Effectiveness of inactivated COVID-19 vaccine against symptom severity in hospitalized COVID-19 patients infected with the Omicron variant

In this study, we analysed the outcomes of 1165 symptomatic patients infected with the SARS-CoV-2 Omicron BA.2 variant and their response to Sinovac-CoronaVac vaccination. We assessed the effectiveness of vaccination against adverse outcomes (severe, critical, or fatal cases). Of these patients, 504 (43.3%) were men, the median age was 71 years, and 391 patients (66.4%) had received the Sinovac-CoronaVac COVID-19 vaccine (Sino Pharma, Beijing, China). The percentages of severe, critical, and fatal cases were 3.9%, 2.8%, and 3.7%, respectively, with significantly lower rates among vaccinated patients (2.8% vs. 14.2%; adjusted odds ratio [aOR], 0.306; 95% confidence interval [95% CI], 0.129–0.727). Age ≥ 70 years and a CRP level > 8 mg/L were independent predictors of an adverse outcome. Out of 630 patients aged ≥ 70 years, 107 (17.0%) were vaccinated. Seven vaccinated patients (6.5%) and 94 unvaccinated patients (18.0%) experienced adverse outcomes. Multivariate analysis indicated that vaccination (OR, 0.401; 95% CI 0.162–0.991) and CRP levels > 8 mg/L (OR, 3.262; 95% CI 1.754–6.067) were independently associated with adverse outcomes in patients aged ≥ 70 years. Inactivated vaccines were effective against symptomatic and severe COVID-19. Even in symptomatic Omicron infections, full vaccination with inactivated vaccines significantly reduced the number of adverse cases, especially in patients aged ≥ 70 years. Systemic inflammation (as measured by the CRP level) was independently correlated with adverse outcomes in patients infected with Omicron BA.2.