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- What: A study published October 9, 2025, in the New England Journal of Medicine details the declining effectiveness of the JYNNEOS vaccine against mpox (formerly monkeypox).
- Where: Data was collected from a large cohort of men who have sex with men (MSM) in the United States during the 2022-2023 mpox outbreak.
- When: The study covers the period from July 2022 through September 2023,with follow-up extending into early 2024.
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MPox Vaccine Effectiveness Wanes Over Time, But Remains Protective
Table of Contents
The JYNNEOS vaccine, the primary tool used to combat the 2022-2023 mpox outbreak, provides meaningful initial protection against infection. However, new research reveals that its effectiveness diminishes over time, though it continues to offer substantial protection against severe disease, hospitalization, and death. This analysis, published on October 9, 2025, in the New England Journal of Medicine, underscores the importance of ongoing monitoring and potential booster strategies.
The Study: Tracking Vaccine performance
Researchers analyzed data from over 32,000 men who have sex with men (MSM) across multiple U.S. cities. This demographic was disproportionately affected during the mpox outbreak, making it a key population for study. The study meticulously tracked infection rates among vaccinated and unvaccinated individuals, accounting for factors like time since vaccination and risk behaviors.
The study period spanned from July 2022,when the outbreak was gaining momentum,through September 2023,allowing researchers to observe the vaccine’s performance over a critical period. Follow-up data extended into early 2024, providing a longer-term view of protection.
Key Findings: Waning Immunity and Continued Protection
Initial vaccine effectiveness was high, reaching approximately 89% within the first six months after completing the two-dose series. Though, this protection began to decline noticeably after that point. By 12 to 18 months post-vaccination, effectiveness dropped to around 64%.
Importantly, even with waning protection against infection, the vaccine continued to provide strong defense against severe outcomes. Hospitalization rates were considerably lower among vaccinated individuals, even those with reduced protection against infection. The study found no deaths among vaccinated participants.
| Time Since Vaccination | Vaccine Effectiveness (against Infection) |
|---|---|
| 0-6 Months | 89% |
| 6-12 Months | 78% |
| 12-18 Months | 64% |
| 18+ Months | Data Limited, but further decline expected |
Why Does Vaccine Effectiveness Wane?
Several factors likely contribute to the decline in vaccine effectiveness. The JYNNEOS vaccine is a modified vaccinia Ankara (MVA) vaccine, which generally elicits a robust but possibly less durable immune response compared to some othre vaccine technologies.
Exposure to the virus, even mild or asymptomatic infections, can also boost immunity. However, the study suggests that natural exposure alone may not provide the same level of protection as vaccination, particularly against severe disease.
Who is Affected? Implications for Public Health
The findings have significant implications for public health strategies. Individuals previously vaccinated, particularly those who completed their series more than a year ago, might potentially be at increased risk