Late-stage studies were stopped in early September after a UK volunteer reported transverse myelitis, an inflammation of the spinal cord that can cause paralysis.
The trials later resumed in the UK, India, South Africa and Brazil but are still on hold in the US, where regulations are tougher.
While this pause was widely reported as soon as it happened, a first halt in July went under the radar.
At the time it was also a UK patient who developed transverse myelitis, however the inflammation was attributed t the fact that she was already suffering from multiple sclerosis.
US scientists are concerned over what made the FTSE 100 firm restart trials the second time.
“The communication around it has been horrible and unacceptable. This is not how the American people should be hearing about this,” Dr Peter Jay Hotez, a virologist with Baylor College of Medicine in Texas, told the New York Times.
AstraZeneca told volunteers these unexplained neurological symptoms were “either considered unlikely to be associated with the vaccine or there was insufficient evidence to say for certain that the illnesses were or were not related to the vaccine”, The Telegraph reported.
“In each of these cases, after considering the information, the independent reviewers recommended that vaccinations should continue,” the pharma giant said.
1/n The AZ vaccine adverse effects are seriously concerning. First, since public money flowed into the project through OWS, we need to have access to the data to figure out what happened. If needed, this should be escalated (legally)
— Siddhartha Mukherjee (@DrSidMukherjee) September 13, 2020
The US medical community is concerned about neurological adverse effects in vaccines after an emergency influenza jab caused 30 deaths and 450 cases of Guillain-Barre syndrome in 1976.
The Guillain-Barre syndrome and transverse myelitis both affect the spinal cord and are both rare.
“If there are two cases, then this starts to look like a dangerous pattern. If a third case of neurological disease pops up in the vaccine group, then this vaccine may be done,” Mark Slifka, a vaccine expert at Oregon Health and Science University, told the New York Times.
Shares in AstraZeneca were trading 2% lower at 8,572p on Monday afternoon.