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Pfizer and BioNTech announced Wednesday that the efficacy portion of their Covid-19 vaccine trial has been completed, showing the vaccine to prevent 95% of cases of the disease.

The companies said that they plan to submit to the Food and Drug Administration for an emergency use authorization “within days,” and will also submit to regulatory agencies around the globe.


The results come little more than a week after the companies said an earlier analysis from the study showed the vaccine reduced infections by more than 90%, and just days after another company, Moderna, said that a similar vaccine reduced infections by 94.5%. Taken together, the results raise hopes that vaccines will be broadly available sometime next year, and also, perhaps, that other vaccines against the disease will also prove effective.

In a conversation at the STAT Summit Tuesday, Pfizer CEO Albert Bourla said that viewing the earlier data on the efficacy of the vaccine had been a high point in his life. “When I heard the over 90% efficacy, I felt I was living a dream,” he said.

Of the 170 cases of Covid-19 Pfizer observed in its trial, 162 occurred in the placebo group and just eight among the group that got its two-dose vaccine. Of the 10 cases of severe Covid-19, nine were in the placebo group, an important finding which suggests the vaccine prevents not only mild cases, but the type of serious disease that leads patients to die or be hospitalized.


In the early look at the Moderna trial, which is not yet complete, there were 11 cases of severe disease, all in the placebo group.

It is not clear how long the efficacy of either vaccine will last, in part because they have been developed at such dramatic speed.

“I take one step at a time,” Anthony Fauci, the director of the National Institute for Allergy and Infectious Diseases, said Tuesday at the STAT Summit. “I’ll take the 94.5% effective for now,” he said of Moderna’s interim analysis this week, “and we’ll worry about the durability of the effect” next.

Pfizer said that no serious safety concerns related to the vaccine were reported in its study, which included 43,661 volunteers. Data on common side effects was tracked in an 8,000-patient portion of the study. The only severe side effects to occur in more than 2% of people were fatigue, which occurred in 3.7% of patients after the second dose, and headache, which occurred in 2%. Older adults had fewer and milder side effects than younger participants. Approximately 19,000 participants in the study have been followed for at least two months since their second dose of the vaccine.

Pfizer’s updated results follow Moderna’s report that its Covid-19 vaccine, using similar technology, reduced the risk of Covid-19 by 94.5% in interim results from a 30,000-volunteer clinical trial. Moderna observed 95 cases of symptomatic Covid-19, only five of which affected participants who received the company’s vaccine.

In Moderna’s study, there were 11 cases of severe Covid-19, all of which occurred in the placebo group. Moderna said there were no significant safety concerns observed in the trial. Severe events that occurred in greater than 2% of patients included fatigue, muscle pain, headache, and achiness. These events were “generally short-lived,” the company said in a press release.

Like Pfizer, Moderna plans to file in the coming weeks for emergency use authorization from the FDA once it has collected further safety data. That would allow its vaccine to be distributed to people at high risk for Covid-19.

Pfizer and Moderna both use messenger RNA, or mRNA, technology in their vaccines. Each strand of synthetic mRNA is designed to encode for a protein found on the surface of SARS-CoV-2, the virus that causes Covid-19. Those mRNA strands enter the body’s cells and instruct them to produce that protein. The immune system then recognizes it as a foreign invader and produces antibodies that protect against Covid-19 if a person is later exposed to the virus.

Supplies of both vaccines are likely to be limited for months to come. Pfizer has said it could produce up to 50 million doses by the end of the year. Moderna, whose vaccination also requires two doses, has promised as many as 20 million in the same time frame. In 2021, Pfizer expects to manufacture about 1.3 billion doses, while Moderna will make between 500 million and 1 billion.

The Pfizer-BioNTech vaccine needs to be kept at very cold temperatures, which has raised concerns about the possible rollout of the vaccine. On Tuesday, Bourla said he expected that distribution issues would be manageable. “I think people will be surprised how smoothly the whole operation will go,” he said.

