What to know about smallpox vaccines for monkeypox virus

Two vaccines are available in the U.S.

by Chia-Yi Hou | Jul. 19, 2022

blurry image of man waiting outside, sign saying vaccination entrance in background

A man waits in line to recieve the Monkeypox vaccine before the opening of a new mass vaccination site at the Bushwick Education Campus in Brooklyn on July 17, 2022, in New York City. – New York, on the US East Coast, has already either administered or scheduled 21,500 vaccines and hopes to speed up the process, promising more than 30,000 jabs for the whole state. KENA BETANCUR/AFP via Getty Images

Story at a glance

Monkeypox cases are approaching 2,000 in the U.S.

One of the first cases in the U.S. was detected in May.

Two smallpox vaccines are available to prevent monkeypox.

There have been 1,972 reported monkeypox (MPX) cases reported in the U.S., according to the Centers for Disease Control and Prevention. The most have been detected in New York (581), with more than 260 in California and Illinois and more than 100 in Florida and Georgia each. There are two vaccine options in the U.S., with important differences in how they are administered and availability of doses.

The MPX virus is spread through physical contact and is not transmitted through the air. Health officials have established a plan to make smallpox vaccines available for people who have had exposures, both presumed and confirmed. There is no vaccine for MPX, but smallpox vaccines are somewhat effective at preventing severe illness because of MPX’s similarity to smallpox.

The older vaccine, called ACAM2000, contains vaccinia virus, which is in the same family as smallpox. It cannot cause smallpox but does lead to mild illness. It’s administered using a two-pronged needle that pokes the vaccine solution into the body, where red sore spots will develop in 3 to 4 days. It can also cause itching, fever and headache.

This virus can replicate in the body, and it’s also possible to pass it on to someone else who is unvaccinated. ACAM2000 is only officially approved by the Food and Drug Administration (FDA) for smallpox but because of the viruses’ similarities, it does confer some protection against MPX.

The newer vaccine, called Jynneos, is given as two shots four weeks apart. The virus contained in this vaccine is attenuated, which can be interpreted as being weaker than the original virus. It cannot make copies of itself once inside the body. Data from clinical trials in Africa suggest that it is 85 percent effective at preventing MPX. Jynneos has been approved by the FDA for prevention of both smallpox and MPX disease.

The benefit to using ACAM2000 is that there is a large stockpile of the vaccine available, set aside by the government in preparation for potential bioterrorism. More than 200 million doses are stockpiled in the U.S. Strategic National Stockpile.

For the Jynneos vaccine, the U.S. Department of Health and Human Services (HHS) announced they will provide 296,000 doses. However, only 56,000 of those doses will be immediately available.

When the first MPX cases were detected in the U.S., health officials were recommending people with confirmed exposures to get vaccinated. But with more cases being reported and inadequate testing, the “ring” strategy of vaccinating close contacts around a case may not be enough to stop transmission.

“Now our strategy has shifted, as we may not be able to actively identify all contacts. So we are casting a wider net,” explained epidemiologist Katelyn Jetelina in her newsletter.

The CDC has been shipping out tests to commercial laboratories. They recommend people who have been exposed or are at higher risk of exposure to get vaccinated. The HHS expects that an additional 500,000 doses will be available this fall.