%0 Journal Article %@ 2369-2960 %I JMIR Publications %V 10 %N %P e56926 %T SARS-CoV-2 Infection Risk by Vaccine Doses and Prior Infections Over 24 Months: ProHEpiC-19 Longitudinal Study %A Torán-Monserrat,Pere %A Lamonja-Vicente,Noemí %A Costa-Garrido,Anna %A Carrasco-Ribelles,Lucía A %A Quirant,Bibiana %A Boigues,Marc %A Molina,Xaviera %A Chacón,Carla %A Dacosta-Aguayo,Rosalia %A Arméstar,Fernando %A Martínez Cáceres,Eva María %A Prado,Julia G %A Violán,Concepción %A , %K SARS-CoV-2 %K COVID-19 %K health care workers %K cohort %K extended Cox models %K coronavirus %K epidemiology %K epidemiological %K risks %K infectious %K respiratory %K longitudinal %K vaccines %K vaccination %K vaccinated %D 2024 %7 22.11.2024 %9 %J JMIR Public Health Surveill %G English %X Background: As the vaccination campaign against COVID-19 progresses, it becomes crucial to comprehend the lasting effects of vaccination on safeguarding against new infections or reinfections. Objective: This study aimed to assess the risk of new SARS-CoV-2 infections based on the number of vaccine doses, prior infections, and other clinical characteristics. Methods: We defined a cohort of 800 health care workers in a 24-month study (March 2020 to December 2022) in northern Barcelona to determine new infections by SARS-CoV-2. We used extended Cox models, specifically Andersen-Gill (AG) and Prentice-Williams-Peterson, and we examined the risk of new infections. The AG model incorporated variables such as sex, age, job title, number of chronic conditions, vaccine doses, and prior infections. Additionally, 2 Prentice-Williams-Peterson models were adjusted, one for those individuals with no or 1 infection and another for those with 2 or 3 infections, both with the same covariates as the AG model. Results: The 800 participants (n=605, 75.6% women) received 1, 2, 3, and 4 doses of the vaccine. Compared to those who were unvaccinated, the number of vaccine doses significantly reduced (P<.001) the risk of infection by 66%, 81%, 89%, and 99%, respectively. Unit increase in the number of prior infections reduced the risk of infection by 75% (P<.001). When separating individuals by number of previous infections, risk was significantly reduced for those with no or 1 infection by 61% (P=.02), and by 88%, 93%, and 99% (P<.001) with 1, 2, 3, or 4 doses, respectively. In contrast, for those with 2 or 3 previous infections, the reduction was only significant with the fourth dose, at 98% (P<.001). The number of chronic diseases only increased the risk by 28%‐31% (P<.001) for individuals with 0‐1 previous infections. Conclusions: The study suggests that both prior infections and vaccination status significantly contribute to SARS-CoV-2 immunity, supporting vaccine effectiveness in reducing risk of reinfection for up to 24 months after follow-up from the onset of the pandemic. These insights contribute to our understanding of long-term immunity dynamics and inform strategies for mitigating the impact of COVID-19. Trial Registration: ClinicalTrials.gov NCT04885478; http://clinicaltrials.gov/ct2/show/NCT04885478 %R 10.2196/56926 %U https://publichealth.jmir.org/2024/1/e56926 %U https://doi.org/10.2196/56926