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

Polls are finding Americans increasingly wary of accepting vaccine. And scientists inside and outside of the government are worried that regulators, pressured by President Trump for results before the US election day on Nov. 3, might release an unproven or unsafe vaccine.

“The release of these protocols seems to reflect some public pressure to do so,” said Natalie Dean, a biostatistician and expert in clinical trial design for vaccines at the University of Florida. “This is an unprecedented situation, and public confidence is such an enormous part of the success of this endeavor.”

Moderna and Pfizer revealed details of their vaccine trials on Thursday.

Experts are particularly concerned about AstraZeneca’s trials due to the company’s refusal to give details about serious neurological illnesses in two participants, both women, who received its experimental vaccine in Britain, where the company’s trials began in April.

Those cases spurred the organization 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. However, there are about 18,000 people worldwide who have received AstraZeneca’s vaccine so far.

The 111-page trial blueprint of AstraZeneca’s, familiar as a protocol, states that its goal is a vaccine with 50 percent effectiveness — the same threshold has been set by the Food and Drug Administration as its guidance for coronavirus vaccines.

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