The US Food and Drug Administration (FDA) approved the ACAM2000 jab for use against mpox on August 29.

Facebook posts claim the vaccine could cause a smallpox outbreak, and approval puts the public in danger.

“This approval puts the general public in significant danger as these LIVE viral vaccines have to the potential to transmit smallpox to individuals who are in close proximity to the vaccinated up to 42 days post-vaccination,” one post said.

Another post claimed: “ACAM 2000 can cause a worldwide smallpox pandemic". 

Infectious diseases expert Professor Peter Collignon said smallpox jabs were live vaccines, but they did not contain the smallpox virus, so they could not transmit it.

He said the jabs contained the vaccinia virus, which was different to the variola virus that causes smallpox.

“So by being similar, but different and very much less aggressive, it protects you but without the effects of smallpox on your body,” Professor Collignon told AAP FactCheck.  

The US FDA and the US Centres for Disease Control and Prevention also confirmed that vaccinia was a virus related to smallpox that caused a milder disease.

 "The FDA ACAM2000 does not contain the smallpox virus and cannot cause smallpox," the FDA website said.

The Therapeutic Goods Administration said the ACAM2000 vaccine, which was only registered to prevent smallpox in Australia, contained a weakened version of the vaccinia virus.

"This is not the virus that causes Smallpox, and it cannot cause smallpox," a TGA spokesperson told AAP FactCheck.

While ACAM2000 had infrequently been used against mpox in the past, the Australian regulator said a different vaccine called JYNNEOS was preferred.

Smallpox was globally eradicated in 1980, the WHO declared.

The live variola virus is only kept in two research laboratories in the US and Russia, the WHO added.