Pfizer, which did not accept research funding from the federal Operation Warp Speed initiative, has agreed to supply 100 million doses to the U.S. government in exchange for $1.9 billion. The U.S. has the option to buy 400 million more. Moderna received about $1 billion in federal funds to support its vaccine development and has agreed to provide 100 million doses to the U.S. for $1.5 billion.

About 30% of U.S. volunteers in Pfizer’s trial are people of color, the company said. Roughly 45% of U.S. participants are between the ages of 56 and 85, a group at particularly high risk for severe Covid-19. About 37% of volunteers in Moderna’s 30,000-volunteer trial were people of color, according to the company.

  • Questions: 1) How does the body know when to STOP producing this protein, since the mrna is injected into your genome. Is the protein being produced constantly? Is your immune system constantly producing these antibodies? 2) How do you know there is no disruption of other genes? 3) Since the body is producing this protein, how does the immune system know it is a “foreign” protein?
    4) How long does the immunity last?

    • 1) The last protein mRNA in any sequence is the STOP protein. Only a new mRNA sequence would produce more virus proteins.
      2) Not something we can really answer now, but that’s why they are studying side effects.
      3) Because it’s a foreign protein. It doesn’t matter where it came from, that’s not what the immune system uses to decide if something is foreign or not.
      4) At least five minutes.

  • I am also in the Pfizer trial. I’m sure I got the placebo. I had no symptoms after either of the shots. I’m hoping that the FDA will allow Pfizer to vaccinate those of us in the placebo group in the first group receiving the vaccine after approval. A friend of mine is in the Moderna trial, and we know she got the actual vaccine, as she was mildly ill for about three days after he second shot. She’s fine now.

  • Assuming that Pfizer had a balanced study, I find it surprising that less than 1% of the placebo group (roughly 22,000 people?) became infected. There is still far too little information about this study. Are we to assume that only 170 subjects out of the 43661 total subjects were infected with COVID-19? How long have the participants been followed? What percent of vaccinated subjects developed antibodies? With as little information as we are being given about the study, it seems that the reporting is fairly skewed on the optimistic side.

    • MaryAnne, I understand the point you are making. But, we don’t know the person time of followup of the 162 placebo cases or 22,000 total people in the placebo group. One thing we do know is that enrollment started at the end of July. So this gives us a ~4-month risk of about 0.7% (162/22,000). When I look at my county (low to moderate incidence, urban and rural) dashboard and calculate the risk of infection over the same time period as the trial, the 4-months risk is about 1.2%. So pretty similar. Then add in the fact that those who participate in a vaccine trial are likely more cautious and informed about protective measures, and indeed are strongly counseled by the study to be cautious to protect themselves from infection, it seems like a fairly reasonable rate of infections occurred. Also, just a week ago Pfizer was at 95 cases, and shot up to 170 as incidence has sped up nationwide.
      My conclusion is that their number of cases are in line with what one would expect.

    • 1) The study has JUST passed 2 months since the median patient received their 2nd dose. 162 / 22,000 is a pretty reasonable infection rate in that context;

      2) Trial participation tends to be strongly skewed to those who take greater precautions;

      3) These are SYMPTOMATIC cases of COVID-19 only (primary endpoint is prevention of DISEASE not sterilising immunity against infection). The general testing statistics would also capture some asymptomatic infections picked up in corporate testing and in close contact testing. I.e. The 162 can probably be grossed up to over 200 when you factor in those who mount a seriously robust T-cell defence and never display symptoms of infection;

      4) This isn’t some two bit bunch of amateurs that have conducted this study. It has been run by a firm that are experts at conducting randomised trials and is the most scrutinised process EVER in pharmaceutical history. Some respect for their professional integrity may be in order;

      5) If you still aren’t happy you can always take the risk of not being vaccinated. I’m going to be getting this or another strongly effective vaccine the minute I can however.

  • Now, I am assuming/hoping the claim is based on sound statistical analysis. Which seems to lie on the assumption that people in both the treated and control group have the same chance of being infected in the “wild”. On the other hand, those positivities related to the control group size of the trial equate to about 800 cases/100k (Moderna’s 600/100k), which are far below the current positivity rate among most of US states. That is, something seems off, or too optimistic.

