Pfizer and BioNTech are starting a large study of a vaccine candidate for Covid-19 aimed at securing approvals — but it’s not the one for which results were released earlier this month.
The companies always said that they planned to pick from among four different candidates, all of which use a technology called messenger RNA to produce a protein on SARS-CoV-2, the virus that causes Covid-19. The vaccine is designed to lead the immune system to recognize the protein, and, it is hoped, to attack the virus.
On July 1, the companies announced results in a preprint publication showing that one vaccine candidate, BNT162b1, prompted an immune response in patients. That vaccine encoded a key portion of the spike protein, which SARS-CoV-2 uses to enter cells. The vaccine being taken forward, BNT162b2, encodes an optimized version of the whole spike protein, which should lead to “more consistent responses across diverse populations and in older adults,” Pfizer said in a statement.
Philip Dormitzer, a Pfizer vice president in charge of vaccines, said the selected vaccine appears to generate a stronger response, in part because it includes more of the spike protein. That could mean less variability in how people respond to the vaccine. It also seemed to cause fewer side effects. “It just seemed a bit milder,” Dormitzer said in an interview. “There were fewer reactions to immunization, and that was an important mark in its favor.”
Why this vaccine candidate would have milder side effects is not clear, but Pfizer scientists speculate it is because of changes made to the messenger RNA that make it easier for mammalian cells to produce it. Whatever the reason, Dormitzer said, this is the candidate Pfizer is testing during the pandemic, though it can’t rule out that other variants might be taken off the shelf in the future. “This really is the final choice,” he said.
The study, which began today, will recruit about 30,000 healthy volunteers between 18 and 85, randomizing half to get the vaccine and the rest to get placebo. Pfizer will recruit from about 120 sites in countries around the world, including the U.S., Germany, and Brazil. The primary goal is preventing patients from developing Covid-19, and the secondary endpoints include preventing severe cases of the disease and protecting volunteers against asymptomatic infections.
The announcement came hours after Moderna began a 30,000-participant trial of its own. Like Pfizer and BioNTech, Moderna’s vaccine uses messenger RNA to compel the body to produce spike proteins, leading to an immune response that will ideally protect subjects from Covid-19.
Pfizer and BioNTech said that they are on track to file for regulatory review as early as October, and that they plan to supply up to 100 million doses by the end of 2020 and 1.3 billion doses by the end of 2021.