The COVID-19 Hospitalization Metric in the Pre- and Post-vaccination Eras as a Measure of Pandemic Severity: A Retrospective, Nationwide Cohort Study

Importance: Since the early days of the pandemic, COVID-19 hospitalizations have been used as a measure of pandemic severity. However, case definitions do not include assessments of disease severity, which may be impacted by prior vaccination. Objective: To measure how the severity of respiratory disease changed among inpatients with documented SARS-CoV-2 infection and to measure the impact of vaccination status on these trends, in order to evaluate the accuracy of the metric of “hospitalization plus a positive SARS-CoV-2 test” for tracking pandemic severity. Design: Retrospective cohort of inpatients with laboratory-confirmed SARS-CoV-2. All data were obtained from electronic health records. Setting: Multi-center, nationwide study conducted in the healthcare system of the US Department of Veterans Affairs (VA) from March 1, 2020, through June 30, 2021. Participants: All VA patients admitted to a VA hospital with a laboratory-confirmed SARS-CoV-2 infection within the 14-days prior to admission or during the hospital admission. Main Outcome: Moderate-to-severe COVID-19 disease, defined by use of any supplemental oxygen or documented SpO2 <94%, during an inpatient hospitalization between one day before and two weeks after a positive SARS-CoV-2 test. Exposure: SARS-CoV-2 vaccination status at the time of hospitalization. Patients were regarded as fully vaccinated starting 14 days after receiving the second of a 2-dose regimen or 14 days after receipt of a single-dose vaccine. Results: Among 47,742 admissions in 38,508 unique patients with laboratory-confirmed SARS-CoV-2, N=28,731 met the criteria for moderate-to-severe COVID-19. The proportion with moderate-to-severe disease prior to widespread vaccine availability was 64.0% (95% CI, 63.1-64.9%) versus 52.0% in the later period (95% CI, 50.9-53.2%), p-value for non-constant effect, <0.001. Disease severity in the vaccine era among hospitalized patients was lower among both unvaccinated (55.0%, 95% CI, 53.7-56.4%) and vaccinated patients (42.6%, 95% CI, 40.6-44.8%). Conclusions and Relevance: The proportion of hospitalizations that are due to severe COVID-19 has changed with vaccine availability, thus, increasing proportions of mild and asymptomatic cases are included in hospitalization reporting metrics. The addition of simple measures of disease severity to the case definition of a SARS-CoV-2 hospitalization is a straightforward and objective change that should improve the value of the metric for tracking SARS-CoV-2 disease burden.