COVID-19 reinfection data on individuals diagnosed with their first SARS-CoV-2 infection

Background: In many settings, a large fraction of the population has both been vaccinated against and infected by SARS-CoV-2. Hence, quantifying the protection provided by post-infection vaccination has become critical for policy. We aimed to estimate the protective effect against SARS-CoV-2 reinfection of an additional vaccine dose after an initial Omicron variant infection.

Methods: We report a retrospective, population-based cohort study performed in Shanghai, China, using electronic databases with information on SARS-CoV-2 infections and vaccination history. We compared reinfection incidence by post-infection vaccination status in individuals initially infected during the April-May 2022 Omicron variant surge in Shanghai and who had been vaccinated before that period. Cox models were fit to estimate adjusted hazard ratios (aHR).

Results: 275,896 individuals were diagnosed with RT-PCR-confirmed SARS-CoV-2 infection in April-May 2022; 199,312/275,896 were included in analyses on the effect of a post-infection vaccine dose. Post-infection vaccination provided protection against reinfection (aHR 0.82; 95% CI 0.79-0.85). For patients who had received one, two or three vaccine doses before their first infection, hazard ratios for the post-infection vaccination effect were 0.84 (0.76-0.93), 0.87 (0.83-0.90) and 0.96 (0.74-1.23), respectively. Vaccination within 30 and 90 days before the second Omicron wave provided different degrees of protection (in aHR): 0.51 (0.44-0.58), and 0.67 (0.61-0.74), respectively. Moreover, for all vaccine types, but to different extents, a post-infection dose given to individuals who were fully vaccinated before first infection was protective.

Conclusions: In previously vaccinated and infected individuals, an additional vaccine dose provided protection against Omicron variant reinfection. These observations will inform future policy decisions on COVID-19 vaccination in China and other countries.

Methods
 During the first wave of the Omicron variant, Shanghai, a city with a population of over 25 million, underwent multiple rounds of SARS-CoV-2 RT-PCR testing from April 1 to May 31, 2022. Both asymptomatic individuals identified through mass screening and symptomatic individuals seen by healthcare professionals were included. Demographic data (sex at birth, age) and infection history were provided by the Shanghai Center for Disease Control and Prevention. The reinfection data collected prior to January 2023, with reinfection-related death defined as death within 30 days of a SARS-CoV-2 reinfection.

Individuals with missing first infection dates or those who died from non-COVID-19 reasons between the two Omicron waves were excluded.The analysis centered on individuals who had received at least one vaccine dose before their first SARS-CoV-2 infection.