Emergency approval of coronavirus vaccines is not guaranteed, FDA commissioner says

Career scientists at the FDA are reviewing raw data from the Pfizer and Moderna trials for the first time, investigating any adverse events or side effects that transpired over the course of the trials, different levels of effectiveness among populations and age groups, and whether individuals who were already infected by the coronavirus might have had any unexpected reactions to the vaccine doses.

Hahn said they will also be looking for evidence that the vaccines lead to increased respiratory infections – a concern with vaccines produced for other coronaviruses, such as SARS and MERS – and whether the use of a relatively new method of vaccine development, known as messenger RNA technology, poses any significant risks.