Pfizer Inc said on Tuesday participants were showing mostly mild-to-moderate side effects when given either the company's experimental coronavirus vaccine or a placebo in an ongoing late-stage study.
The company said in a presentation to investors that side effects included fatigue, headache, chills and muscle pain. Some participants in the trial also developed fevers - including a few high fevers. The data is blinded, meaning Pfizer does not know which patients received the vaccine or a placebo.
Kathrin Jansen, Pfizer's head of vaccine research and development, stressed that the independent data monitoring committee "has access to unblinded data so they would notify us if they have any safety concerns and have not done so to date."
The company has enrolled more than 29,000 people in its 44,000-volunteer trial to test the experimental Covid-19 vaccine it is developing with German partner BioNTech.
Over 12,000 study participants had received a second dose of the vaccine, Pfizer executives said on an investor conference call.
The comments follow rival AstraZeneca's Covid-19 vaccine trials being put on hold worldwide on September 6 after a serious side effect was reported in a volunteer in Britain.
AstraZeneca's trials resumed in Britain and Brazil on Monday following the green light from British regulators, but remain on hold in the United States.
Pfizer expects it will likely have results on whether the vaccine works in October.
"We do believe - given the very robust immune profile and also the preclinical profile ... that vaccine efficacy is likely to be 60 per cent or more," Pfizer's Chief Scientific Officer Mikael Dolsten said.