A leading COVID-19 vaccine candidate has been shown to be more than 90% effective, developer Pfizer and its partner BioNTech announced Monday.

Trial participants received two injections of the vaccine about three weeks apart. There were 90% fewer cases in those who received the vaccine versus those who received a placebo. The drugmakers expect to apply to the Food and Drug Administration for emergency use authorization in the third week of November, once all required safety data are collected.

But key questions remain while the full data is under wraps. It is not yet known whether the vaccine can prevent severe cases or whether it will prevent transmission by asymptomatic carriers, reported medical news outlet STAT.

The study is ongoing, and final vaccine efficacy percentage may vary, the drug companies noted. Participants will continue to be monitored for long-term protection against COVID-19 and safety for an additional two years after their second dose.

The vaccine is one among others being readied by the U.S. government for possible rapid distribution even as they continue to wend their way through clinical trials. If and when approved, the vaccines are scheduled to be on their way to high-risk groups within 24 hours, according to Operation Warp Speed, the White House’s vaccine delivery campaign.