The analysis found that the updated vaccine, which targets the BA.4 and BA.5 Omicron variants and the original virus strain, helped prevent illness in people who had previously received two to four doses of the original COVID-19 vaccine, CDC said.
The analysis looked at cases from Dec. 1 until Jan. 13, a time period in which U.S. circulation of XBB and XBB.1.5 increased. The subvariants are descendants of Omicron, the most contagious variant of the virus causing COVID-19.
The CDC said the updated vaccine worked similarly against BA.5-related infections and XBB/XBB.1.5-related infections. It was 52% effective at preventing infections against BA.5 and 48% against XBB/XBB.1.5 among those aged 18-49. Effectiveness fell to 37% against BA.5 and 43% against XBB/XBB.1.5 among those aged 65 years and older.
The CDC also said that the study shows how the vaccines work on a population level, but individuals might not see the same benefit. People should stay up to date with recommended COVID-19 vaccines, the CDC said.
XBB.1.5 was estimated to make up nearly half of U.S. cases in the week ended Jan. 21, government data showed.
The CDC analysis comes ahead of a meeting on Thursday at which outside experts to the U.S. Food and Drug Administration are expected to discuss whether and how the United States should offer the COVID vaccine as an annual shot.