    • But positivity rate > % infected, because of sample bias. It seems reasonable to assume that the population that gets tested in the US tends to have higher risk than the population that doesn’t (the population that gets tested includes more frontline workers, people with symptoms, people with known exposures, etc.).

      As an extreme example, in April New York state had a positivity rate approaching 40%, but that doesn’t meant that 40% of New Yorkers have had COVID!

    • In this cohort of cases, Pfizer is reporting individuals who are infected with COVID in the first report, 7 days after the 2nd shot. As time passes, they will likely report many more infected in the Placebo group. The design study seeks to capture those infected just after the 2nd shot, and with it running 2 years, it will continue to measure the difference in infected/non-infected. This is well explained in Medcram video

  • I’m a participant in the Pfizer trial, and just had a antibody test this week with Labcorp. 62 days after the 2nd shot, I have positive results for IgG antibodies. I’m very interested to learn more about the case of severe COVID in the one patient that had the shot and still developed COVID.

    • Hi Steve, had you taken an antibody test before being vaccinated? I understand that the likelihood of having asymptomatic COVID and this being the reason for testing positive is low.

      Would you know if all commercially available antibody tests can detect antibodies following vaccination? I had my 1st shot just a week ago and the next one is in two weeks. I don’t know if there is any point in testing myself before at least two weeks after the second shot but since I had some side effects as in 100.2F temperature, muscle aches for a few days and fatigue I am intrigued!

    • Hey John, I did have an antibodies test at Labcorp. It was $10 out of pocket with the rest covered by insurance or the government. They did test for S-Protein which the vaccine generates. Which vaccine did you take? If it was mRNA, then you can probably get tested at Labcorp to see. Based on your symptoms, it sounds like you got the actual vaccine. They recommend waiting 10-14 days after the 2nd shot to get max antibodies, and the Pfizer trial didn’t withdraw blood until 30 days after.

    • John, I misread your question initially. When I got the vaccine they did do a PCR test and blood draw, and so far as I know, it was negative because that would have knocked me out of the trial. I also gave blood just before, and with that blood donation, I tested negative for antibodies. My antibody test was 2 months after the 2nd shot and came back positive.

    • Thank you Steve. I had the Novavax vaccine (or placebo but unlikely) so, not an mRNA one but looking at phase I results, they tested for IgG anti–spike protein response so, I assume that I should get a positive result but need to be more patient!

      Regarding the severe COVID case, there are so many parameters to consider such as the health status of the affected individual, occupation (healthcare professional, hence more exposure to the virus?) and of course whether their outcome, despite the severity of the case, was positive and they got better.

    • Steve, I also found this while searching for an answer as to which commercially available tests could be used: “The Abbott SARS-CoV-2 IgG test is also limited in that it detects only IgG antibodies directed against nucleocapsid and cannot be used for recombinant spike protein vaccine studies.”

      So, not every test is fit for (this) purpose!

    • Steve, I too am a participant of Pfizer trial. My 2nd shot was at the end of August. I had zero side effects, so I assumed I am part of the placebo group. But curious where you got an antibody test. Are all antibody tests accurate enough or are there a lot of false readings? Just curious

    • Steve, just to let you know, if you had tested positive for antibodies at the first appointment when you recieved your first shot you wouldn’t necessarily have been knocked out of the trial… They only excluded symptomatic, laboratory confirmed (or probably) COVID cases. Asymptomatic cases were allowed to be included in the trial (presumably to test safety and efficacy in this population as well).

    • After the first dose of the vaccine, doesn’t it take 10 days or so for the immune response to develop ( especially the B cells making antibodies, a bit slower than T cell response)? You could still be infected the day after your first shot, before your immune system is ready.

      Also, not every ‘infectious dose’ of the COVID virus is the same. Your vaccinated-prepared immune system might easily defeat exposure to a few aerosols with 10,000 viruses each, but get overwhelmed by exposure to several 90 micron respiratory droplets swimming with 100 million viruses each. Exponential replication has to be defeated right away.

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