Researchers have failed to find any evidence of HIV or SIV infection in samples of oral polio vaccine held by the Wistar Institute, developers of a vaccine alleged to have introduced HIV into the human population during studies in Central Africa in the late 1950s.
The tests were carried out following controversy fuelled by the publication of The River, an investigation of the oral polio vaccine theory by journalist Edward Hooper. The writer called on the Wistar Institute to submit its remaining stock of the vaccine, labelled CHAT, to independent analysis by a panel led by Dr Claudio Basilico of New York University. Dr Basilico presented his group's findings at a meeting on the origins of HIV organised by the Royal Society to review the evidence in support of competing theories.
"There is nothing in the results from these tests to support the theory that HIV entered the human population during the late 1950s poliovirus clinical trials in Africa," Dr. Basilico said today. "The different tests performed by the three independent laboratories did not find any evidence of SIV or HIV in the samples nor did they find chimpanzee DNA. In fact, the laboratories were able to determine that all of the Wistar samples were grown in monkey cell cultures rather than chimpanzee cell cultures."
The detection limits of the assays used were 100 copies of HIV RNA and 1,000 copies of SIV RNA. A presentation from Philippe Lena of the Pasteur Institute in Paris used more sensitive DNA tests to examine whether SIV could persist in macaque kidney cells prepared in the fashion employed by vaccine developers. Kidney tissue from a macaque with high viral load contained SIV DNA which persisted after 6 passages, and M. Lena concluded that whilst the quantities of DNA were possibly too small to be infectious in this case, macrophages which could harbour retrovirus infection were detectable in the kidney cells, and replication competent virus was also detectable.
However, John Garrett of the UK's National Instititute of Biological Standards and Control assessed the viability of HIV and SIV in monkey kidney monolayer exposed to trypsin, an agent used in the preparation of the cells used to culture polio vaccine in macaque kidneys. HIV and SIV did not replicate in the presence of trypsin in the NIBSC experiments. Edward Hooper has suggested that trypsin was not routinely used in the late 1950s.
Hooper also questioned whether the vaccine samples tested were relevant, and questioned Dr Stanley Plotkin's claim that the sample tested from Batch 10a/11 was actually used in Africa. Hooper claims to have evidence in a statement from a vet employed by a polio vaccine laboratory in Stanleyville that chimp kidneys from Congo were sent to the Wistar Institute by researchers who now deny that chimp kidneys were used in any vaccine preparation undertaken by the Institute. The question of which primate tissues were used is crucial because only chimpanzees have been shown to carry SIV.
Mr Hooper's arguments were challenged from an epidemiological standpoint by Kevin de Cock of the US Centers for Disease Control, who said that the association between oral polio vaccination events and subsequent AIDS outbreaks was purely "ecological". "The theory depends on the chance survival of human sera from Kinshasa and an Ebola investigation in northern Congo. There is no data on the Congo vaccinees to allow us to compare the AIDS risk of those exposed and unexposed, and the presence or absence of SIV in champs today tells us nothing about the potential for cross-species transfer at particular times or particular places in the past".
Beatrice Hahn of the University of Alabama echoed this view. "The reason we study sequences is because we don't rely on anyone to tell us anything - we rely on the sequence. It is like a fossil". Dr Hahn reported on the prevalence of SIVcpz in chimpanzees living in the wild in several different regions; a low level of SIVcpz was found in Cote d'Ivoire and an undisclosed location in East Africa, and Dr Hahn plans to analyse the sequence from East Africa to analyse its relationship to other SIV isolates found in animals captured in Gabon and Cameroon, and to HIV isolates. It is hoped that this analysis will provide more information about when HIV group M might have diverged from SIV - in other words, when it crossed the species barrier.
But on this question too, Mr Hooper is unconvinced, despite two presentations from molecular epidemiology groups which have dated the divergence to 1930-41. Hooper argues that the technique can only identify the last common ancestor of known SIVcpz isolates and HIV group M isolates with known isolation dates, but not the point at which the cross-species transfer itself occurred. Subsequent divergence of HIV group M could have occurred in humans, or it could have occurred as the consequence of multiple introductions of SIVcpz in different locations, and neither possibility excludes the oral polio vaccine transfer theory, Hooper contends.
The meeting continues on Tuesday September 12 with further presentations on the timing of cross-species transfer.
This article was originally published in September 2000. Click here for the second report from the meeting.