Antibodies derived from chimpanzees may help treat smallpox and the potentially deadly side effects caused by the existing vaccine, US scientists say.
The last smallpox case was in 1977
Natural smallpox is thought to have been eradicated but there are fears it could be used as a biological weapon.
Proceedings of the National Academy of Sciences reports that lab tests suggest the new treatment can be used in cases where instant protection is needed.
Rodent tests showed it could also treat vaccine complications, the report adds.
The World Health Assembly declared the world free of naturally occurring smallpox in May 1980 after a highly effective campaign to eradicate the deadly disease. Since then, there has not been a single case.
However, there is concern that the virus could be used as a devastating biological weapon in a terrorist attack.
The UK government has a stockpile of 40m doses of the current smallpox vaccine - which works well but can trigger severe side effects - for use in the event of such an attack.
The smallpox virus causes symptoms such as headache, delirium and vomiting, followed the development of a trademark rash. It kills approximately one in three of those infected.
The current vaccine blocks infection by the smallpox virus variola by targeting it with another virus, vaccinia.
However, vaccinia in turn can trigger severe side effects in a small minority of people.
These can be treated using a mixture prepared from the blood of vaccinated individuals - but this carries a small risk of contamination with prion diseases such as vCJD.
A team led by Dr Robert Purcell, at the US National Institute of Allergy and Infectious Diseases, set out to test the potential of non-human antibodies.
They focused on those taken from the bone marrow of chimpanzees because of their close similarity to human forms.
The animals were immunised with a more robust strain of vaccinia than that used in the human smallpox vaccine, and produced particularly powerful antibodies in response.
Two of these appeared to be potential candidates for use on humans, and one was used to create a hybrid human/chimp version.
In tests on cell cultures it was found to inhibit the spread of both vaccinia and variola.
It also protected mice from complications associated with the vaccinia virus more effectively than the current treatment.
Dr Purcell said: "These antibodies will not replace smallpox vaccines for most purposes.
"However, such vaccines don't work in immunosuppressed people.
"Furthermore, smallpox vaccines require at least a week and preferable two weeks for protection to develop.
"Potent antibodies can provide instant protection after exposure to the virus."
Professor John Oxford, an expert in virology at Queen Mary School of Medicine, London, told the BBC News website that the hybrid vaccine would probably be too expensive for widespread use.
But he said: "Having antibodies to treat people who get seriously ill after receiving the vaccine would be rather good.
"It might also help to treat people who have already been infected with smallpox."
Professor Oxford said new smallpox vaccines that carried a lower risk of complications were currently under development.