Risks and Hazards of Post-Vaccine SARS-CoV-2 Infection in Antibody Naive Populations during the Mass Vaccination Campaign Against COVID-19 in Mongolia

Introduction In this study, we aimed to compare the risks and hazards of new infection and hospitalization in seronegative before-vaccination population groups and its association with sociodemographic, initial, and booster vaccination patterns. Materials and Methods We enrolled a total of 1,709 vaccinees who tested negative for anti-SARS-CoV-2 receptor binding domain (RBD) specific antibodies before receiving two doses of one of four COVID-19 vaccines. Data on vaccinations, new infections, hospitalizations and fatal outcomes during the 80-weeks follow-up were obtained from national health registry operators using participants' national registration IDs. Results We observed a new infection rate of 49.1% among the antibody-naïve population over an 80-week follow-up period. We found frontline workers (FWs), especially those who worked in rural health facilities and those who worked in direct contact with infected patients (red-zone FWs) at increased risk and hazard for new infections and followed hospitalizations beginning with milestone of 24 weeks compared to rest population groups prioritized for mass immunization. Both the Pfizer BioNTech and Sputnik V vaccines demonstrated greater effectiveness in preventing new infections while the Sinopharm BBIBP vaccine showed lower preventive effects of hospitalization. We identified age cut points adjusted for sex, showing that COVID-19 significantly affected younger females.