LOS ANGELES - The worldwide eradication of smallpox in the mid-20th century was a remarkable public health achievement, but it may have set the stage for the HIV pandemic of the latter half of the century, researchers reported yesterday.

Lab tests suggest that immunity to smallpox triggered by the smallpox vaccine can inhibit the replication of the AIDS virus. Such vaccination could have kept HIV transmission partly under control in the early days of the outbreak, but withdrawal of the vaccine in the 1950s would have freed HIV to spread unfettered, the researchers said.

The most common form of HIV is thought to have evolved from a simian immunodeficiency virus found in chimpanzees of southern and western Africa sometime around 1931. It spread slowly until the mid- to late-1950s, when it began to spread exponentially.

Wars, misuse of medical equipment and contamination of a polio vaccine have been suggested as possible causes of the spread, but such theories have either been disproved or do not sufficiently explain the behavior of the HIV pandemic, said Dr. Raymond S. Weinstein of the biodefense program at George Mason University in Manassas, Va.

Weinstein and his colleagues noted that the progression of an HIV infection can be mitigated by a co-infection with certain other viruses, such as human herpesvirus 6 or 7 or the paramyxovirus that causes measles. Such viruses interfere with a cellular receptor of white cells that is also used by HIV. The vaccinia virus also blocks this receptor.

Given the great difficulties researchers have encountered in trying to develop an HIV vaccine, the ironic fact is that we may once have had a vaccine that is more effective against the virus than anything that has since been developed, and we threw it away.

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