Did the Bivalent COVID-19 Booster Work Against XBB? Early CDC Estimates, January 2023

An MMWR Early Release from January 25, 2023. Vaccines and variants have changed since; check CDC's current advice.

The Omicron sublineage XBB was first found in the United States in August 2022. With its offshoot XBB.1.5, it made up more than half of sequenced cases in the Northeast by the end of December 2022, and 52% nationwide by January 21, 2023. How well the updated bivalent mRNA boosters — built on the original strain and on Omicron BA.4/BA.5 — protected against these BA.2-descended variants was unknown, and early lab studies had found weaker neutralisation of XBB.

How it was measured

The study used tests from the Increasing Community Access to Testing (ICATT) programme, which offers testing at pharmacies and community sites in socially vulnerable areas. Adults with COVID-like symptoms tested between December 1, 2022, and January 13, 2023, reported their vaccine history, symptoms, past infections and conditions.

A quirk of the lab test separated the variants: in one widely used PCR kit, BA.5-related sublineages (including BQ.1 and BQ.1.1) fail to show the spike (S) gene target, while BA.2-related ones, including XBB and XBB.1.5, show it. Positive tests were sorted that way and compared with negative tests. The study counted only immunocompetent adults who had received 2 to 4 doses of the original (monovalent) mRNA vaccine, and compared those who then got a bivalent booster with those who did not.

Of 29,175 eligible tests, 13,648 were positive: 10,596 BA.5-related and 3,052 XBB/XBB.1.5-related. Bivalent boosters were reported by 34% of those testing negative, against 21–22% of those testing positive.

Results

* The report's summary gives 48% for this group; its results section gives 49%.

• Protection was generally similar against both groups of variants, and similar to earlier estimates from the same network during BA.5 and BQ.1 circulation.
• There was little sign of waning by 2–3 months after the booster, though the estimates were imprecise.
• Analyses starting later in December, when XBB.1.5 made up more of the sample, gave similar results.

What it meant

The bivalent booster added protection against symptomatic XBB/XBB.1.5 infection for at least the first three months in people who had had two, three or four earlier doses, supporting the push to raise booster coverage. CDC advised everyone to stay up to date, including a bivalent booster when eligible, and stressed that effectiveness must keep being monitored as new variants appear. The vaccine programme's main goal remained preventing severe disease, but protection against symptomatic infection gives an early read on new variants.

Limits

Vaccination, past infection and health conditions were self-reported; past infection is likely under-reported and gives some protection itself, which could bias estimates toward no effect. Booster coverage was low (6–39% across adult age groups by January 14, 2023), and early boosters may differ from later ones. Exposure, mask use and testing habits weren't measured. Time since the last original dose wasn't controlled for, though that protection wanes fast. The XBB-type group also included some other BA.2-related sublineages.

Sources

• Link-Gelles R, Ciesla AA, Roper LE, et al. "Early Estimates of Bivalent mRNA Booster Dose Vaccine Effectiveness in Preventing Symptomatic SARS-CoV-2 Infection Attributable to Omicron BA.5– and XBB/XBB.1.5–Related Sublineages Among Immunocompetent Adults — Increasing Community Access to Testing Program, United States, December 2022–January 2023." MMWR, CDC. https://www.cdc.gov/mmwr/volumes/72/wr/mm7205e1.htm
• Graphic: CDC.
• Rewritten in hubnx's own words.