Yes, a smallpox vaccine protects against monkeypox, but it comes with its own set of risks

• Monkeypox has spread to more than a dozen countries including several in Europe, the US, and Canada.
• The smallpox vaccine is 85% effective against monkeypox, but no longer widely available.

Google searches for "smallpox vaccine" have rocketed since monkeypox cases started turning up in people across Europe and North America who have no recent travel history to Africa.

The search curiosity is not unprompted — President Joe Biden promoted the US' smallpox vaccine stockpile when he told reporters on Monday he doesn't think the monkeypox outbreak merits the same level of concern as COVID-19.

"Look, we've had this monkeypox … in the past," Biden said, referring to an outbreak that occurred across six states and infected dozens of people in 2003.

"The smallpox vaccine works for it."

However, while a smallpox vaccine may be around 85% cross-protective against monkeypox, not everyone can just run to their local pharmacy and get a shot right now.

There are two smallpox vaccines in the US. For the newer, safer one, supply is scarce — representatives from the Centers for Disease Control and Prevention said on Monday that upwards of 1,000 doses of the Jynneos vaccine are available right now, and that "we expect that level to ramp up very quickly in the coming weeks."

The US has also stockpiled more than 100 million doses of another, older smallpox vaccine, called ACAM2000, but infectious disease experts warn it carries a higher risk of heart problems, brain swelling, blindness, and in the most severe cases (one in a million) — death.

Eradicating smallpox may have had 'unintended consequences' for monkeypox

Routine smallpox vaccination was stopped in the US in 1972, more than 30 years after the last smallpox cases were diagnosed in the country. In 1980, smallpox was declared eradicated worldwide — it's the only human disease that vaccines have successfully wiped off the planet entirely.

Babies today aren't vaccinated against smallpox, and the last people vaccinated in large numbers are around 50 years old, or older. (You can see tell-tale signs that some older adults had a smallpox vaccine, because they have characteristic scars on their arms.)

While the eradication of smallpox through vaccination has saved more than 200 million lives and many billions of dollars over the past four decades, infectious disease experts have been warning for several years now that the lack of vaccine-induced immunity to smallpox worldwide may carry some "unintended consequences."

It's estimated that roughly 70% of the world has no immunity to smallpox today, and experts suspect that may be at least part of the reason why monkeypox outbreaks have become more common in some western African countries, like Nigeria, in recent years.

Older smallpox vaccines leave scars, and carry risks of spreading the disease

The US keeps a strategic national stockpile of smallpox vaccine on hand at all times, just in case of a bioterror attack, but during peacetime the CDC only recommends the smallpox vaccine to lab workers who work with smallpox, or smallpox-like orthopoxviruses, including monkeypox.

According to the CDC, there are enough vaccines to vaccinate "everyone who would need it" if a smallpox disaster occurred. (Unlike COVID-19 vaccines, if you take a smallpox vaccine in the week after you're exposed to the virus, it can still offer some protection.) But the US's national stockpile includes many doses of the older, more dangerous ACAM2000 vaccine, which experts are understandably cautious about taking off the shelf unless it is absolutely necessary.

The UK — where the first monkeypox case of the 2022 outbreak was announced on May 7 — also uses the newer, safer smallpox vaccine from Bavarian Nordic (called Jynneos in the US) but in Europe it's referred to as Imvanex. (The UK began offering Imvanex to healthcare workers that may have been exposed to the virus on May 20.)

Infectious disease expert Andrea McCollum, who investigates monkeypox outbreaks for the CDC, said she decided to get vaccinated with ACAM2000 because it was recommended for her work, at a time when the newer, safer Jynneos vaccine wasn't available yet.

"I felt as if the benefit of vaccination far outweighed any risk associated with handling specimens, not vaccinated," she told Insider.

ACAM2000 is a live virus vaccine (Jynneos is not) and as such it prompts a smallpox-like "pock" to form on the arm, in the days after it is injected.

"With ACAM2000 you do develop, it's called a 'take,' you develop the lesion on the arm — it's an indication of successful vaccination," McCollum said.

For her, it wasn't a big deal, because she knew what to expect — feeling "crummy" for a "couple days" — and she knew how to take care of the blister, which scabs, falls off, and eventually leaves a small scar.

"It has to remain covered, and you have to do frequent handwashing and appropriately take care of the site and any of the bandages," to avoid infecting others in the roughly three weeks after vaccination, McCollum said.

Correction: A previous version of this story mistakenly said that Imvanex and Jynneos are different vaccines — they are not, they are just different brand names for the same product in the US and Europe.