Researchers monitoring Pfizer Inc.’s giant trial of an experimental COVID-19 vaccine have reported no safety problems even after more than 12,000 people received their second of two doses.

"So far there has been no safety signal reported,” said Mikael Dolsten, Pfizer’s chief scientific officer, in an online meeting with investors. The trial, now targeted at enrolling 44,000 people, has reached its initial goal of 30,000 subjects, Pfizer executives said. The drugmakers have expanded the trial to to allow inclusion of teenagers and people with certain medical conditions, like HIV.

The trial compares two shots of the vaccine Pfizer is co-developing with Germany’s BioNTech SE to two doses of a placebo. While the doctors and subjects in the trial don’t know who got the vaccine — a measure to avoid bias — a committee of outside experts that knows which people received the shot is reviewing safety data on a weekly basis, Pfizer executives said during the investor meeting.