Abstract

Effectiveness of mRNA vaccination against COVID-19-associated hospitalization in a large healthcare system during the Omicron dominant era

Christopher Kabir, MSPH¹, Eric Beck, PhD², Maureen Shields, MPH³, Jeffrey Ording, MD⁴ (1)Academic Affairs, Advocate Aurora Health, (2)ACL Laboratories, Aurora West Allis Medical Center, (3)Advocate Aurora Research Institute, Advocate Aurora Health, (4)Internal Medicine Department, Advocate Illinois Masonic Medical Center

APHA 2022 Annual Meeting and Expo

Observational studies utilizing electronic medical record data provide essential epidemiologic information on risk factors and SARS-CoV-2 infections in near real-time. While Vaccine Effectiveness (VE) against COVID-19 hospitalizations is acknowledged to be very high among messenger RNA (mRNA) vaccines, this presentation reports a sub-analysis of healthcare system COVID-19 VE data limited to individuals aged >16 years of age who were hospitalized during the Omicron dominant era of the pandemic. Data was extracted from a large midwestern hospital system between December 20, 2021 and July 23, 2022. Key eligibility criteria included COVID-19-like symptomatic patients with PCR-confirmed test results; patients with partial vaccination or non-mRNA vaccines were excluded. This retrospective test-negative study design compared SARS-CoV-2 virus detection in boosted (3 doses) vs unvaccinated patients and fully vaccinated (2 doses) vs unvaccinated patients from 17,262 individuals aged 16-64 years and 17,817 individuals >65 years of age. Among the older age group, VE was higher in the boosted group than in the fully vaccinated group, when comparing boosted vs unvaccinated and fully vaccinated vs unvaccinated (VE: 63.0% and 43.4%, respectively). Similar differences in VE were also identified among patients aged 16-64 years, when comparing boosted vs unvaccinated and fully vaccinated vs unvaccinated (VE: 61.8% and 42.9%, respectively). These results suggest a notable reduction in COVID-19-associated illness leading to hospitalizations between boosted and unvaccinated patients during the Omicron era, with evidence of immunity when comparing fully vaccinated and unvaccinated patients. Vaccine effectiveness at preventing hospitalizations remains important to measure, and VE estimate adjustment should consider varying dose schedules, in addition to time-sensitive factors for waning natural and vaccine-acquired immunity, dominant variant in circulation, local transmission trends, and public health or mandate decisions. Despite limitations, these VE statistics present unique data from an integrated health system and regional evidence of vaccine protection during a newly emerging viral variant.