Associated factors in SARS-CoV-2 infection among close contacts during the zero-COVID policy from 2020 to 2022 in the northeast of Shenzhen, China: a retrospective cohort study

Background Identifying high-risk groups and developing specific interventions is essential to combat pandemic including COVID-19. We estimated key factors of demographic characteristic, exposure-related variables, and vaccination status in secondary infection among close contacts throughout the zero-COVID policy. Methods We used contact tracing data from 622 primary cases and 31,278 close contacts between February 2020 and December 2022 in the northeast of Shenzhen, China. The multivariate logistic regression was utilized to identify factors affecting SARS-CoV-2 infection of close contacts. Results The secondary attack rate (SAR) of close contacts was 1.4% (95% CI: 1.3–1.5%), and the associated factors included over 50 (OR = 1.47, 95% CI: 1.13–1.91), living in urban village housing (OR = 1.51, 95% CI: 1.04–2.19), closer relationship with primary cases (e.g., household members: OR = 44.06, 95% CI: 34.45–56.36), last exposure occurring before the illness onset of cases (e.g., >2 days before: OR = 0.52, 95% CI: 0.27–1.00), exposed to cases with moderate symptoms (OR = 0.62, 95% CI: 0.40–0.95), and better COVID-19 vaccination status (e.g., booster vaccination within 6 months before the last exposure: OR = 0.38, 95% CI: 0.24–0.60). Conclusion Our findings should be helpful to develop targeted surveillance and interventions for these high-risk groups to understand ongoing COVID-19 issue and improve future pandemic management.