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The updated 2023-2024 COVID-19 vaccine was approximately 54% effective against symptomatic SARS-CoV-2 infection in adults, and was also effective against the JN.1 variant, which became predominant in January, CDC researchers said.

Overall, vaccine effectiveness against symptomatic COVID was 57% for people ages 18 to 49 years and 46% for people ages 50 and older, reported Ruth Link-Gelles, PhD, of the CDC's National Center for Immunization and Respiratory Diseases, and colleagues in the .

The updated vaccine is a monovalent XBB.1.5-derived vaccine, and there have been few estimates regarding its effectiveness, the authors noted. This study is the first to look at the vaccine's effectiveness against symptomatic COVID caused by the JN.1 variant, a derivative of BA.2.86.

The data came from the CDC's Increasing Community Access to Testing program that provided no-cost SARS-CoV-2 nucleic acid amplification tests (NAAT) to uninsured people at participating CVS and Walgreens pharmacies from Sept. 21, 2023 to Jan. 14, 2024.

Vaccine effectiveness was 58% among those who received testing 7 to 59 days after receiving the updated vaccine, and 49% among those who received testing 60 to 119 days after receipt.

In a subanalysis, the researchers also looked at spike gene amplification results from reverse transcription-polymerase chain reaction (RT-PCR) COVID tests to differentiate XBB lineages from JN.1 and other Omicron BA.2.86 lineages.

For those who had received the updated vaccine 60 to 119 days earlier, vaccine effectiveness was 49% for tests indicating infection with JN.1 lineages, and 60% for tests indicating infection with non-JN.1 lineages.

"Whereas the goal of the U.S. COVID-19 vaccination program is to prevent severe disease, vaccine effectiveness against symptomatic infection can provide useful insights into protection early after introduction of updated vaccines and during the emergence of new lineages," Link-Gelles and colleagues wrote.

The study only looked at data through 119 days since vaccination -- a relatively brief period of time, the authors pointed out. "Waning of effectiveness is expected with additional elapsed time since vaccination, especially against less severe disease," they noted.

In September 2023, the CDC's Advisory Committee on Immunization Practices (ACIP) recommended that everyone 6 months of age and older get the 2023-2024 updated COVID vaccine.

This analysis looked at 9,222 NAAT COVID test results over the study period among people with COVID-like symptoms. Most people tested were women (61%), about 40% were white, 30% were Hispanic or Latino, and 16% were Black or African American.

Of available NAAT tests, 36% were positive for SARS-CoV-2. The researchers calculated vaccine effectiveness by comparing odds of receipt versus nonreceipt of the updated COVID vaccine among those who tested positive (case patients) and those who tested negative (control patients). Of the 1,125 individuals who had received an updated COVID vaccine at least 7 days before testing, 14% were SARS-CoV-2-negative and 9% were positive.

Notably, only 12% of all people tested reported that they had received an updated vaccine dose, and over 26% had never received any COVID vaccine. Sixty percent reported a previous SARS-CoV-2 infection more than 3 months before the current test.

Link-Gelles and team pointed out that vaccination status, previous infection history, and underlying medical conditions were self-reported and subject to bias. Since there is a high prevalence of infection-induced SARS-CoV-2 immunity among adults in the U.S., previous infection was probably underreported and likely provided some protection against repeat infection.

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