The successful eradication of smallpox 30 years ago and the subsequent ending of the mass vaccination campaign of the 1960s and 1970s may have unwittingly created the conditions that allowed the explosive spread of Aids in Africa, scientists have claimed.

A test tube study on blood samples of people who had recently been vaccinated against smallpox has found that their blood cells are remarkably resistant to infection with HIV. The researchers suggested that the smallpox vaccine may have conferred some resistance against HIV, which was lost when the vaccination campaign ended in the late 1970s.

Raymond Weinstein of the George Mason University in Manassas, Virginia, said that the vaccinia virus used to make the smallpox vaccine may have altered certain proteins – known as CCR5 receptors – on the surface membrane of white blood cells.

These receptors are needed by HIV to infect the cells and so destroy the human immune system.

"There have been several proposed explanations for the rapid spread of HIV in Africa, including wars, the re-use of unsterilised needles and the contamination of early batches of polio vaccine," Dr Weinstein said. "However, all of these have either been disproved or do not sufficiently explain the behaviour of the HIV pandemic.

"Our finding that prior immunisation with vaccinia virus may provide an individual with some degree of protection to subsequent HIV infection suggests that the withdrawal of such vaccination may be a partial explanation," he added.

The findings, published in the journal BMC Immunology, are only based on laboratory experiments and are still too preliminary to recommend the general use of vaccinia immunisation for fighting HIV, Dr Weinstein said.