Almost pounds 500m was wiped off the value of the UK company and at one point the share price fell more than 85p in hectic selling, though it recovered to close down 51p at 692p, valuing the group at just under pounds 6bn.
The stock market, which has been nervous of drug shares over the past few months, reacted badly to results of a clinical trial showing that AZT, sold by Wellcome under the brand name Retrovir, is not effective at preventing Aids in healthy people infected with HIV.
The leading anti-Aids drug, which accounts for about 12 per cent of the company's annual pounds 2bn sales, is a key contributor to Wellcome's profits. When initial results in August 1989 suggested the drug could delay the onset of Aids, the share price leapt by a third in one day.
Wellcome fought a rearguard action against the result of the Anglo-French 'Concorde' trial, published yesterday in the Lancet. It criticised the interpretation of the data, which was 'preliminary and incomplete', said Trevor Jones, company research director.
The company said the results, which showed that taking AZT was no better than a placebo at preventing Aids, had become 'diluted' during the course of the three-year investigation because some volunteers taking a placebo had switched to AZT. Dr Jones said the conclusion was therefore 'misleading'.
Scientists involved in the study discounted the possibility of any bias being introduced that would alter significantly the trial conclusions. 'I don't think it's misleading at all,' said Professor Ian Weller, of the Middlesex Hospital in London, a principal co-ordinator of the trial.
Professor Richard Peto, an expert in clinical trials at Oxford University, said that Wellcome was 'exaggerating'. He said the aim of the trial was to see whether healthy HIV-positive people benefited from AZT and the answer was 'no'.
He said the Concorde results showed more people in the group taking AZT died during the three-year period than in the placebo group. 'The fundamental question is not so much whether there is a progression to Aids, but whether you would do better by holding off from AZT until the first symptoms appear. The answer seems to be 'yes'.'
The London Lighthouse, an Aids charity, said its advice to people considering taking AZT is 'not to jump to any conclusions and consult their doctor'. The Concorde results do not affect the evidence suggesting that AZT is useful for people who already have Aids.
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