AstraZeneca revealed details of its large coronavirus vaccine trials on Saturday, the third in a wave of rare disclosures by drug companies under pressure to be more transparent about how they are testing products that are the world’s best hope for ending the pandemic.

Experts have been particularly concerned about AstraZeneca’s vaccine trials, which began in April in Britain, because of the company’s refusal to provide details about serious neurological illnesses in two participants, both women, who received its experimental vaccine in Britain.

Those cases spurred the company to halt its trials twice, the second time earlier this month. The studies have resumed in Britain, Brazil, India and South Africa, but are still on pause in the US. About 18,000 people worldwide have received AstraZeneca’s vaccine so far.

AstraZeneca’s 111-page trial blueprint, known as a protocol, states that its goal is a vaccine with 50 per cent effectiveness. To determine with statistical confidence whether the company has met that target, there will have to be 150 people ill with confirmed coronavirus among participants who were vaccinated or received placebo shots.

However, the plan anticipates that a safety board will perform an early analysis after there have been just 75 cases. If the vaccine is 50 per cent effective at that point, it might be possible for the company to stop the trial early and apply for authorisation from the government to release the vaccine for emergency use.

Dr Eric Topol, a clinical trials expert at Scripps Research in San Diego, said AstraZeneca’s plan, like those of Moderna and Pfizer, had a problematic feature: All count relatively mild cases of Covid-19 when measuring efficacy, which may hamper efforts to determine whether the vaccine prevents moderate or severe illness.

Such plans are not usually shared with the public “due to the importance of maintaining confidentiality and integrity of trials”, Michele Meixell, a spokesperson for AstraZeneca, said in a statement.

The company has released few details about the two cases of serious illness in its trial. The first participant received one dose of the vaccine before developing inflammation of the spinal cord, known as transverse myelitis, according to a participant information sheet for AstraZeneca’s vaccine from July. The condition can cause weakness in the arms and legs, paralysis, pain and bowel and bladder problems.

The case prompted a pause in AstraZeneca’s vaccine trials to allow for a safety review by independent experts. A company spokesperson told The Times last week that the volunteer was later determined to have a previously undiagnosed case of multiple sclerosis, unrelated to the vaccine, and that the trial resumed shortly thereafter.

The company said it had not confirmed a diagnosis in the second case, a participant who got sick after the second dose of the vaccine. A person familiar with the situation who spoke with The Times on the condition of anonymity said the participant’s illness had been pinpointed as transverse myelitis. The trial was paused again on September 6 after she fell ill.

The condition is rare, but serious, and experts said that finding even one case among thousands of trial participants could be a red flag. Multiple confirmed cases, they said, could be enough to halt AstraZeneca’s vaccine bid entirely.

“If there are two cases, then this starts to look like a dangerous pattern,” said Mark Slifka, a vaccine expert at Oregon Health and Science University. “If a third case of neurological disease pops up in the vaccine group, then this vaccine may be done.”

The company’s chief executive told investors about the problems but did not discuss them publicly until the information was leaked and reported by the new website STAT. Dr Paul Offit, a professor at the University of Pennsylvania and a member of the US Food and Drug Administration’s advisory committee on vaccines, and other experts noted that transverse myelitis is rare, diagnosed in only about one in 236,000 Americans a year. The trial in Britain involved only about 8,000 volunteers.

If a serious side effect was definitively linked to AstraZeneca’s vaccine, scientists would need to determine if its root cause stemmed from the adenovirus vector it uses, or perhaps the coronavirus genes it carried.

New York Times News Service