AstraZeneca PLC promised the world more vaccines, at a lower cost, than any of the more established vaccine makers. It is finding how hard living up to those commitments can be.

A public spat this week with U.S. officials over the precise effectiveness of its Covid-19 shot has upset regulators and chipped away at the drugmaker’s reputation in its biggest market. The fight has heightened doubts among senior U.S. medical officials about the company’s data analysis, threatening to lengthen the regulatory review. That is already months behind schedule after a separate standoff with the Food and Drug Administration in the fall over disclosures about the vaccine’s U.K. trials.