Study population and setting
Between March 11 and July 14, 2021, a case control study to assess the effectiveness of mRNA COVID-19 vaccines to prevent COVID-19 hospitalization up to 24 weeks post immunization was performed across 21 hospitals in 18 states in the US. Cases were adults aged 18 and older (1,194 patients) with COVID-19-like illness and a positive RT-PCR or antigen test, and controls (1,895 patients) were similarly aged adults with or without COVID-19-like illness and negative PCR or antigen test results. Inclusion criteria included having a COVID-19 test within 10 days of hospitalization and being hospitalized within 14 days of symptom onset. Patients were considered fully vaccinated if they had received two doses of either the Pfizer or Moderna COVID-19 mRNA vaccine, with the second dose having been administered at least 14 days prior to onset of illness. Whole genome sequencing was performed on specimens from 454 of the case-patients in order to determine which COVID-19 variant caused infection. Vaccine effectiveness (VE) against COVID-19 hospitalizations was estimated using logistic regression analysis adjusting for potential confounding factors, including hospital admission date (bi-weekly intervals), U.S. Department of Health and Human Services region, age, sex, and race/ethnicity. Subgroup analyses were conducted among older adults (65 years+), immunocompromised patients, and patients with three or more chronic medical conditions. The VE for the Delta variant was estimated and compared to other variants
Summary of Main Findings
The median age of participants was 59 years; 49% were female, and 21% were immunocompromised. Overall, 141 (12%) of cases were fully vaccinated, compared to 988 (52%) of the controls. The overall VE was estimated to be 86% over the entire surveillance period, and 90% after excluding immunocompromised patients. There was no significant difference between the pre- and post-Delta VE estimates (87% vs 84%). There was so significant difference in VE estimates 2-12 weeks post-vaccination (86%) and 13-24 weeks post-vaccination (84%).
This surveillance study included hospitalized patients without COVID-19 as controls, increasing the confidence that they came from the same underlying population as cases. It also included immunocompromised patients, older adults, and people with multiple comorbidities, allowing for estimates of VE among these groups at high risk for severe COVID-19 disease. The VE estimates were adjusted for potential confounding factors, including hospital admission date (bi-weekly intervals), U.S. Department of Health and Human Services region, age, sex, and race/ethnicity.
This study focused on Pfizer and Moderna’s mRNA vaccines, so the findings may not reflect the VE of other vaccines.
This study provides evidence of continued high effectiveness of mRNA vaccines to prevent COVID-19 hospitalization in the US, up to 24 weeks post-vaccination.
This review was posted on: 21 September 2021