@Article{info:doi/10.2196/56926, author="Tor{\'a}n-Monserrat, Pere and Lamonja-Vicente, Noem{\'i} and Costa-Garrido, Anna and Carrasco-Ribelles, Luc{\'i}a A and Quirant, Bibiana and Boigues, Marc and Molina, Xaviera and Chac{\'o}n, Carla and Dacosta-Aguayo, Rosalia and Arm{\'e}star, Fernando and Mart{\'i}nez C{\'a}ceres, Eva Mar{\'i}a and Prado, Julia G and Viol{\'a}n, Concepci{\'o}n", title="SARS-CoV-2 Infection Risk by Vaccine Doses and Prior Infections Over 24 Months: ProHEpiC-19 Longitudinal Study", journal="JMIR Public Health Surveill", year="2024", month="Nov", day="22", volume="10", pages="e56926", keywords="SARS-CoV-2; COVID-19; health care workers; cohort; extended Cox models; coronavirus; epidemiology; epidemiological; risks; infectious; respiratory; longitudinal; vaccines; vaccination; vaccinated", abstract="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 ", issn="2369-2960", doi="10.2196/56926", url="https://publichealth.jmir.org/2024/1/e56926", url="https://doi.org/10.2196/56926" }