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On the heels of the World Health Organization's (WHO) global emergency declaration, the FDA expanded the approval of the smallpox vaccine (ACAM2000) for use in people at high risk for mpox, on Thursday.

"Mpox has progressed to become an uncontrolled epidemic in Africa -- prompting the WHO to declare a second public health emergency of international concern -- creating an enormous need to use all effective tools to extinguish it as a threat," said infectious diseases expert Amesh Adalja, MD, of Johns Hopkins Bloomberg School of Public Health in Baltimore, in the company's press release.

"ACAM2000, a direct descendant of the Jenner vaccine (humanity's first) which was used to eradicate smallpox, and now with the broadened indication, will be an invaluable tool in this endeavor," added Adalja.

The new FDA approval was based on previously available human safety data and data from a well-controlled animal study in which the vaccine was shown to be effective in protecting against mpox, according to the manufacturer.

It marks the second vaccine to receive full FDA approval against mpox. The third-generation live orthopoxvirus vaccine (Jynneos), , was the first.

ACAM2000 was . In 2022, the FDA made the vaccine available for use against mpox under an Expanded Access Investigational New Drug protocol following the global outbreak of clade II mpox that led to more than 95,000 cases across 115 non-endemic countries.

Most recently, the has led to an increase in cases in Congo and other parts of Africa, posing a global threat. In response, the WHO declared mpox a global public health emergency for the second time in 2 years. A case of this more infectious strain was in Sweden as well.

ACAM2000 is a . It is administered as a single, percutaneous dose by multiple punctures in the upper arm using a bifurcated needle.

Although the U.S. has a large supply of ACAM2000, this vaccine has more side effects and contraindications than Jynneos, according to the CDC.

Because it is a live vaccine, ACAM2000 is contraindicated in individuals with severe immunodeficiency because it can cause severe localized or systemic infection with vaccinia in persons with weakened immune systems. Other potential side effects include myocarditis, encephalitis, severe vaccinia skin infections, ocular vaccinia, and fetal death, among others.

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