Infectious Disease

CDC publishes first estimates of bivalent boosters’ effectiveness against XBB.1.5

January 25, 2023

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The authors report no relevant financial disclosures.

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The CDC for the first time has estimated how effective the bivalent COVID-19 vaccines have been against the newest dominant omicron subvariant of SARS-CoV-2.

The subvariant, XBB.1.5, which has been described as the most transmissible SARS-CoV-2 subvariant yet, was responsible for around 49% of new COVID-19 cases in the United States last week, according to CDC tracking.

IDN0123LinkGelles_Graphic_01_WEB

Data derived from Link-Gelles R, et al. MMWR Morb Mortal Wkly Rep. 2023;doi:10.15585/mm7205e1_w.

In new estimates published Wednesday in MMWR, CDC researchers reported that vaccination with a bivalent messenger RNA booster had a relative vaccine effectiveness of 49%, 40% and 43% against symptomatic infection related to XBB or XBB.1.5 among people aged 18 to 49 years , 50 to 64 years and 65 years or older, respectively, compared with receiving only two to four monovalent mRNA vaccine doses.

Vaccine effectiveness against symptomatic BA.5-related infection was 52% in the youngest age group, 43% in the middle age group and 37% in the older age group. (BA.5 accounted for only 2% of new infections last week.)

The estimates were based on data from the Increasing Community Access to Testing national pharmacy program.

According to the authors of the report, there was minimal evidence of waning vaccine effectiveness 2 to 3 months after receiving a bivalent booster, although they called the estimates “imprecise.”

The bivalent boosters were developed to target both the original wild strain of SARS-CoV-2 and the omicron BA.4 and BA.5 subvariants. XBB.1.5 is a sublineage of XBB, which is a recombinant of two BA.2 sublineages.

Although prior studies have shown that the bivalent boosters have provided added protection against symptomatic COVID-19 and reduced the risk for severe outcomes from the disease compared with receiving two to four doses of monovalent mRNA vaccines, only around 15% of the eligible US population has received one of the shots, according to CDC tracking.

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Amesh A. Adalia, MD)

Amesh A. Adalja, MD

It is not surprising that bivalent COVID-19 vaccines provide moderate levels of effectiveness against infection for some period of time with newer variants, such as XBB.1.5. There is one major limitation of the study in that those who elected to receive the bivalent booster are probably more likely to have also adhered to other COVID-19 prevention measures, such as masking, and be relatively more cautious, which could bias the findings in favor of the booster. Nonetheless, it is not clear to me that boosting low-risk people to transiently prevent mild illness from an endemic respiratory virus is an optimal policy. Boosters, in my opinion, are key to the prevention of severe disease and, therefore, should be targeted to those who have a baseline risk for severe disease (this is the approach taken by Denmark and the United Kingdom).

Amesh A. Adalja, MD

Senior scholar

Johns Hopkins Center for Health Security

Disclosures: Adalja reports serving on a speakers bureau for Pfizer.

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