Epidemiology, risk factors, and vaccine effectiveness for SARS-CoV-2 infection among healthcare workers during the omicron pandemic in Shanghai, China

Background: The COVID-19 pandemic has exposed healthcare workers (HCWs) to serious risk of infection. The aims of our study were to investigate the epidemiological characteristics and risk factors of SARS-CoV-2 infection among HCWs, and evaluate the vaccine effectiveness (VE) during the Omicron pandemic in Shanghai, China. Methods: Active surveillance of COVID-19 was performed among HCWs who worked in Shanghai General Hospital from December 2022 to January 2023. A case-control study was conducted by questionnaire survey to analyse the infection-related risk factors. A retrospective cohort study was explored to evaluate VE against primary infection. Results: During the Omicron outbreak, 2,008 of 2,460 (81.6%) HCWs were infected with SARSCoV-2. The infection rate was higher in women, younger age groups, nurses and medical technicians. Among the 1,742 participants in the questionnaire, 1,463 (84.0%) were tested positive, and 95.1% of them developed symptoms. Most of the infections (53.0%) were acquired outside the hospital. The risk factors associated with higher odds of infection were working in the emergency department (aOR 3.77, 95% CI 1.69-8.38) and medical examination area (aOR 2.47, 95% CI 1.10-5.51). The protective factors associated with lower odds of infection were previous infection with SARS-CoV-2 (aOR 0.01, 95% CI 0-0.07) and receiving four doses of vaccines (aOR 0.40, 95% CI 0.17-0.97). For frontline HCWs, those who had oral-nasal exposure to coworkers were more likely to be infected (aOR 1.74, 95% CI 1.21-2.51). In VE analysis, the risk of primary infection was lower in HCWs who received the emergency heterologous booster (the fourth dose) during the epidemic (aHR 0.25, 95% CI 0.15-0.40), resulting in an adjusted-VE of 75.1%. Conclusions: In response to future pandemic, it is important for public health policies to aim at protecting HCWs through risk-differentiated infection control measures, strengthening personal protection and recommending vaccination to vulnerable individuals before the arrival of Omicron wave.