When mpox spread in 2022, mainly through sexual contact among gay, bisexual and other men who have sex with men, the United States rolled out JYNNEOS, a vaccine licensed as a two-dose series whose protection against mpox had been judged only from immune-response studies in people and challenge studies in animals. A study by the New York State Department of Health measured it in the outbreak itself: two doses were 88.5% effective against diagnosed mpox.
The rollout
| 2022 | |
|---|---|
| June 2 | First mpox case reported in New York State outside New York City |
| June 28 | The federal Administration for Strategic Preparedness and Response announces a phased rollout of JYNNEOS from the Strategic National Stockpile, prioritizing post-exposure prophylaxis and vaccination of people at recent or ongoing risk |
| July 6 | New York receives its first 2,206 vials |
| July 29 | A state executive order makes reporting of doses mandatory |
| By September 12 | 35,666 vials delivered |
| June 2–December 31 | 375 mpox cases reported outside New York City; 27,385 doses recorded — 16,769 first doses (61%) and 10,616 second doses (39%) |
The vaccine, made by Bavarian Nordic, is given as two doses four weeks apart.
How effectiveness was measured
The study compared men 18 and older diagnosed with mpox from July 24 to October 31, 2022, with "control" men diagnosed in the same period with rectal gonorrhea or primary syphilis who reported sex with men — people with similar risks who were also being tested. Both groups' records were matched to the state immunization registry, and the results were adjusted for week of diagnosis, region, age, and race and ethnicity.
Of 252 men with mpox, just 22 (8.7%) had been vaccinated: 10 with one dose less than 14 days earlier, 10 with one dose at least 14 days earlier, and 2 with both doses. The comparison group had 255 men.
| Vaccination | Effectiveness against diagnosed mpox (95% CI) |
|---|---|
| One dose (at least 14 days earlier) or two doses | 75.7% (48.5%–88.5%) |
| One dose | 68.1% (24.9%–86.5%) |
| Two doses | 88.5% (44.1%–97.6%) |
| One dose less than 14 days earlier | No protection |
Credit: CDC, Morbidity and Mortality Weekly Report*.*
The short-interval result may mix people vaccinated after an exposure with people exposed before their immune response could develop; the study was not designed to measure post-exposure protection.
Checking the result
Four sensitivity analyses tested the assumptions. Leaving out men 50 and older, who may have had smallpox vaccine before routine U.S. vaccination ended in 1972, gave nearly identical results. Adding 71 secondary syphilis cases to the controls gave 64.8%; limiting controls to men tested because of symptoms or a partner's referral gave 63.6%; limiting to men of known race and ethnicity gave 80.5%. Every version showed moderately high protection.
Other studies had found similar protection: CDC's multi-jurisdiction data showed unvaccinated at-risk men had 9.6 times the incidence of two-dose recipients and 7.4 times that of one-dose recipients, a U.K. analysis found 78% one-dose effectiveness, and an Israeli study 86%. But such cohort studies struggle to account for differences in risk; a case-control design like this one builds in control for risk behavior and testing.
Limits
Some confounding may remain. Doses could be undercounted — reporting was optional before July 29, and doses given out of state are not recorded — which would make the vaccine look less effective. Behavior after vaccination could not be measured, the outbreak ended too quickly to measure how long protection lasts, and there were too few data to compare injection routes or look at HIV. The study measured diagnosed, symptomatic mpox, not symptomless infection or onward spread. How much of the outbreak's rapid decline in 2022 came from vaccination, changed behavior or seasonal patterns is unknown.
The bottom line: the findings support the recommended two-dose JYNNEOS series, in line with CDC and New York State guidance, and folding the vaccine into broader sexual health services. With many people at risk still unvaccinated, the authors warned that mpox could spread again in summer 2023.
Sources
Based on Eli S. Rosenberg, Vajeera Dorabawila, Rachel Hart-Malloy, et al., "Effectiveness of JYNNEOS Vaccine Against Diagnosed Mpox Infection — New York, 2022," Morbidity and Mortality Weekly Report, volume 72, Centers for Disease Control and Prevention; 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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