JYNNEOS (modified vaccinia Ankara vaccine, Bavarian Nordic) is licensed in the United States as a two-dose smallpox and mpox vaccine, the doses 28 days apart. One dose was known to reduce mpox lesions and hospitalization; the effect of two doses — especially for people with HIV, who are at higher risk of severe mpox — hadn't been measured. Observational studies had put its effectiveness at 66% to 89% for two doses and 36% to 75% for one.
From May 12, 2022, to May 18, 2023, 5,765 Californians had mpox, and 250 were hospitalized. California began distributing JYNNEOS at the end of May 2022; about 300,000 doses have since been given, reaching an estimated 64% of the state's at-risk population with one dose and 40% with two.
The study
The California Department of Public Health matched interviews with people who had laboratory-confirmed mpox to the California Immunization Registry and to its Office of AIDS HIV records. 1,154 people (20.0%) were excluded because their hospitalization status was missing, leaving 4,611 — 94.4% male, 45.2% Hispanic or Latino, and 69.1% gay, lesbian or same-gender loving.
| Group | Definition |
|---|---|
| One dose | one pre-exposure dose at least 14 days before illness (or two doses, if the second was less recent or the doses less than 24 days apart) |
| Two doses | two doses at least 24 days apart, the second at least 14 days before illness |
| Post-exposure (PEP) | a first dose after a known or suspected exposure, less than 14 days before illness |
Results
| Vaccination | Mpox cases | Hospitalized | Odds of hospitalization vs unvaccinated |
|---|---|---|---|
| Unvaccinated | 3,845 (83.4%) | 233 | — |
| One dose | 233 (5.0%) | 4 | 0.27 |
| Two doses | 79 (1.7%) | 1 | 0.20 |
| Post-exposure | 457 (9.9%) | 12 | 0.42 |
In all, 250 (5.4%) of the 4,611 were hospitalized.
People with HIV
People with HIV were 40.7% of patients but 56.0% of hospitalizations.
| Vaccination | Hospitalized | Odds vs unvaccinated |
|---|---|---|
| Unvaccinated | 131 of 1,585 (8.3%) | — |
| One dose | 2 of 81 (2.5%) | 0.28 |
| Two doses | 0 of 19 | — |
| Post-exposure | 7 of 193 (3.6%) | 0.42 |
Among people without HIV, the odds for one dose, two doses and post-exposure vaccination were 0.29, 0.36 and 0.41.
What it means
- Vaccination — before or after exposure — lowered the odds of hospitalization, whether or not people had HIV.
- Too few cases to prove two doses beat one overall — but among people with HIV, none with two doses was hospitalized.
- Everyone eligible at risk of mpox — especially people with HIV — should complete both doses for lasting protection.
- Since no vaccine is 100% effective, infections in vaccinated people will rise as coverage grows; more work is needed on how JYNNEOS affects disease severity, and how HIV viral suppression and CD4 counts shape the response.
Limits
People who got sick within 14 days of a first dose were assumed to have had PEP, but exposure timing couldn't be confirmed; misclassification, confounding and selection bias are possible — if sicker people were less likely to be vaccinated, the benefit may be overstated; vaccinated patients may have had better access to care; and excluded patients differed in race and ethnicity, sexual orientation, HIV and age.
Sources
Based on Schildhauer S, Saadeh K, Vance J, et al., "Reduced Odds of Mpox-Associated Hospitalization Among Persons Who Received JYNNEOS Vaccine — California, May 2022–May 2023," MMWR volume 72, number 36, Centers for Disease Control and Prevention, as corrected by its erratum; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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