A product taken from the blood of people who have been vaccinated for smallpox was approved on Friday to use to treat reactions from the shot, the U.S. Food and Drug Administration said.

The FDA said it had approved DynPort Vaccine Co. LLC’s Vaccinia Immune Globulin Intravenous. It is made out of human plasma.

“This plasma contains increased levels of protective antibodies against the vaccinia virus, the live virus used in the currently available smallpox vaccine,” the FDA said in a statement.

The smallpox vaccine is decades old and is based on even older technology. It uses a live virus called vaccinia, which is a relative of smallpox that does not usually cause disease in people.

But it does in some people, mostly localized skin reactions, but when it was used in the past it did kill about 1 in a million people who got the shot.

DynPort’s immune globulin has been shown to help treat some of those reactions, by boosting the body’s immune response to the vaccinia virus.

“Those at increased risk for these complications include people with eczema or other skin conditions and people whose immune systems are suppressed due to diseases or medications, such as steroids or therapies for cancer,” the FDA said.

Smallpox was eradicated in 1979 by vaccination campaigns, but experts fear it could be used as a biological weapon, so governments, especially the United States are stockpiling vaccine.

A U.S. government effort to vaccinate health and emergency workers has moved very slowly, in part because of fears about complications from the shot.

Companies are working to develop safer smallpox vaccines.