Analysis of re-infection cases and influencing factors post first severe COVID-19 wave in Jiangsu Province, China.

Introduction: This study aimed to assess COVID-19 re-infection rates among individuals previously infected between 2020 and November 2022, particularly during the first wave of high-intensity transmission, and to identify the risk factors associated with re-infection in Jiangsu Province, China.

Methodology: Epidemiological investigations were conducted through telephone interviews and face-to-face visits in February and March 2023. Statistical analyses included the Chi-square or Fisher`s exact test for categorical data, Student's t-test for numerical data, Poisson regression for influencing factors, and Kaplan-Meier for cumulative re-infection risk.

Results: Among 12,910 individuals surveyed, 957 (7.4%) cases of re-infection were identified. Re-infection rates varied significantly by initial infection period: 42.5% in January-February 2020, 15.5% in July-August 2021, 6.7% in March-April 2022, and 1.1% in September-October 2022. Females and individuals aged 18-50 years were more susceptible to re-infection. A reduced risk of re-infection was observed in those who received four vaccine doses, with a relative risk of 0.25 (p = 0.019).

Conclusions: For populations prone to COVID-19 re-infections, particularly females and young adults aged 18-50 years, receiving four or more vaccine doses effectively reduces the likelihood of repeated infections. These findings emphasize the need to prioritize vaccination and protect high-risk groups in COVID-19 prevention efforts.