2024 COVID-19 vaccine, formulated to target the XBB.1.5 Omicron subvariant, offered significant additional protection against medically attended COVID-19 cases in adults, according to a recent study published in JAMA Network Open on June 24. The research, conducted by the U.S. Centers for Disease Control and Prevention (CDC), underscores the continued value of updated booster shots amid evolving coronavirus strains.

Led by Dr. Ruth Link-Gelles and a team of CDC researchers, the case-control study analyzed data from over 450,000 adult emergency department (ED) and hospital visits across the United States during the periods dominated by the Omicron XBB and JN.1 variants. The study aimed to measure the effectiveness of the updated 2023–2024 monovalent vaccine in preventing COVID-19-associated emergency care, hospitalization, and critical illness.

Among 345,639 eligible ED and urgent care (UC) visits from adults with COVID-19-like symptoms, around 11 percent tested positive for the SARS-CoV-2 virus. The updated vaccine demonstrated an overall effectiveness (VE) of 24 percent against ED and UC visits related to COVID-19 between 7 and 299 days after vaccination.

For hospitalizations, the findings were even more reassuring. Among 111,931 eligible cases, 9 percent of patients tested positive for COVID-19. The vaccine reduced the risk of COVID-19-related hospitalization by 29 percent and the risk of critical illness by 48 percent during the same timeframe.

Notably, protection was strongest in the early weeks following vaccination. During the first 7 to 59 days, the vaccine was 49 percent effective at preventing ED and UC visits, 51 percent effective against hospitalization, and 68 percent effective at reducing the risk of critical illness. However, vaccine effectiveness waned over time. By 180 to 299 days post-vaccination, protection dropped to −7 percent for ED visits, −4 percent for hospitalizations, and 16 percent for critical illness, suggesting diminishing benefits as time passes.

"The 2023–2024 COVID-19 vaccines were associated with additional protection against COVID-19-associated ED and UC encounters, hospitalization, and critical illness beyond existing protection provided by prior vaccination or SARS-CoV-2 infection," the study authors wrote.

The findings highlight the evolving challenge of maintaining immunity against COVID-19 in the face of rapidly changing variants. While the updated vaccine does provide meaningful short-term protection, especially against severe outcomes, the decline in effectiveness over time may prompt discussions around the need for further boosters or new formulations in the future.

The study also noted that several of its authors have disclosed affiliations with pharmaceutical companies, a standard declaration in medical research to ensure transparency.

As health authorities continue to assess future vaccination strategies, this research reinforces the importance of timely booster doses, particularly for highhigh-risk populations, to maintain protection during periods of variant surges.