Pfizer’s COVID-19 vaccine is 90% effective, preliminary trial data show

The vaccine protects people from getting COVID-19, according to an analysis of 94 cases

volunteer receives vaccine in Pfizer and BioNTech’s trial

A volunteer in Florida gets an injection in early September as part of Pfizer and BioNTech’s trial to determine how well their COVID-19 vaccine works.

Eva Marie Uzcategui/Bloomberg via Getty Images

The race to greenlight a COVID-19 vaccine in the United States has entered its final sprint, with one leading candidate becoming the first to release preliminary results showing its vaccine is more than 90 percent effective at preventing people from getting sick from the coronavirus.

The long-awaited announcement came in a Nov. 9 news release detailing the results from an interim analysis of an ongoing Phase III clinical trial comparing the vaccine developed by global pharmaceutical company Pfizer and German biotech company BioNTech with a placebo.

COVID-19 cases are soaring globally, and some countries are reverting to lockdowns and other drastic measures to curb the virus’ spread. As of November 9, more than 50 million people have been infected worldwide — including more than 10 million in the United States — and more than 1.2 million people have died from the disease.

“We are a significant step closer to providing people around the world with a much-needed breakthrough to help bring an end to this global health crisis,” Pfizer chairman and CEO Albert Bourla said in the release.

The preliminary data have yet to be peer reviewed by other scientists and the specifics have not yet been released. But among people who received the vaccine, there were more than 90 percent fewer symptomatic cases of COVID-19 than among people given a placebo, the companies reported. It’s still unclear how the vaccine might perform among different age groups, including children and older people, and whether getting the vaccine affected the severity of the disease.

The trial has so far enrolled more than 43,000 people. Of those participants, 38,955 have completed the vaccine regimen, which consists of two injections spaced three weeks apart. At the time of the analysis — which looked at data seven days after participants received a second dose — a total of 94 people in the trial had confirmed coronavirus cases.

The publicly released data did not include how many cases were recorded in each group or what portion of the nearly 39,000 people who got both vaccine doses were included in the analysis. The companies had previously said they would conduct a preliminary analysis when 62 people had COVID-19. The reported efficacy rate suggests that most of those who have gotten sick so far were in the placebo group. Researchers plan to perform a final analysis when a total of 164 cases have been recorded and will follow participants for two years.

The U.S. Food and Drug Administration recommends that COVID-19 vaccines have at least 50 percent efficacy, meaning a vaccine should reduce COVID-19 cases in vaccinated people compared with a placebo by half (SN: 10/4/20). The preliminary analysis hints that the Pfizer and BioNTech vaccine could exceed that benchmark, but the clinical trial is ongoing so final efficacy could change.

“It’s early, but we can be cautiously optimistic,” says Mark Slifka, a viral immunologist at Oregon Health & Science University in Portland. “By hitting greater than 90 percent, that’s where we want to see successful vaccines.”

To apply for an emergency use authorization, the FDA requires that half of participants have at least two months of data regarding a COVID-19 vaccine’s safety after they receive the final dose. For Pfizer and BioNTech’s vaccine, that point is expected to come in the third week of November.

Vaccines from other companies, including one from Cambridge, Mass.–based Moderna, Inc., are expected to be close behind (SN: 5/18/20). And countries like China and Russia have approved other vaccines for limited use in their countries, although none have released data from Phase III trials to show how effective those might be (SN: 8/11/20). 

Slifka notes that it’s unknown whether the Pfizer-BioNTech vaccine’s effectiveness will hold over time.” That 90 percent efficacy is “only until day seven [after the last of the two injections]. We just need to see if it maintains above, at or near that level and if so for how long. And even if it goes down, that doesn’t necessarily mean the vaccine is a failure. It just tells us when we would need to do another booster.”

The Pfizer and BioNTech vaccine contains a viral mRNA, a genetic molecule that cellular machinery reads in order to build proteins. In this case, that mRNA holds the genetic instructions for making the coronavirus’ spike protein, which helps the virus attach to and enter host cells. Nanoparticles encase the mRNA and deliver it to the host cells. Those cells then build replicas of the spike protein so the immune system can recognize it and build up a memory of the virus. If a vaccinated person then encounters the virus for real, their body is primed to send in antibodies and other immune cells to fight the virus. The vaccine does not contain any infectious virus that can infect cells.

Previous clinical trials showed that the mRNA vaccine produced immune responses in people that could potentially protect against the virus. It and other vaccines also passed initial safety tests indicating that the vaccine did not cause serious side effects (SN: 7/21/20). The vaccine does cause some mild to moderate symptoms, such as pain at the injection site, fever and chills.

“These are certainly very promising results,” says Brianne Barker, an immunologist at Drew University in Madison, N.J. But the news release suggests that the vaccine protects participants from getting sick with COVID-19 — that is, having the symptoms of the disease like loss of smell, fever or trouble breathing (SN: 7/2/20) — but not from infection with the coronavirus. That could have implications regarding whether the vaccine can eventually eliminate the virus from circulating among people. “It’s possible that a vaccine could prevent disease but still allow you to get infected,” Barker says.

The news release also doesn’t mention if there was a difference in how sick people in each group got. “That could be really important in terms of how this vaccine works in the population,” Barker says.

Erin I. Garcia de Jesus is a staff writer at Science News. She holds a Ph.D. in microbiology from the University of Washington and a master’s in science communication from the University of California, Santa Cruz.

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