From the article:

Late-stage vaccine testing remains crucial, as large trials can turn up rare but serious side effects that would surface only if many thousands of people received a vaccine.

“This is the whole point of doing these Phase 2, Phase 3 trials,” said Dr. Phyllis Tien, an infectious disease physician at the University of California, San Francisco. “We need to assess safety, and we won’t know the efficacy part until much later. I think halting the trial until the safety board can figure out whether or not this was directly related to the vaccine is a good idea.” Emphasis added

Which is why the phase three part of the trial, which typically takes at least a year, is so important.