MILWAUKEE (AP) — The federal government has given conditional approval to the first vaccine licensed to fight a deadly pig virus.

Porcine epidemic diarrhea has killed millions of baby pigs since it showed up in the United States last year and is partly responsible for recent increases in pork prices.

The U.S. Department of Agriculture gave conditional approval last week to a vaccine produced by Iowa-based Harrisvaccines. The vaccine is being used mostly to boost immunity in sows that have had the disease so they can better pass on antibodies to their young.

Company spokesman Joel Harris said Wednesday that one challenge is immunity to PED doesn’t seem to last like it does with many viruses. He says it might last only a few months.

Other companies also have been working on vaccines.