Effectiveness and Impact of Autumn 2023 COVID-19 Vaccination in Preventing Hospitalizations in Navarre, Spain,...

We aimed to estimate autumn 2023 COVID-19 vaccine effectiveness (CVE) in preventing hospitalizations due to COVID-19 until September 2024.

We performed a test-negative case-control study nested in the cohort of adults aged ≥ 45 years with indication of autumn 2023 COVID-19 vaccination in Navarre, Spain. The study included patients hospitalized for severe acute respiratory infection (SARI) and tested by polymerase chain reaction between October 2023 and September 2024. The COVID-19 vaccination statuses in the current and previous seasons were compared between confirmed COVID-19 cases and test-negative controls. CVE was estimated as (1 - adjusted odds ratio) × 100.

Of 4051 SARI hospitalized patients included in the study, 474 (12%) were confirmed for COVID-19. CVE to prevent COVID-19 hospitalizations was 32% (95% confidence interval [CI], 11%-48%) on average for the year and 38% (95% CI, 17%-54%) among people aged ≥ 65 years. However, estimates for current-season vaccination were 51% (95% CI, 30%-66%), 50% (95% CI, 16%-70%), and 0% (95% CI, -42% to 30%) for 7 to 89, 90 to 179, and ≥ 180 days between vaccination and COVID-19 diagnosis, respectively. The residual effect of previous-season vaccination was not statistically significant (14%; 95% CI, -20% to 39%). CVE was moderate in preventing COVID-19 hospitalizations between October 2023 and March 2024 (50%; 95% CI, 28%-65%), and null between April and September 2024 (6%; 95% CI, -41% to 38%). The vaccine averted 19% of COVID-19 hospitalizations. On average, 963 doses of vaccine were necessary to prevent one COVID-19 hospitalization.

CVE was moderate in preventing COVID-19 hospitalizations during the 2023-2024 season, but decreased 6 months after vaccination.