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Effectiveness of the 2023-to-2024 XBB.1.5 COVID-19 Vaccines Over Long-Term Follow-up : A Target Trial Emulation
George N Ioannou1, Kristin Berry2, Nallakkandi Rajeevan3
1Research and Development and Division of Gastroenterology, Veterans Affairs Puget Sound Health Care System, and Division of Gastroenterology, University of Washington, Seattle, Washington (G.N.I.).
Background:
Monovalent COVID-19 vaccines targeting the XBB.1.5 Omicron variant were introduced in September 2023. In the absence of randomized controlled trials demonstrating their efficacy, information on real-world vaccine effectiveness (VE) is needed.
Objective:
To determine XBB.1.5 COVID-19 VE and the extent to which it declines over time.
Design:
Target trial emulation.
Setting:
U.S. Veterans Health Administration.
Participants:
Eligible XBB.1.5 vaccine recipients were matched 1:1 to unvaccinated persons in 7 sequential biweekly trials with enrollment from 2 October 2023 through 3 January 2024.
Intervention:
XBB.1.5 COVID-19 vaccination versus no XBB.1.5 vaccination.
Measurements:
Outcomes were ascertained through 10 May 2024 and included any positive result on a SARS-CoV-2 test from day 10 after the matched index date, subsequent hospitalization within 1 day before or 10 days after the positive result, or death within 30 days after the positive result. Vaccine effectiveness was estimated as 100 × (1 - risk ratio).
Results:
Participants (91.3% male; mean age, 69.9 years) included 587 137 pairs of vaccinated and matched unvaccinated persons. Over a mean follow-up of 176 days (range, 118 to 211 days), VE was -3.26% (95% CI, -6.78% to -0.22%) against documented SARS-CoV-2 infection, 16.64% (CI, 6.47% to 25.77%) against SARS-CoV-2-associated hospitalization, and 26.61% (CI, 5.53% to 42.32%) against SARS-CoV-2-associated death. When estimated at 60, 90, and 120 days, respectively, VE against documented infection (14.21%, 7.29%, and 3.15%), hospitalization (37.57%, 30.84%, and 25.25%), or death (54.24%, 44.33%, and 30.25%) showed substantial waning.
Limitation:
Potential for residual confounding and incomplete capture of COVID-19 vaccination and SARS-CoV-2-related outcomes.
Conclusion:
COVID-19 vaccines targeting the XBB.1.5 variant of Omicron were not effective in preventing infection and had relatively low VE against hospitalization and death, which declined rapidly over time.
Primary Funding Source:
U.S. Department of Veterans Affairs.
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