There is no cure for AIDS, but cocktails of drugs can keep the disease at bay for many years.

The human immunodeficiency virus that causes AIDS infects 33.3 million people globally, according to the United Nations agency UNAIDS. It has killed more than 25 million people.

Because it is spread in so many ways - during sex, on needles shared by drug users, in breast milk and in blood - there is no single easy way to prevent infection.

A vaccine is the best hope, and many drug companies and scientific research groups are working on various ways to try to develop one.

“The breakthrough here is in using a viral-delivered vaccine that persists - essentially using an engineered virus to thwart a pathogenic virus,” said Robin Shattock, a professor of mucosal infection and immunity at Britain’s Imperial College, who was not involved in the research.

“Before this ... scientists had pretty much given up on the idea of a vaccine that could control HIV replication (but) this puts it firmly back on the agenda.”

Efforts so far to make an AIDS vaccine have not been successful, but a 2009 study in Thailand involving 16,000 people showed for the first time that a vaccine could safely prevent HIV infection in a small number of volunteers.

Picker said the next step is to make a weaker version of the CMV virus to make sure it does not cause any problems in people.

“The concern would be if we move a virus that is not modified that in some small number it might cause disease,” he said.

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By Julie Steenhuysen and Kate Kelland