Massachusetts-based Moderna says it has enrolled 25,296 people in the late-stage clinical trial of its COVID-19 vaccine that kicked off in July, one of three such studies currently underway in the US.

The company will most likely have results from the trial indicating whether the shot works in November, Bancel said. He added that it’s possible but “unlikely” that the results could arrive in October, when fellow drugmaker Pfizer expects to know whether its own vaccine works.

“If the infection rates in the country was to slow down in the next weeks, it could potentially be pushed out as a worst-case scenario, I would say, of December,” Bancel told CNBC in an interview.

The timeline depends on infection rates because researchers have to count how many people participating in the trial get sick to determine the vaccine’s efficacy, according to Bancel. Half the participants are receiving the vaccine while the other half are getting a placebo, he said.

“It’s sad to say, but that’s how clinical research goes — we need people getting disease from the virus to be able to scientifically, in a very rigorous process, calculate how many people get the disease with a vaccine, how many people get the disease on placebo to calculate the efficacy,” Bancel told CNBC.

Drugmakers around the world are racing to produce a vaccine to help end the global COVID-19 pandemic. Moderna, Pfizer and AstraZeneca have all started so-called Phase 3 studies meant to show whether their vaccines are safe and effective. The three companies are among nine drugmakers that have pledged to put safety first in their vaccine development efforts.

Centers for Disease Control and Prevention director Robert Redfield said Wednesday that a vaccine will be ready sometime between November and December but not widely available until mid-2021 — a claim President Trump later disputed.

Moderna shares were down 0.4 percent at $68.55 as of 11:20 a.m. Thursday.