This week in mSphere, scientists at Tonix Pharmaceuticals report on studies suggesting that the horsepox virus in the experimental vaccine is substantially more attenuated — and thus less likely to trigger a systemic infection — than the vaccinia virus used in the single-dose vaccine already approved by the U.S. Food and Drug Administration (FDA).

“We are trying to go for the immunity of the old vaccines and safety profile of the new ones, and so far the data are very promising,” said virologist Farooq Nasar, Ph.D, senior author on the study. “We would like to be in the middle.”

Smallpox, which is caused by the variola virus, was a highly contagious disease with a fatality rate between 30% and 97%. Historical records suggest it began infecting people thousands of years ago, and the World Health Organization (WHO) estimated that smallpox killed between 300-500 million people in the 20th century. An effective vaccination campaign that began in the 1950s stopped the spread, and in 1980 the WHO declared the disease eradicated.

Mpox, formerly known as monkeypox, is a related disease caused by the monkeypox virus, which like variola is a member of the genus Orthopoxvirus. Multiple mpox outbreaks have occurred since 2022 and estimates of case fatality rate vary from 0.2% to 11%. The 2 FDA-approved vaccines for smallpox are also approved for use to prevent mpox infection.

The approved live-virus smallpox vaccine includes the vaccinia virus, a less harmful member of the Orthopoxvirus family that helps the body develop defenses against smallpox infection. A few years ago, however, researchers discovered that smallpox vaccines used before and during the American Civil War contained a virus more than 99% genetically similar to horsepox — another Orthopoxvirus. That discovery launched investigations into whether a vaccine with live, attenuated horsepox could confer immunity without the unwelcome side effects of the live vaccinia virus-based vaccine.

Previous studies by researchers at Tonix Pharmaceuticals and elsewhere have reported that the experimental vaccine provoked an antibody response but did not cause disease in non-human primates; in addition, the vaccine protected non-human primates against a lethal exposure to the monkeypox virus with no occurrence of clinical disease.

The new study, led by virologists Nasar and Stefanie Trefry, Ph.D., builds on those findings. Testing in both human cell lines and mouse models, the researchers compared virus replication in models with vaccinia virus strains to those given the experimental horsepox virus.

They found that the horsepox virus is attenuated up to 1,000-fold more than the vaccinia virus strains, which means it contained a much lower concentration of infectious particles. Attenuation weakens a virus, which means it is less likely to trigger a systemic infection in the host. Mice given the vaccinia-based vaccine strains often developed severe symptoms, while mice given the experimental horsepox vaccine showed no adverse effects.

The researchers are now planning phase I human clinical trials to test the safety of the vaccine in people. Smallpox now only exists in laboratories in the United States and Russia, and researchers continue to investigate smallpox vaccines as a defense against smallpox as a bioterror agent. The more urgent public health concern right now, said Nasar, is preventing future mpox outbreaks. The new vaccine hopes to serve both purposes.

“We’re focused on making a product that can be used against both mpox and smallpox,” he said.