An immune cell infected with HIV

The surprising small-scale findings prompted an 10% rise in the share price of vaccine maker Acambis when they were released to the Stock Exchange.

However, other researchers are unconvinced by the results of lab experiments on blood cells.

They say it is improbable that the results would be reproduced in humans.

The study was carried out by researchers from George Mason University in Virginia, and involved 10 people who had been vaccinated against smallpox and 10 who had not.

Cells taken from blood samples were exposed to HIV in the laboratory.

Resistant

The researchers suggested there had been a "statistically significant" increase in resistance to HIV infection in the vaccinated cells, although the precise difference was not made public.

It was also unclear whether the 10 smallpox vaccinated volunteers had been given other vaccinations during their life, or how soon after smallpox vaccination the experiments had been carried out.

There are significant practical problems with the idea of using the existing smallpox vaccine in Africa, even if other studies suggest that there may be a positive effect. Julian Meldrum, National Aids Manual

A theory that a decline in immunity to smallpox following the end of mass immunisation in the 1970s may have allowed the spread of HIV in Africa and elsewhere prompted the George Mason University study.

However, other experts believe there is little evidence to support this theory.

Julian Meldrum from National Aids Manual said: "There is no evidence that levels of smallpox immunity had any impact on HIV spread."

He said that while the finding was "interesting", he said the small size of the study meant it was hard to be clear whether the effect was genuine, and that results in test tubes were notoriously difficult to reproduce in living animals or humans.

He said: "There are significant practical problems with the idea of using the existing smallpox vaccine in Africa, even if other studies suggest that there may be a positive effect.

"You shouldn't give smallpox vaccine to people who have HIV, and, given that testing is far from available in many places, there are many people who have HIV but do not know it."

He suggested that those hoping for a mass vaccination programme against HIV might be better waiting for one of the current HIV vaccine development projects to come to fruition.