Previous Smallpox Vaccine Provides Immunity to Mpox, Scientists Discover

Extracellular, brick-shaped monkeypox virions (colorized pink). Backlighting shows surface membranes of the virions and the outlines of nucleocapsids. Courtesy of NIAID.

Throughout the previous year's Mpox outbreak, the virus disseminated beyond Africa for the initial instance, resulting in more than 85,000 recorded instances of the ailment until now. The majority of infections can be attributed to men engaging in sexual relations with other men.

The Mpox-causing virus belongs to the category of orthopoxviruses and bears a striking resemblance to the virus responsible for smallpox, which remained prevalent until its eradication in the mid-1970s.

Given the available data suggesting the effectiveness of the previous smallpox vaccine in providing protection against Mpox, scientists at Karolinska Institutet began contemplating whether individuals who received the vaccine many years ago for smallpox might still possess some level of immunity against Mpox due to a residual memory response.

-"The findings from our study demonstrate that this holds true, indicating that the memory cells exhibit remarkable longevity and possess the ability to identify closely related viruses like the Mpox virus. They can offer overlapping immunity or cross-reactive protection," says Marcus Buggert, the corresponding author of the study and a docent and researcher at the Center for Infectious Medicine, Karolinska Institutet.

Through the examination of the T-cell immune response in 105 healthy blood donors, the researchers discovered that individuals born prior to 1976 displayed a notably robust immune response against both viral types. Furthermore, the researchers evaluated the immune response in 22 men who recently contracted Mpox and found that they also demonstrated a vigorous immune response to the virus. This suggests the potential for future immunity among those individuals.

Although the current study had limited sample size, it was insufficient to determine the extent of protection conferred by prior smallpox vaccination. However, Buggert alludes to a recently published British observational study that investigated the impact of a smallpox vaccine administered to males at risk in 2022. This study may shed further light on the subject.

Buggert remarks, "According to this study, the smallpox vaccine has demonstrated the potential to offer approximately 80 percent protection against Mpox."

Disclosure: Marcus Buggert serves as a consultant for Oxford Immunotec, Mabtech, BMS (Bristol-Myers Squibb), and MSD (Merck Sharp & Dohme).

Source: Karolinska Institute