While other vaccine developers have used human adenoviruses for such vaccines, the Oxford researchers chose an adenovirus found in chimpanzees.
They felt this would reduce the likelihood that an individual's immune system would attack the vector virus due to prior exposure rather than the intended target.
Reuters reviewed six research papers that detailed safety data of vaccines using the engineered chimpanzee adenovirus called ChAdOx1 for diseases including tuberculosis, prostate cancer and influenza.
In one of those trials, one serious adverse event cited by researchers was deemed unrelated to the vaccine.
The type of review being conducted by the FDA is generally intended to scrutinize raw data for other side effects, the sources said.
The US government's effort to speed development of a Covid-19 vaccine — and promises by Trump that one could be available prior to the November 3 presidential election — has led to concerns of political interference in the regulatory process at the expense of safety.
The FDA has denied this.
AstraZeneca's Covid-19 vaccine development could also be slowed by a change to its clinical trial protocols to show that it has met pre-determined safety and efficacy standards.