Participants of a phase 3 clinical trial of American biopharmaceutical company Pfizer's COVID-19 vaccine candidate were experiencing mild-to-moderate side effects, said the company on Tuesday.

The company has enrolled more than 29,000 people in its 44,000-volunteer trial to test the experimental COVID-19 vaccine, BNT162b2, that Pfizer is developing with German partner BioNTech.

In a presentation to investors, Pfizer said side effects included fatigue, headache, chills and muscle pain. Some participants in the trial also developed fevers, including a few high fevers.

Over 12,000 study participants had received a second dose of the vaccine, Pfizer executives said on an investor conference call.

Depending on current infection rates, a conclusive readout on efficacy of the vaccine candidate is expected by the end of next month, according to the company.

The latest comments by Pfizer came after multinational pharmaceutical company AstraZeneca's COVID-19 vaccine trials were put on hold globally on Sept. 6 following a report of serious side effect 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, according to a report of Fox Business.

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