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Pfizer and partner BioNTech said Monday that their vaccine against Covid-19 was strongly effective, exceeding expectations with results that are likely to be met with cautious excitement — and relief — in the face of the global pandemic.

The vaccine is the first to be tested in the United States to generate late-stage data. The companies said an early analysis of the results showed that individuals who received two injections of the vaccine three weeks apart experienced more than 90% fewer cases of symptomatic Covid-19 than those who received a placebo. For months, researchers have cautioned that a vaccine that might only be 60% or 70% effective. 


The Phase 3 study is ongoing and additional data could affect results.

In keeping with guidance from the Food and Drug Administration, the companies will not file for an emergency use authorization to distribute the vaccine until they reach another milestone: when half of the patients in their study have been observed for any safety issues for at least two months following their second dose. Pfizer expects to cross that threshold in the third week of November.

“I’ve been in vaccine development for 35 years,” William Gruber, Pfizer’s senior vice president of vaccine clinical research and development, told STAT. “I’ve seen some really good things. This is extraordinary.” He later added: “This really bodes well for us being able to get a handle on the epidemic and get us out of this situation.”


Although it is a bright spot in the battle against the pandemic and a triumph for Pfizer and BioNTech, a German company, key information about the vaccine is not yet available. There is no information yet on whether the vaccine prevents severe cases, the type that can cause hospitalization and death.

Nor is there any information yet on whether it prevents people from carrying the virus that causes Covid-19, SARS-CoV-2, without symptoms. 

Without more information, it’s too early to start predicting how much of an impact the vaccine could make, said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy.

“I don’t want to dampen any enthusiasm for this vaccine. I just want us to be realistic,” Osterholm said. “For a vaccine to really have maximal impact, it’s going to have to also reduce severe illness and death. And we just don’t know yet.”

Because the vaccine has been studied for only a matter of months, it is impossible to say how long it will protect against infection with the virus. The vaccine does cause side effects, including aches and fevers, according to previously published data. Gruber said that he believed the side effect profile was comparable to standard adult vaccines, but probably worse than Pfizer’s pneumonia vaccine, Prevnar, or a flu shot.

The results have not been peer-reviewed by outside scientists or published in a medical journal, and even Pfizer and BioNTech have been given no other  details about how the vaccine performed by the independent monitors overseeing the study.

Initial supplies of the vaccine, if authorized, will be limited. Pfizer says up to 50 million doses could be available globally. by the end of the year, with 1.3 billion available in 2021. There are also expected to be distribution challenges. The vaccine must be stored at super-cold temperatures, which could make it extremely difficult to deliver to many places. Pfizer has said it is confident those issues can be managed.

Although the estimate of the efficacy of the vaccine could change as the study is completed, it is close to a best-case scenario. That also bodes well for other vaccines in the late stages of testing, including those developed by Moderna, AstraZeneca, and Johnson & Johnson. 

“If that headline really number really holds up, that is huge. That is much better than I was expecting and it will make a huge difference,” said Ashish Jha, the dean of the School of Public Health at Brown University. He cautioned, however, that it is always difficult to evaluate science via press release and that researchers will need to see the full results. He noted that side effects are something to watch, because even if there are no serious long-term complications, people feeling sick for a day or two could lead some to be hesitant to take a vaccine.

“This really bodes well for us being able to get a handle on the epidemic and get us out of this situation.”

William Gruber, Pfizer senior vice president of vaccine clinical research and development

Both Pfizer’s vaccine and Moderna’s use messenger-RNA, or mRNA, technology, which uses genetic material to cause the body to create a protein from the virus; the immune system then recognizes the virus and learns to attack. Other vaccines in the late stages of development use genetically engineered viruses for a similar purpose, or pieces of protein that are directly injected. No mRNA product has ever been approved by regulators.

The story of how the data have been analyzed seems to include no small amount of drama. Pfizer, seeing an opportunity to both help battle a pandemic and demonstrate its research prowess, made decisions that were always likely to make its study the first of a Covid-19 vaccine to produce data — including its decision to have an independent group of researchers, known as a data safety and monitoring board, take an early look at the data in the 44,000-volunteer study before its completion. 

