Effects of COVID-19 vaccination type and vaccination-infection interval on viral clearance dynamics in Omicron BA.5 patients at Guangzhou Fangcang shelter hospitals: A propensity score matched multicenter retrospective cohort study

The Omicron BA.5 subvariant exhibited pronounced immune evasion and was associated with widespread breakthrough infections. Unlike settings where mRNA vaccines predominated, China mainly used inactivated vaccines, with the adenovirus type 5-vectored vaccine Ad5-nCoV also administered at scale. Real-world evidence on post-infection viral clearance associated with these platforms remains limited. This multicenter retrospective cohort study enrolled 16,533 mild or asymptomatic Omicron BA.5 cases from two Fangcang shelter hospitals in Guangzhou, China, from Novernber 5 to December 8, 2022. The primary objective was to compare viral-clearance time between unvaccinated and vaccinated participants. The secondary objective was to compare inactivated-vaccine recipients, or and Ad5-nCoV recipients, containing regimens in a propensity score-matched cohort; the last-dose-to-infection interval analysis was exploratory. Propensity score matching was performed using age, sex, marital status, occupation, and province as covariates to improve between-group comparability. All patients underwent standardized daily RT-PCR testing containing both N and ORF1ab genes, with Ct values > 35 defined as negative. The outcome was time to viral clearance, defined as two consecutive negative nasopharyngeal swabs ≥ 24 hours apart. In the overall cohort, vaccinated patients had significantly shorter shedding duration (9.05 vs… 10.42d; P < .001) compared with unvaccinated patients. The matched cohort included 228 unvaccinated, 228 inactivated vaccine recipients, and 114 Ad5-nCoV recipients. Median time to viral clearance differed across groups with 9.42, 8.56, and 8.48d, respectively. Prior vaccination was associated with shorter viral clearance but with no significant difference observed between inactivated vaccine and Ad5-nCoV recipients among the matched regimens.