Can Mpox Vaccines Reduce Transmission or Hospitalization

Since the May 2022 Mpox outbreak began, people have been lining up to receive a smallpox vaccine authorized by the U.S. Food and Drug Administration (FDA).

Since then, numerous studies have been conducted to quantify if Bavarian Nordic's JYNNEOS® (MVA-BN) vaccine is effective in reducing virus transmission or hospitalizations against the mpox West African clade IIb subtype.

To better understand JYNNEO's actual effectiveness, the U.S. Centers for Disease Control and Prevention (CDC) Morbidity and Mortality Weekly Report published on September 8, 2023, found that from May 12, 2022–May 18, 2023, of the Californians in this study, 250 (4.3%) reported mpox-associated hospitalizations, of which 233 were unvaccinated.

The odds of hospitalization among persons with mpox who had received 1 or 2 JYNNEOS doses were 0.27 (95% CI = 0.08–0.65) and 0.20 (95% CI = 0.01–0.90), respectively, compared with unvaccinated mpox patients.

In mpox patients with HIV infection, the odds of hospitalization among those who had received 1 JYNNEOS vaccine dose was 0.28 (95% CI = 0.05–0.91) times that of those who were unvaccinated.

And no mpox-associated hospitalizations were identified among persons infected with HIV who had received 2 JYNNEOS vaccine doses. 

These findings underscore the benefit to persons with HIV infection of completing the 2-dose JYNNEOS vaccination series, wrote the CDC.

Efforts should understand better the relationship between HIV and the Mpox virus, including how viral suppression and CD4 counts might affect the immune response to mpox in response to vaccination.

However, the current CDC study could not determine this because of the limited number of mpox infections among vaccinated persons with HIV.

In a separate CDC study (Volume 29, Number 10—October 2023) published on September 1, 2023, among 38 patients with no known exposure to a person with mpox, behaviors preceding illness included sexual activity (45%), close face-to-face contact (37%), attending large social gatherings (29%), and being in occupational settings (26%).

Additionally, the CDC reported while the 2022–2023 mpox outbreak predominantly affected adult men, 1.3% of reported cases were in adolescents less than 18 years of age.

And because the JYNNEOS vaccine is not 100% effective in preventing transmission, the number of mpox reinfections occurring in vaccinated persons will likely increase as more are vaccinated, wrote the CDC. During 2023, breakthrough infections have been confirmed in Chicago and Denver.

As of August 22, 2023, 1,257,943 JYNNES doses had been administered in 57 U.S. Jurisdictions, and vaccinations remain available in various urban centers in the U.S.

Furthermore, the CDC is not recommending a third JYNNEOS dose as of September 2023.

However, the CDC's vaccine committee unanimously recommended JYNNEOS boosters in persons who received the smallpox vaccine ACAM2000 as the primary mpox vaccine.