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On September 12, 2023, CDC's Advisory Committee on Immunization Practices recommended an updated 2023–2024 COVID-19 vaccine for everyone 6 months and older. Unlike earlier vaccines, it contained only a component from the Omicron XBB.1.5 lineage — no original strain. Most people 5 and older needed one dose.
Then a new variant arrived. JN.1, an Omicron BA.2.86 lineage detected in the U.S. in September 2023, carries more than 30 spike mutations compared with XBB.1.5, which might help it evade antibodies. XBB lineages dominated until December; by the two weeks ending January 6, 2024, JN.1 caused 69% of U.S. infections. Would the new vaccine still work?
Image from CDC's report
How it was measured
CDC's Increasing Community Access to Testing (ICATT) program offers free COVID-19 tests at CVS and Walgreens pharmacies nationwide to uninsured people, prioritizing socially vulnerable areas. CDC compared vaccination among adults with symptoms who tested positive versus negative between September 21, 2023 and January 14, 2024. People who were immunocompromised, recently infected or recently vaccinated with older doses were excluded.
Telling JN.1 apart: in one widely used PCR test, JN.1 and other BA.2.86 viruses cause S-gene target failure (SGTF) — the spike gene doesn't amplify — while XBB viruses don't. So SGTF served as a stand-in for likely JN.1.
Results
Of 9,222 tests, 3,295 (36%) were positive. 9% of positive cases and 14% of negative controls had the updated vaccine.
| Group | Added protection against symptomatic infection |
|---|---|
| All adults (median 52 days after vaccination) | 54% |
| Ages 18–49 | 57% |
| Ages 50 and older | 46% |
| 7–59 days after vaccination | 58% |
| 60–119 days after vaccination | 49% |
Against JN.1 — 60–119 days after vaccination:
| Likely variant | Protection |
|---|---|
| JN.1 (SGTF) | 49% |
| Other lineages, mainly XBB (no SGTF) | 60% |
The confidence intervals overlapped, so the difference is not statistically significant. Laboratory studies also show the updated vaccine produces antibodies that neutralize both XBB lineages and JN.1.
What it means
- The updated vaccine gave about 54% extra protection on top of whatever immunity people already had from past vaccination or infection — and most U.S. adults have infection-derived immunity.
- It protected against JN.1 as well as the other circulating lineages.
- Protection is expected to wane over time, especially against milder illness; these data cover only the first 119 days. CDC will keep tracking effectiveness, including against severe disease — the main goal of the vaccination program.
- Everyone 6 months and older should get an updated dose. As of January 13, 2024, only about 22% of adults had.
Limits: vaccination, past infection and health conditions were self-reported; some people may have had more than one updated dose; people who seek testing may differ from others; early vaccine takers may differ from later ones; SGTF is only a proxy for JN.1; and time since earlier doses wasn't controlled for.
Sources
Based on Ruth Link-Gelles, Allison Avrich Ciesla, Josephine Mak and others, "Early Estimates of Updated 2023–2024 (Monovalent XBB.1.5) COVID-19 Vaccine Effectiveness Against Symptomatic SARS-CoV-2 Infection Attributable to Co-Circulating Omicron Variants Among Immunocompetent Adults — Increasing Community Access to Testing Program, United States, September 2023–January 2024," MMWR, volume 73, Centers for Disease Control and Prevention; a work of the United States government in the public domain.
Licenza: CC0 1.0 (pubblico dominio) · Tratto da www.cdc.gov
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