89.9) against pneumonia, and 100% (95%CI: 36.6-100) against severe/critical illness. For the two inactivated vaccines, combined VE was 74.6% (95% CI, 36.0-90.0) against symptomatic COVID-19, 76.7% (95% CI: 19.3-93.3) against pneumonia, and 100% (95% CI: 47.6-100) against severe/critical COVID-19. There were no statistically significant differences in VE between twoinactivated vaccines for symptomatic COVID-19 and for pneumonia, nor were there statistically significant differences between inactivated and ad5-vectored VE in any of the three outcomes. The median durations of RT-PCR positivity were 17 days for fifteen people vaccinated with an inactivated vaccine, 18 days for forty-four people vaccinated with the Ad5 vectored vaccine, and 26 days for eleven unvaccinated individuals. INTERPRETATION: These results provide reassuring evidence that the three vaccines are effective at preventing Delta-variant COVID-19 in short term following vaccination campaign, and are most effective at preventing more serious illness. The findings of reduced duration of RT-PCR positivity and length of hospital stay associated with full vaccination suggests potential saving of health-care system resources.