Effectiveness of Smallpox Vaccination to Prevent Mpox in Military Personnel

This test-negative, case-control study evaluated the effectiveness of prior smallpox vaccination in preventing mpox (monkeypox) among 1014 U.S. military personnel and veterans during the 2022 outbreak. Vaccination data spanning Dryvax (first-gen) and ACAM2000 (second-gen) were linked with orthopoxvirus testing outcomes across VA and DoD records. Key findings:

Positive test rate among previously vaccinated participants was lower:

Dryvax: OR = 0.28 (95% CI: 0.13–0.58) → 72% estimated efficacy

ACAM2000: OR = 0.25 (95% CI: 0.15–0.42) → 75% estimated efficacy

Median time from vaccination to diagnosis: 13 years

HIV-positive individuals had higher odds of mpox infection:

OR = 2.34 (95% CI: 1.65–3.35)

Older age groups showed lower infection risk, especially ≥65 years (OR = 0.04)

Within the VA cohort: 10% hospitalization, no fatalities, complete recovery Authors conclude that long-term protection is retained for over a decade post-vaccination with both vaccine generations, reinforcing utility of historical immunizations in outbreak contexts.

This test-negative, case-control study evaluated the effectiveness of prior smallpox vaccination in preventing mpox (monkeypox) among 1014 U.S. military personnel and veterans during the 2022 outbreak. Vaccination data spanning Dryvax (first-gen) and ACAM2000 (second-gen) were linked with orthopoxvirus testing outcomes across VA and DoD records. Key findings:

Positive test rate among previously vaccinated participants was lower:

Dryvax: OR = 0.28 (95% CI: 0.13–0.58) → 72% estimated efficacy

ACAM2000: OR = 0.25 (95% CI: 0.15–0.42) → 75% estimated efficacy

Median time from vaccination to diagnosis: 13 years

HIV-positive individuals had higher odds of mpox infection:

OR = 2.34 (95% CI: 1.65–3.35)

Older age groups showed lower infection risk, especially ≥65 years (OR = 0.04)

Within the VA cohort: 10% hospitalization, no fatalities, complete recovery Authors conclude that long-term protection is retained for over a decade post-vaccination with both vaccine generations, reinforcing utility of historical immunizations in outbreak contexts.