The smallpox vaccine will be made available to people exposed to the monkeypox virus in the United States, health officials have said.
US authorities have banned the sale of prairie dogs
The Centres for Disease Control and Prevention in Atlanta, Georgia, will release the vaccine to veterinarians and health workers involved in fighting the outbreak of the virus.
And in another move aimed at limiting the spread of monkeypox, the US Government has banned the sale of prairie dogs and prohibited the import of African rodents.
The latest moves come as the federal investigation into the monkeypox outbreak - the first in the Western hemisphere - was expanded to eight more states, bringing the total to 15.
There are more than 50 suspected cases, mostly in the US mid-west where the first cases were diagnosed last week.
So far there have been no fatalities. Symptoms include a high temperature, headache, backache, enlarged lymph nodes, sore throat, cough, shortness of breath and a rash.
The smallpox vaccine is being issued because it can prevent monkeypox up to two weeks after exposure to the virus, although it is most effective in the first four days.
"We're optimistic we can deliver the vaccine to these people in time to do good," said Dr David Fleming, a senior official at the disease control centres.
He said the smallpox vaccine is 85% effective against monkeypox.
The Department of Agriculture will enforce the prairie dog
ban, which also prohibits transporting the rodents that are native to the American Plains.
Gambian rats and five other types of large African rodents were banned because a Gambian rat is believed to have spread the virus to prairie dogs at a pet distributor near the city of Chicago in Illinois.
Health officials are trying to trace 115 customers - both individuals and pet stores - who have bought animals from the business since 15 April.
The US Centres for Disease Control and Prevention says it believes most of the people suspected of having contracted monkeypox got it through contact with infected animals.
In Wisconsin, a family has been ordered to stay at home after becoming infected with suspected monkeypox from prairie dogs bought as pets.
Monkeypox is related to smallpox and causes rashes, fevers and sores, though it is not usually fatal.
Although there have been no reported fatalities yet from this US
outbreak, there have been mortality rates of up to 10% with outbreaks of the disease in Africa.