The vaccine proved to be more than 90% effective in the first 94 subjects who were infected by the new coronavirus and developed at least one symptom, the companies said Monday.
The positive, though incomplete, results bring the vaccine a big step closer to getting cleared for widespread use.
Pfizer said it is on track to ask health regulators for permission to sell the shot before the end of this month, if pending data indicate the vaccine is safe.
The timetable suggests the vaccine could go into distribution this month or next, though U.S. health regulators have indicated they will take some time to conduct their review. Then it will take months for the companies to make enough doses for the general population.
The findings arrived on the timetable that the companies had been projecting. The results came too early for researchers to assess the safety of the vaccine, which the U.S. Food and Drug Administration says must include two months of monitoring at least half the study’s subjects for side effects.
Pfizer said it remained on track to collect at least two months of safety data during the third week of November and could file for an emergency authorization shortly thereafter.
So far, no serious safety issues have arisen in the study, the companies said. The study has enrolled nearly 44,000 subjects in the U.S. and other countries.
It is unclear how long the protection the vaccine appears to provide lasts, since researchers haven’t been studying volunteers for very long.
“You have to be encouraged by this,” said Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, who is on an FDA vaccine-advisory committee. “It certainly looks like things are trending in the direction for protection.”
Dr. Offit said it would be important to learn how well the vaccine works in high-risk groups, such as the elderly and diverse populations. Researchers say that in order for vaccines to be effective they must work across age groups, races and ethnicities.
The interim review of the vaccine’s effectiveness and safety was performed by an outside panel of independent experts known as a data-safety monitoring committee, which then shared its findings with Pfizer and BioNTech.
“You never know what the outcome is, but we had a feeling that we did everything we could possibly do,” Dr. Jansen said.
The vaccine is among the most-advanced in development in the West, with others in late-stage testing from Moderna Inc., Johnson & Johnson and AstraZeneca PLC.
Covid-19 vaccines developed by researchers in China and Russia have already been given to people in those and certain other countries.
The positive results don’t mean other vaccines will also prove to work, but they do suggest people can be protected against Covid-19.
“The most important message is that you can make a vaccine against this critter,” said Prof. John Bell, a health-policy adviser to the U.K. government from the University of Oxford who led Oxford in reaching its Covid-19 vaccine deal with British drugmaker AstraZeneca PLC.
Prof. Bell predicted that Oxford and AstraZeneca “won’t be far behind” in releasing their late-stage trial results out of the U.K. The Oxford-AstraZeneca results are expected within weeks.
Pfizer and BioNTech’s vaccine uses a new and unproven technology, known as mRNA, short for the molecular couriers called messenger RNA that carry genetic instructions to cells.
The shots deliver mRNA that prompts cells to make a synthetic version of the spike protein that juts from the surface of the new coronavirus. That protein triggers the immune system to defend against the virus.
Moderna’s vaccine also uses the mRNA technology. “This is great news as it shows mRNA can work,” Moderna Chief Executive Stéphane Bancel said. “Great day for the world, as we are all waiting for vaccines.”
He said that given the high Covid-19 cases rate in the U.S., Moderna is on track for an interim efficacy analysis and a two-month safety follow-up for at least half the people in its clinical trial this month.
After its vaccine appeared to work safely in a smaller and earlier-stage study, Pfizer and Germany’s BioNTech began in July seeking thousands of healthy volunteers for the large final-phase trial to determine whether it could be given to the public.
Like most vaccine trials, just a fraction of the subjects must become sick to evaluate whether the two-dose shot from Pfizer and BioNTech works.
For the final analysis, 164 study subjects need to become infected and develop at least one symptom. Researchers, however, designed the trial to take peeks at how the shot is performing after smaller numbers get sick.
Researchers originally planned for a first interim analysis after 32 subjects became sick. After talking with the FDA, Pfizer agreed to conduct the early peek after at least 62 subjects became sick, Dr. Jansen said.
By the time the two sides came to an agreement, the number of subjects who developed Covid-19 symptoms reached 94, Dr. Jansen said.
Pfizer officials learned about the early, or interim, analysis Sunday after speaking with the data-safety monitoring committee, Dr. Jansen said.
She said Pfizer has shared the outcome of the analysis with the FDA, though the World Health Organization and the European Medicines Agency, the region’s health regulator, said they haven’t seen the data yet.
The EMA has said it would clear a vaccine for use in the European Union even if it isn’t 50% effective and the WHO had set similar expectations, while the FDA has said it won’t authorize a vaccine unless it is at least 50% effective. The U.S. agency and companies wanted to see an even higher rate during an early look at an initial set of subjects to be sure it really works.
In its first look, however, the Pfizer and BioNTech vaccine worked even better than the FDA and two companies had been seeking.
The two-dose vaccine was found to be more than 90% effective at seven days after the second dose, Pfizer said, meaning that subjects were protected four weeks after their first shot.
Pfizer didn’t disclose the breakdown of how many of the 94 subjects in the analysis received the vaccine or a placebo. In the study, half receive the vaccine, while the other half receive a placebo.
Although specific safety information wasn’t available, Dr. Jansen said the data-safety monitoring committee told Pfizer officials that any side effects were similar to those in earlier testing of the vaccine.
Previously, Pfizer said some subjects in its early-stage study of the vaccine reported side effects such as fatigue, headaches and chills, and they eventually recovered. There weren’t serious side effects.
The latest timetable for the vaccine to become widely available is consistent with what Pfizer Chief Executive Albert Bourla and BioNTech co-founder and Chief Executive Ugur Sahin have suggested.
Pfizer plans to monitor patients for two years after their second dose for safety and vaccine duration.