AstraZeneca and Oxford University on Wednesday acknowledged a manufacturing error that is raising questions about preliminary results of their experimental COVID-19 vaccine. A statement on the error 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 UK and Brazil. AstraZeneca said the vaccine appeared to be 90% effective in low-dose group and 62% effective in two full doses group. Combined, the it said the vaccine appeared 70% effective.