Bigger problems soon surfaced.
An AstraZeneca executive told Reuters on Monday that the company had not intended for any participants to receive the half dose. British researchers running the trial there had meant to give the full dose initially to volunteers, but a miscalculation meant they were mistakenly given only a half dose. The executive, Menelas Pangalos, described the error as “serendipity,” allowing researchers to stumble onto a more promising dosing regimen.
To many outside experts, that undercut the credibility of the results because the closely calibrated clinical trials had not been designed to test how well a half-strength initial dose worked.
The company’s initial announcement didn’t mention the accidental nature of the discovery.
In the statement attributed to Oxford, Ms. Meixell, the AstraZeneca spokeswoman, said the error stemmed from an issue, which has since been fixed, with how some of the vaccine doses were manufactured.
Then, on Tuesday, Moncef Slaoui, the head of Operation Warp Speed, the U.S. initiative to fast-track coronavirus vaccines, noted another limitation in AstraZeneca’s data. On a call with reporters, he suggested that the participants who received the half-strength initial dose had been 55 years old or younger. Ms. Meixell declined to say whether that was the case, noting that the data would be published soon in a peer-reviewed journal.
If the initial half-strength dose wasn’t tested in older participants, who are especially vulnerable to Covid-19, it would undermine AstraZeneca’s case to regulators that the vaccine should be authorized for emergency use.
Stephanie Caccomo, a spokeswoman for the Food and Drug Administration, declined to comment on whether the dosing error would hurt the vaccine’s chances of being authorized. The F.D.A. has said it expects vaccines to be at least 50 percent effective in preventing or reducing the severity of the disease, a bar that the vaccine appears to have cleared even in the group that got the two full doses.
AstraZeneca’s shares have fallen about 5 percent this week, while broader stock indexes have climbed to record highs. Investors seem to be disappointed with the murky results, especially compared with the much clearer data released by two of AstraZeneca’s chief rivals in the race for a coronavirus vaccine.