Many questions remain, including whether the vaccine can prevent severe disease or complications, how long it will protect against infection and how well it will work in the elderly
Scientists on Monday said initial trial results for Pfizer Inc and BioNTech's coronavirus vaccine far outpaced their expectations for protection against a completely new disease, but that many questions remain unanswered.
The drugmakers said their vaccine was more than 90 percent effective at preventing Covid-19, based on data from the first 94 people in the trial to become infected with the coronavirus.
The efficacy rate means that the overwhelming majority of infections occurred among people who received a placebo rather than the vaccine.
The 44,000-volunteer study was initially designed for a first interim analysis of whether the vaccine was working after 32 participants developed Covid-19.
Dr. William Gruber, Pfizer's senior vice president of vaccine clinical research and development, said in an interview the companies changed the study plan after discussions with US regulators and ultimately ended up with data on 94 people.
"It gives you more power and more confidence," said John Moore, a professor of microbiology and immunology at Weill Cornell Medical College in New York. "When you triple the numbers and you get a large difference between them, it's much more likely to be real."
Others cautioned that many questions remain, including whether the vaccine can prevent severe disease or complications, how long it will protect against infection and how well it will work in the elderly.
They noted that required safety data will not be available until later this month. In addition, Pfizer and BioNTech have yet to submit their data for peer review by other scientists, a key step in determining the quality of the results.
'Really Encouraging' But Early
"These are really encouraging but they are the earliest of results possible," said Dr Gregory Poland, a virologist and vaccine researcher with the Mayo Clinic in Rochester, Minnesota.
The full study is designed to show the vaccine is effective after 164 people fall ill. Pfizer said that may happen in the first or second week of December, when a panel of outside advisors to the Food and Drug Administration reviews the study results and decides whether to recommend authorizing its use.
Gruber said he did not yet have a breakdown of how many of the first 94 infections occurred among Black or Latino participants, two communities hit hard by the disease. He also did he know exactly how many elderly people in the trial got sick. Older people with weakened immune systems are particularly vulnerable to severe Covid-19
So far, none of the 94 people in the initial analysis developed severe Covid-19. The FDA initially had requested that the interim data include at least five severe cases, but recently relaxed that requirement, Gruber said.
"If a vaccine is to reduce severe disease and death, and thus enable the population at large to return to their normal day-to-day lives, it will need to be effective in older and elderly members of our society," said Eleanor Riley, professor of immunology and infectious disease at Edinburgh University.
Gruber expects there will be some cases of severe disease before the trial ends.
"Bottom line is we're going to have to use the data we have and the high efficacy to give us confidence that we're going to prevent severe infection," he said.
Scientists also want to understand whether the Pfizer vaccine fully prevents people from getting infected with the new coronavirus - a huge advantage in reducing transmission - or if it simply reduces the degree to which they become ill.
"Ideally, we want to be able to completely protect from infection, but I think we all accept that these so-called first generation vaccines are more likely to prevent disease," said Lawrence Young, a professor of molecular oncology at Britain's University of Warwick. "And the subtlety there, which is important, is if you're infected then you can still transmit the virus."
Pfizer intends to seek a broad approval for individuals aged 16 to 85, but the FDA and an advisory group to the US Centers for Disease Control and Prevention would make decisions about who should be first in line to get the initial doses, which will be scarce.
If Pfizer wins emergency use authorization, Gruber said the company feels ethically bound to inform eligible trial participants who received a placebo that they are not protected against infection, and to offer them the vaccine in hopes of keeping them from dropping out of the trial.