Effectiveness of mRNA vaccines in preventing COVID-19 hospitalization by age and burden of chronic medical conditions among immunocompetent US adults, March-August 2021

In a multi-state network, vaccine effectiveness (VE) against COVID-19 hospitalizations was evaluated among immunocompetent adults (≥18-years) during March-August 2021 using a case-control design. Among 1669 hospitalized COVID-19 cases (11% fully vaccinated) and 1950 RT-PCR-negative controls (54% fully vaccinated), VE was higher at 96% (95% CI: 93-98%) among patients with no chronic medical conditions than patients with ≥3 categories of conditions (83% [95% CI: 76-88%]). VE was similar between those aged 18-64 years vs ≥65 years (p\u3e0.05). Vaccine effectiveness against severe COVID-19 was very high among adults without chronic conditions and lessened with increasing burden of comorbidities. Keywords: COVID-19; chronic medical conditions; preexisting conditions; vaccine effectiveness