The first analysis was to occur after 32 volunteers — both those who received the vaccine and those on placebo — had contracted Covid-19. If fewer than six volunteers in the group who received the vaccine had developed Covid-19, the companies would make an announcement that the vaccine appeared to be effective. The study would continue until at least 164 cases of Covid-19 — individuals with at least one symptom and a positive test result — had been reported.

Scientists can now design genetic material called mRNA to help us build immunity to certain viruses, including SARS-CoV-2, the coronavirus that causes Covid-19. Find out how mRNA vaccines work in this video. Hyacinth Empinado/STAT

That study design, as well as those of other drug makers, came under fire from experts who worried that, even if it was statistically valid, these interim analyses would not provide enough data when a vaccine could be given to billions of people. 

In their announcement of the results, Pfizer and BioNTech revealed a surprise. The companies said they had decided not to conduct the 32-case analysis “after a discussion with the FDA.” Instead, they planned to conduct the analysis after 62 cases. But by the time the plan had been formalized, there had been 94 cases of Covid-19 in the study. It’s not known how many were in the vaccine arm, but it would have to be nine or fewer.

Gruber said that Pfizer and BioNTech had decided in late October that they wanted to drop the 32-case interim analysis. At that time, the companies decided to stop having their lab confirm cases of Covid-19 in the study, instead leaving samples in storage. The FDA was aware of this decision. Discussions between the agency and the companies concluded, and testing began this past Wednesday. When the samples were tested, there were 94 cases of Covid in the trial. The DSMB met on Sunday.

This means that the statistical strength of the result is likely far stronger than was initially expected. It also means that if Pfizer had held to the original plan, the data would likely have been available in October, as its CEO, Albert Bourla, had initially predicted.

Gruber said that there will not be another interim analysis conducted in the study. He also said that Pfizer’s estimate that it could file for authorization of the vaccine by the third week of November was based on the assumption that the FDA would be willing to accept two-month safety data on half the volunteers in the study as initially planned, when it was to include 30,000 volunteers, not more than 44,000, as is now the case. Those discussions are ongoing.

But Gruber said he now expects that by the time of the planned meeting of the FDA’s vaccine advisory committee in December, the study’s efficacy portion could be completed, having reached 164 cases of Covid-19.

He also emphasized that although there will only be a few months of data from this study, results from earlier studies make him optimistic that immunity from the vaccine will not wane rapidly.

The study has enrolled  43,538 volunteers the companies said, and 38,955 have received their second dose. About 42% of global participants and 30% of U.S. participants have racially and ethnically diverse backgrounds.

Bourla, Pfizer’s CEO, said the results mark “a great day for science and humanity,” in a statement, saying they provide “initial evidence of our vaccine’s ability to prevent Covid-19.” He added: “We look forward to sharing additional efficacy and safety data generated from thousands of participants in the coming weeks.”

Helen Branswell contributed reporting.

  • This Pfizer/BioNTech “news” has zero connection with a departing or incoming president. The companies created a hopeful wave that is premature: in the 3rd week of November (at the earliest) will half of trial patients be followed for 2 months after their second shot. It is also still not clear if this vaccine really reduces severe illness and death (per expert Michael Osterholm). So what is the difference between yesterday and today? Nothing but empty commotion. The companies did put in big money, an investment that generated big orders. For a product that is almost impossible to get to many other countries where super-low storage temperatures are impossible. There are other R&D companies advancing better manageable vaccines (one shot, or a tablet), with likely better research / proof. I would recommend that everyone stays calm and patient, there is no Hallelujah thing today, and lawsuits against these companies ought to be expected.

  • I’m amazed at the politically rabid and irrelevant tenor of some of these comments. Well done to the development team. Let’s hope the positive results are borne out and the vaccine is delivered safely to as many people as possible. Let’s hope too that we can develop a little more tolerance and grace along the way, though; otherwise, frankly, what’s the point?!

