NEW YORK • Pfizer said on Monday it had 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.

The American pharmaceutical corporation expanded the size of the trial from 30,000 people over the weekend. It has said it could have results next month on whether the vaccine works.

Fellow US biotechnology company Moderna's late-stage data and AstraZeneca's efforts with the University of Oxford are not far behind. AstraZeneca's trial in Britain has resumed after a pause of less than a week after a participant fell ill.

The trials will gauge how effective a vaccine is based on infection rates in those getting a placebo compared with those getting a shot and may produce interim results as soon as the first 30 or 40 people become infected.

"It's truly a numbers game," said analyst Yaron Werber at Cowen & Co. He and a handful of other Wall Street analysts view the Pfizer and BioNTech vaccine as the most promising so far.

The timing of a vaccine has come under increasing scrutiny as the United States presidential election nears and President Donald Trump appears to be pressuring the Food and Drug Administration for a shot, despite assurances from the agency as well as some drug-makers that decisions will be based on science.

REUTERS, BLOOMBERG