The first hiatus, in July, was not publicly disclosed and the trial was relaunched after it was determined the volunteer had multiple sclerosis, a condition that can cause the same neurological reaction.
The second break, widely reported two weeks ago, followed the second suspected case of the disease in a volunteer who was reportedly hospitalized but now cured.
Traces of the vaccine have since been relaunched in Britain, Brazil, India and South Africa, but are still on hiatus in the United States where the legal and historical background means that regulators tend to take a more stance. tough.
AstraZenica released new data on its testing protocols on Saturday, but US experts continued to raise concerns.
Dr Peter Jay Hotez, a virologist at Baylor College of Medicine in Houston, Texas, told The New York Times: “The communication around him has been horrible and unacceptable. This is not how the American people should hear about it.
Dr Hotez, whose views have been echoed by other US experts, also criticized statements released by UK government officials, including regulators, which he said failed to provide a clear rationale for the resumption of their trials.
“Tell us why you made this decision,” he reportedly said.
In a 9/11 information sheet for trail volunteers, AstraZenica explains the risks as follows: “Reactions from the nervous system are also extremely rare, but can include a condition called Guillain-Barré syndrome, a condition in which people can develop severe weakness and can be fatal.
“In the current trial, we have undertaken safety reviews when volunteers participating in the trials [of the vaccine] developed unexplained neurological symptoms, including change in sensation or limb weakness, and discontinued the study during a safety review.
“Upon independent review, these diseases were either considered unlikely to be associated with the vaccine or there was not enough evidence to say for sure whether or not the diseases were vaccine-related.
“In each of these cases, after reviewing the information, the independent reviewers recommended that the vaccinations continue.”
The United States Food and Drug Administration (FDA), the main drug regulator, has not commented, but would request additional data on the two side effects of AstraZenica.
On Saturday, following calls for more transparency, the company released related details about its testing protocols and how it will calculate whether the vaccine is working.
He said his goal was a vaccine with 50% efficacy, the minimum threshold for FDA approval.
To determine that it achieved this goal with statistical confidence, at least 150 confirmed cases of Covid-19 would need to be registered among trial participants.
But once 75 cases have been recorded, he added, the trial’s safety committee would perform an early analysis, perhaps giving it enough data to apply for an early emergency use license.
Concerns about neurological side effects are particularly acute in the United States.
In 1976, an emergency influenza vaccine caused 450 cases of Guillain-Barré syndrome and 30 people died after 45 million people were vaccinated.
Guillain-Barré syndrome and transverse myelitis are closely related conditions, both of which cause inflammation of the spinal cord.
Experts worry that such conditions are relatively rare, with transverse myelitis being diagnosed in only about one in 250,000 people per year, making it difficult to spot.
Dr Paul Offit, a professor at the University of Pennsylvania and a member of the FDA’s Vaccine Advisory Board, told the New York Times it was not clear how AstraZenica – or UK regulators – determined that the second suspected case of transverse myelitis he reported was unrelated to the vaccine.
The lawsuit in Britain only involved around 8,000 volunteers when it was reported.
Mark Slifka, vaccine expert at Oregon University of Health and Sciences, said: “If there are two cases, it starts to look like a dangerous pattern. If a third case of neurological disease appears in the vaccinated group, then this vaccine can be made. “