Smallpox Vaccination Protected Military Personnel from Mpox

During the ongoing mpox outbreak, older smallpox vaccines were used to prevent infection and reduce disease severity in those at increased risk for infection.

However, the effectiveness of smallpox vaccination against mpox was unknown.

On September 21, 2023, the New England Journal of Medicine published a Correspondence that stated previous vaccination at a median of 13 years earlier with either a first- or second-generation smallpox vaccine reduced the likelihood of testing positive for orthopoxvirus, including mpox, among current or former military personnel for whom vaccination data were available.

The estimated vaccine efficacy was 72% for Dryvax (1st generation) and 75% for ACAM2000 (2nd generation).

Among the participants who tested positive for orthopoxvirus, 121 (41%) had been diagnosed with HIV infection (odds ratio, 2.34; 95% CI, 1.65 to 3.35).

Within the VA portion of the study, 19 of 186 participants (10%) required hospitalization; there were no deaths from mpox, and all the participants had an uneventful recovery.

The Department of Defense electronic laboratory data and the Veterans Affairs Corporate Data Warehouse identified eligible participants. The study was approved by the institutional review board of Emory University and was granted an exemption for consent.