Vaccine experts -- including those who serve on advisory committees for the US Food and Drug Administration and the US Centers for Disease Control and Prevention -- have questions about data released Monday by AstraZeneca about its Covid-19 vaccine.
In a press release Monday, the pharmaceutical giant announced that its vaccine is on average 70% effective. However, the company did not state the data that led them to that conclusion.
"Absent knowing this, it's hard to know the significance of their findings," said Dr. Paul Offit, a member of the FDA's Vaccines and Related Biological Products Advisory Committee, which will review Covid-19 vaccines before they are put on the market.
When two other pharmaceutical companies, Pfizer and Moderna, released their efficacy results earlier this month, they did include the data that led to their results.
In its Monday press release, AstraZeneca presented an analysis of 23,000 participants in its Phase 3 trial.
Some participants received the Covid-19 vaccine, while others received a different kind of vaccine or placebo injections — shots of saline that do nothing.
Then an independent board of experts determined to what extent the group that received the Covid-19 vaccine was protected against the disease.
In its press release, AstraZeneca said a total of 131 study participants developed Covid-19 but did not say how many of those people had received the Covid-19 vaccine and how many did not.
AstraZeneca is running the vaccine trials in collaboration with the University of Oxford in the UK. Spokespeople for the company and the university did not respond to questions from CNN on Monday.
The experts also had questions about the safety of the AstraZeneca vaccine.
The AstraZeneca trial was put on hold twice because government regulators were concerned about two study participants who became seriously ill. Regulators later allowed the trial to resume.
"I'd like to know the data specifically about those serious adverse reactions that caused the trial to go on pause," said Dr. William Schaffner, a member of the CDC's Advisory Committee on Immunization Practices, which will also be reviewing the vaccines before they're allowed on the market.
In its press release, AstraZeneca stated that "no serious safety events related to the vaccine have been confirmed" and that the vaccine was "well tolerated."
The experts had another question about the AstraZeneca trial.
Among the study participants who received the Covid-19 vaccine, there were two different dose regimens.
In one regimen, 2,741 participants received a half-dose of the vaccine and then a full dose at least a month later. In the second group, 8,895 participants received a full dose followed by another full dose at least a month later.
The group that received the half-dose initially was 90% protected against Covid-19, and the group that received two full doses was only 62% protected.
One of the lead Oxford researchers, Dr. Adrian Hill, said it would take "probably weeks and months" to understand why the lower dose yielded much better results.
"We don't fully understand that, but there's several ideas around as to how it might work, and we're exploring those," Hill told CNN's Becky Anderson Monday.
Trials for the vaccine are still underway in several countries and will eventually include under 60,000 study participants by the end of the year, according to the AstraZeneca press release.
Dr. Saad Omer, a vaccine specialist at the Yale School of Medicine, noted that the group with the 90% efficacy rate was relatively small -- just 2,741 study participants -- and those results might not hold up when more people are given this regimen.
He noted a lack of clarity about several aspects of AstraZeneca's data.
"I hate to criticize fellow academics, or anyone for that matter, but releasing information like this is like asking us to try and read the tea leaves," Omer said.