AstraZeneca and Oxford University have acknowledged a manufacturing error that is raising questions about preliminary results of their experimental COVID-19 vaccine.

For our comprehensive coverage and latest updates on COVID-19 click here.

A statement describing the error on November 25 came days after the company and the university described the shots as "highly effective" and made no mention of why some study participants didn't receive as much vaccine in the first of two shots as expected. In a surprise, the group of volunteers that got a lower dose seemed to be much better protected than the volunteers who got two full doses.

In the low-dose group, AstraZeneca said, the vaccine appeared to be 90 percent effective. In the group that got two full doses, the vaccine appeared to be 62 percent effective. Combined, the drugmakers said the vaccine appeared to be 70 percent effective. But the way in which the results were arrived at and reported by the companies has led to pointed questions from experts.

The partial results announced on November 23 are from large ongoing studies in the UK and Brazil designed to determine the optimal dose of vaccine, as well as examine safety and effectiveness. Multiple combinations and doses were tried in the volunteers. They were compared to others who were given a meningitis vaccine or a saline shot.