Atlanta—As of early February, the SARS-CoV-2 Omicron BA.2–related sublineage XBB.1.5 became the predominant Omicron lineage in the United States, accounting for an estimated 66.4% of all new cases, according to the CDC’s COVID Data Tracker Weekly.

The only major lineage currently increasing in proportion in the country, XBB.1.5 is a sublineage of the XBB lineage, which is a combination of two earlier Omicron lineages: BM.1.1 and BJ.

At the same time, the BQ.1 and BQ.1.1 sublineages of the BA.5 lineage of the Omicron variant are continuing to decrease in proportion nationally, presenting just more than one-fourth of cases by the second week in February.

Those changes raise an important question: How effective are bivalent COVID-19 vaccines against XBB and XBB.1.5?

The answer is in line with other recent variants. Working with Walgreens, the CDC used spike S-gene target presence as a proxy for BA.2 sublineages, including XBB and XBB.1.5, in December and January. The results indicated that a bivalent mRNA booster dose provided additional protection against symptomatic XBB/XBB.1.5 infection for at least 3 months after vaccination in those who had previously received two to four monovalent vaccine doses.

“As new SARS-CoV-2 variants emerge, continued vaccine effectiveness [VE] monitoring is important,” according to the authors of the study published in the Morbidity & Mortality Weekly Report. “All persons should stay up to date with recommended COVID-19 vaccines, including receiving a bivalent booster dose when eligible.”

The researchers analyzed data from the Increasing Community Access to Testing national pharmacy program for SARS-CoV-2 testing to estimate VE of updated (bivalent) mRNA COVID-19 vaccines against symptomatic infection caused by BA.5-related and XBB/XBB.1.5–related sublineages among immunocompetent adults from December 1, 2022, to January 13, 2023.

“This report provides the first estimates of bivalent mRNA COVID-19 VE against symptomatic SARS-CoV-2 infection with XBB-related sublineages,” according to the study team. “These preliminary estimates from national pharmacy testing conducted during December 1, 2022-January 13, 2023, showed relative bivalent booster dose VE (compared with two to four monovalent doses) to be similar for XBB/XBB.1.5 sublineage–related infections and BA.5 sublineage–related infections. VE estimates for both sublineages included in this analysis were similar to estimates from the same ICATT network published during a period of Omicron BA.5, BQ.1, and BQ.1.1 sublineage circulation in fall 2022.”

Concerns about a reduction in VE against the emergency variants had been raised based on early immunogenicity studies indicating lower neutralizing activity against XBB compared with other Omicron sublineages after receiving a bivalent booster dose compared with other Omicron sublineages. The article explains that bivalent boosters contain mRNA encoding the S-gene from the SARS-CoV-2 ancestral strain and Omicron BA.4/BA.5 sublineages, pointing out that XBB and XBB.1.5, however, are descendants of the Omicron BA.2 sublineage.

“Findings from this study suggest that bivalent booster doses are continuing to provide additional protection against symptomatic infection for at least the first 3 months after vaccination in persons who had previously received 2, 3, or 4 monovalent vaccine doses, which supports recommendations to continue to increase bivalent booster coverage,” the CDC noted.

The authors emphasized that results from the analysis of national pharmacy testing data argue for everyone to stay up to date on recommended COVID-19 vaccines, including receiving a bivalent booster dose when eligible.

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