More than 12,000 study participants had received a second dose of the vaccine, Pfizer officials said on an investor conference call.
The company has enrolled more than 29,000 people in its 44,000-volunteer trial, developing it with German partner BioNTech to experimentally test the COVID-19 vaccine.
Pfizer said it was continuously investigating the safety and tolerability of the vaccine in its study.
An independent data monitoring committee can recommend stopping the study at any time, but to date it has not been done.
Rival AstraZeneca’s COVID-19 vaccine is being tested worldwide on 6 September after a volunteer in the UK reported serious side effects.
AstraZeneca’s trial resumed in the UK and Brazil on Monday following a green light from British regulators, but remained on hold in the United States.
(Reporting by Manas Mishra in Bengaluru; Editing by Arun Koyur and Shaunak Dasgupta)