  • Then why did you say you got covid. You didnt take the medicine you made. Why is it illinois covid jumped up so high. Guess you got biden to bail you out of our debt. Pelosi is not gonna be the head. So now what you got. Sorry but cant wait till all your money wont keep you out prison. All you dems govs go to prison. Thanks to mike madigan is good. Your next amen

    • None of it is manufactured or developed in China. They didn’t even host a clinical trial.

      Chinese pharma companies have a few vaccines in late stage trials that should start reporting out soon. This vaccine is not one of them.

    • That’s the big question, isn’t it? Yay, we can still do some R&D in the US. Since Clinton, we have had zero capacity to mass produce vaccines of any kind in the US. 100% of vaccine manufacture, and 98% of drug manufacture, have been outsourced to India and China, for the higher profit margins you get when you can use political prisoners and people who live in mud huts and get paid two cents a month as cheap labor. Trump was supposed to do something about this.

      How many products from China or India have come into the US that were actually in spec, that actually lived up to the promises in the contract or the advertisement? Didn’t we import an enormous batch of pet food–why the hell are we importing pet food, by the way?–from China just a couple years ago that was contaminated with toxic waste and killed tens of thousands of pets? Ask any metallurgist about “precision machined steel parts” made in India and how much they look like what’s on the spec sheet.

      I’m not an anti-vaxxer. I fully accept the idea that the human immune system can gain functionality to protect against new diseases this way. But if you think I’m going to let anybody shoot me or my kids up with something made in China or India or any other primitive, backward, polluted Third World country, you’re out of your mind.

  • If this vax is the best Med breakthrough in 100 years as CEO claims why no reporting on number of vaxed which are positive? Where are age/underlying conditions/racial-ethnic/exposure-at-risk activities ratios or estimates/institutionalized populations information given? Are weekly tests conducted? Are number of asymtomatics known? How can u ascertain estimates of efficacies if ppl studied haven’t had known exposure to positives? Assumptions that subjects were exposed is absurdly problematic. Guessing that is tantamount to conspiratorial types saying u did this to make money! You expect high risk populations to take this unsound illogical outrageously and disturbingly problematic pronouncement that it’s the greatest Med development in 100 years as anything other than fantastical thinking! Stuff your ego and get back to work!

    • This is just a press release. As a publicly traded company, there is a lot of pressure to release market moving news quickly (it prevents accusations of insider trading). The real data and analysis will be published through peer review, most likely in a couple of months.

      I would like to push back though against your suggestion that assuming vaccinated subjects were exposed is problematic. Nobody involved in the study knew who had the placebo vs the vaccine. The fact that roughly 85 of the placebo group came down with COVID is strong evidence that the population the trial vaccinated was getting exposed. Otherwise, how would all the vaccinated trial members avoid exposure while the placebo trial members didn’t, even nobody knew who was in which group?

    • Highly unlikely. Market moving information like this is typically held just to people who need to know in order to avoid insider trading. It seems unlikely either individual would have been informed prior to the the release. Not sure if even the FDA would have been notified.

    • Probably not since biden had absolutely nothing to do with it. The vaccine was developed and funded by the Trump administration. I am sure biden is eager to take credit for it.

  • “The companies said an early analysis of the results showed that individuals who received two injections of the vaccine three weeks apart experienced more than 90% fewer cases of symptomatic Covid-19 than those who received a placebo.”

    Am I missing something here. I thought one of the main reasons that made controlling the spread of the virus so difficult was asymptomatic spreading. This study only talks about symptomatic cases decreasing…

    • If everyone in the population has the vaccine, so very few come down with symptoms, then we can allow the virus to spread unhindered with little negative effect. It is not as if we shut down the economy, social distance, and wear masks for a cold. So even a vaccine with no impact on replication would have a huge effect on the world.

      I think it would be really hard to design a clinical trial to measure this. I suspect a general impression for the impact on contagiousness will be gotten though post marketing monitoring.

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