How effective were COVID-19 vaccines in adults in 2023-2024?

The SARS-CoV-2 virus continues to evolve, with population immunity changes, leading to updating of COVID-19 vaccine formulations. With all of these changes, it is important to monitor the effectiveness of COVID-19 vaccines on an ongoing basis. To address this issue, US researchers evaluated the effectiveness of 2023-2024 COVID-19 vaccines against COVID-19–associated emergency department (ED) and urgent care (UC) encounters, hospitalizations and critical illness, including during XBB- and JN.1-predominant periods.The test-negative design vaccine effectiveness case-control study was conducted using data from September 21, 2023, to August 22, 2024, from EDs, UC centres and hospitals in six US healthcare systems. The study included adults (≥ 18 years) with COVID-19–like illness and molecular or antigen testing for SARS-CoV-2. Case patients had a positive molecular or antigen test result, while control patients had a negative molecular test result. Vaccine exposure was defined as receipt of a 2023–2024 (monovalent XBB.1.5) COVID-19 vaccination with products approved or authorized for use in the United States. The main outcomes were COVID-19–associated ED and UC encounters, hospitalizations and critical illness (admission to the intensive care unit or in-hospital death). Vaccine effectiveness was estimated by comparing the odds of receipt of the 2023-2024 COVID-19 vaccine with no vaccine receipt among case and control patients.Results showed the following:The study included 345,639 eligible ED and UC encounters in immunocompetent adults with COVID-19–like illness and available test results (median age, 53 years; 60% female); 11% had a positive SARS-CoV-2 test result.Vaccine effectiveness against COVID-19–associated ED and UC encounters was 24% (95% CI, 21%–26%) during the seven- to 299-day period after vaccination.Among 111,931 eligible hospitalizations in immunocompetent adults with COVID-19–like illness and available test results (median age, 71 years), 9% had a positive SARS-CoV-2 test result. During the seven- to 299-day period after vaccination, vaccine effectiveness was 29% (95% CI, 25%–33%) against COVID-19–associated hospitalization and 48% (95% CI, 40%–55%) against COVID-19–associated critical illness.Vaccine effectiveness was highest seven to 59 days after vaccination (vaccine effectiveness against ED and UC encounters 49% [95% CI, 46%–52%]; hospitalization, 51% [95% CI, 46%–56%]; critical illness, 68% [95% CI, 56%–76%]. Vaccine effectiveness then waned (vaccine effectiveness 180–299 days after vaccination against ED and UC encounters, −7% [95% CI, −13% to −2%]; hospitalization, −4% [95% CI, −14% to 5%]; and critical illness, 16% [95% CI, −6 to 34%]).The authors conclude that in this case-control study, 2023–2024 COVID-19 vaccines were estimated to be effective against medically attended COVID-19, with the highest and most sustained efficacy against critical illness. As found with earlier formulations of COVID-19 vaccines, vaccine effectiveness waned over time. The authors maintain that their results highlight the importance of adults receiving recommended COVID-19 vaccinations.Reference:Link-Gelles R, Rowley EAK, Irving SA, et al. Estimated 2023-2024 COVID-19 vaccine effectiveness in adults. JAMA Netw Open 2025;8;(6):e2517402. Published online June 25, 2025. (accessed June 27, 2025).

How effective were COVID-19 vaccines in adults in 2023-2024?

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