Data collection
Once one case was confirmed by laboratory testing, the epidemiological and clinical criteria and medical records were submitted and accepted by LACDC immediately. After the receipt of the submitted reports, the LACDC performed an epidemiological investigation within 24 h, including verification of the activity trajectory, close contact determination, and tracing. General information (such as name, sex, age, ID number, last exposure time, frequency of exposure, mode of exposure, and location of exposure) of confirmed and asymptomatic infections between 4 April 2022 and 20 April 2022 in Lu’an was extracted from the epidemiological survey reports and National Notifiable Diseases Registry System (NNDRS), which is a web-based computerized reporting system and has been in operation since 2004 in China[25]. Clinical information was obtained from epidemiological survey reports. We obtained the following information from the Anhui Immunization Information Management System (AIIMS) through the ID numbers and names of the individuals: vaccine type, vaccination dose, vaccination site, vaccine manufacturer, vaccination date, vaccination status, and all vaccination records. We considered vaccinations to be valid only if documented in either the national or the provincial immunization information system.
Vaccination data
Three types of vaccines including inactivated, Ad5-nCoV, and recombinant protein vaccines against SARS-CoV-2 were authorized and widely used in China[26]. Vaccination was considered incomplete if the time from the last dose of vaccination to the last exposure time was less than 14 days. We divided all participants into four groups: unvaccinated (zero doses, or first dose<14 days from time of last exposure), partial vaccination (one-dose inactivated vaccine or two-dose recombinant protein vaccine, with the last dose of vaccination >14 days from the time of last exposure), full vaccination (one-dose Ad5-nCoV vaccine or two-dose inactivated vaccine or three-dose recombinant protein vaccine, with the last dose of vaccination >14 days from the time of last exposure), and booster vaccination (three-dose of any COVID-19 vaccine, or two-dose Ad5-CoV vaccine, or three-dose recombinant protein vaccine, or two-dose any inactivated vaccine with a one-dose Ad5-CoV vaccine/recombinant protein vaccine, with the last dose of vaccination >14 days from the time of last exposure)[27,28]. Additionally, homogenous booster vaccination (same as the primary vaccine) and heterologous booster vaccination (third dose with an Ad5-nCoV or recombinant protein vaccine) following a two-dose regimen of inactivated vaccines were included in our study[24,29].