On September 12, 2023, CDC’s Advisory Committee on Immunization Practices recommended an updated 2023–2024 COVID-19 vaccine, based on the Omicron XBB.1.5 variant, for everyone aged 6 months and older. Uptake stayed low, even among adults 65 and older — while thousands of Americans were still hospitalized with COVID-19 each week, about 31,000 in the week of January 7–13, 2024.
CDC measured how well the updated dose protected immunocompetent adults from September 2023 to January 2024, using two networks and a test-negative design, which compares vaccination among people with COVID-like illness who tested positive with those who tested negative.
The networks
- VISION: health records from 369 emergency departments and urgent care centers and 229 hospitals in eight states.
- IVY: 26 hospitals in 20 states, enrolling patients prospectively, with central testing and genetic sequencing of the virus.
In both, people who got the updated dose were compared with people who did not — whether never vaccinated or given only earlier COVID-19 vaccines.
Results
Emergency department and urgent care visits (VISION) — 128,825 visits, 17,229 of them COVID-positive:
| Time since updated dose | Effectiveness (95% CI) |
|---|---|
| 7–59 days | 51% (47%–54%) |
| 60–119 days | 39% (33%–45%) |
The pattern held for adults 18–64 (52%, then 45%) and 65 and older (49%, then 37%).
Hospitalization:
| Network | Effectiveness |
|---|---|
| VISION (37,503 hospitalizations) | 53% at 7–59 days; 50% at 60–119 days; 54% and 50% for adults 65+ |
| IVY (4,117 patients) | 43% overall (median 47 days after the dose); 48% for adults 65+ |
Adding back patients who had tested positive or indeterminate for influenza did not meaningfully change the results.
Against a changing virus
The updated vaccine targeted XBB.1.5, the dominant variant in the first half of 2023. By fall, other lineages that better escape antibodies had spread: among 952 specimens sequenced in IVY from September 21 to December 15, 58% were EG.5-like, 20% HK.3-like, 16% XBB.1.5-like and 6% JN.1-like — close to national surveillance. JN.1, with more than 30 spike changes from XBB.1.5, was first detected in the U.S. in September 2023 and made up approximately 65% of lineages by early January 2024. The vaccine still protected during this mix, but the rapid turnover of lineages argues for ongoing monitoring and periodic vaccine updates.
How it compares
The two networks’ estimates agreed with each other and with another CDC study and a United Kingdom report. Earlier estimates from Denmark and the Netherlands were somewhat higher, likely because they measured protection sooner after vaccination (at most 25 days and 2 months) or used different methods.
Protection against emergency visits waned over time. In 2022–2023, protection against emergency visits and hospitalization waned substantially — by six months it was no longer significantly different from zero in some groups — but protection against critical illness (intensive care, ventilation or death) lasted well over a year.
Limits: some case-patients may have been treated for reasons other than COVID-19; vaccination records may be incomplete, likely underestimating protection; prior infections weren’t accounted for, so results show the added benefit of an updated dose in a largely immune population; and other factors, such as antiviral treatment, may not be fully adjusted for.
Recommendation at the time
CDC concluded the results supported its recommendation that everyone aged 6 months and older get the updated 2023–2024 vaccine, and said it would keep monitoring effectiveness.
Sources
Based on "Interim Effectiveness of Updated 2023–2024 (Monovalent XBB.1.5) COVID-19 Vaccines Against COVID-19–Associated Emergency Department and Urgent Care Encounters and Hospitalization Among Immunocompetent Adults Aged ≥18 Years — VISION and IVY Networks, September 2023–January 2024," MMWR Vol. 73, No. 8, CDC; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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