SARS-CoV-2 Infection Risk by Vaccine Doses and Prior Infections Over 24 Months: ProHEpiC-19 Longitudinal Study

Area of Science:

• Epidemiology
• Immunology
• Public Health

Background:

• Assessing the long-term impact of COVID-19 vaccination is crucial as campaigns progress.
• Understanding immunity against new infections and reinfections is vital.

Purpose of the Study:

• To evaluate the risk of new SARS-CoV-2 infections.
• To analyze the influence of vaccine doses, prior infections, and clinical factors on infection risk.

Main Methods:

• A cohort of 800 healthcare workers was studied over 24 months (March 2020-December 2022).
• Extended Cox models (Andersen-Gill and Prentice-Williams-Peterson) were employed.
• Covariates included sex, age, job title, chronic conditions, vaccine doses, and prior infections.

Main Results:

• Vaccination significantly reduced infection risk: 66% (1 dose), 81% (2 doses), 89% (3 doses), and 99% (4 doses) compared to unvaccinated individuals.
• Each prior infection reduced risk by 75%.
• Individuals with 2-3 prior infections required a fourth dose for significant risk reduction (98%). Chronic conditions slightly increased risk (28-31%).

Conclusions:

• Both vaccination and prior infections are key to SARS-CoV-2 immunity.
• Vaccine effectiveness is supported in reducing reinfection risk up to 24 months.
• Findings inform strategies for mitigating COVID-19's impact and understanding long-term immunity.