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An experimental Covid-19 vaccine being developed by the drug giant Pfizer and the biotech firm BioNTech spurred immune responses in healthy patients, but also caused fever and other side effects, especially at higher doses.

The first clinical data on the vaccine were disclosed Wednesday in a paper released on medRXiv, a preprint server, meaning it has not yet been peer-reviewed or published in a journal. 

“We still have a ways to go and we’re testing other candidates as well,” said Philip Dormitzer, the chief scientific officer for viral vaccines at Pfizer’s research laboratories. “However, what we can say at this point is there is a viable candidate based on immunogenicity and early tolerability safety data.”


The study randomly assigned 45 patients to get one of three doses of the vaccine or placebo. Twelve received a 10-microgram dose, 12 a 30-microgram dose, 12 a 100-microgram dose, and nine a placebo. The 100-microgram dose caused fevers in half of patients; a second dose was not given at that level.

Following a second injection three weeks later of the other doses, 8.3% of the participants in the 10-microgram group and 75% of those in the 30-microgram group developed fevers. More than 50% of the patients who received one of those doses reported some kind of adverse event, including fever and sleep disturbances. None of these side effects was deemed serious, meaning they did not result in hospitalization or disability and were not life-threatening.


The vaccine generated antibodies against SARS-CoV-2, the virus that causes Covid-19, and some of these antibodies were neutralizing, meaning that they appear to prevent the virus from functioning. Levels of neutralizing antibodies were 1.8 to 2.8 times the level of that in the recovered patients.

It’s not certain that higher antibody levels will lead to immunity to the virus. To prove that, Pfizer will need to conduct large studies that aim to prove that people who have received the vaccine are at least 50% less likely to become infected. Those studies are expected to begin this summer, mostly in the United States. Pfizer and BioNTech are testing four different versions of the vaccine, but only one will advance to larger studies.

The current study did not include pregnant women, and no other information on the ethnic diversity of participants was noted, although the paper does say that future studies will need to include a more diverse group.

The second dose, a booster shot, was required for immunity. The patients who received the single 100-microgram dose had lower antibody levels than those who received two shots of the lower doses.

Fourteen Covid-19 vaccines are currently in human trials, according to the Milken Institute, including entrants from Inovio, CanSino, AstraZeneca, and Moderna. More are expected to start soon, including entrants from Merck, Johnson & Johnson, and Sanofi. In total, 178 vaccines are in various stages of development.

The Pfizer/BioNTech vaccine, like the Moderna vaccine, is based on a technology called messenger RNA, which uses a key genetic messenger found in cells to create protein that the immune system then learns to attack. Moderna has not yet published data on its vaccine but is expected to do so soon.

  • Great going ladies and gentlemen, keep up the good work, let’s annihilate this damn virus.

  • Inovio’s trial for its Covid vaccine candidate INO-4800 (a DNA platform) delivered better results than Pfizer/BioNTech: Inovio scored desired results for 34 out of 36 participants, without any high fevers or other side effects – and without the need for a booster shot. Inovio is also already doing the (for all other vaccine developers also still required) animal trials, and will therefore in the final approval race be ahead of loudmouth companies like Moderna and Pfizer (both mRNA platforms), AstraZeneca (adenovirus base) etc. It should be noted that the US Department of Defense ordered millions of Inovio’s Cellectra delivery devices (electro pulser that opens cells so the vaccine enters immediately). This is an innovative, fast and thorough company. Buy INO stock at this incorrectly clobbered low entry point !!!

  • Well done Pfizer/BioNTech! Good work! No-one expected a miracle perfect vaccine overnight, however, you have made a great step forward for the World so far. Thanks to all the staff of the two companies who dedicate their time and best efforts for all of us.

    • Great job? Are you kidding? INO has gone through both animal studies and P1 trials with no fevers. How many fevers here? Where are the animal studies Fpisser? Unethical. And of course the news networks will take this companies money to spread its lie, without mentioning the side-effects. This company has killed more people in its history than covid-19 ever will.

  • Does the protein !!attack the virus by preventing it from going intracellular which stops the cascade

    • I will pick a fever and some mild side effects any day to have immunity, even if it’s partial immunity and in fact I might even be willing to volunteer to get it early as a test subject depending on the vaccine. It’s a great thing to have any options that may help facilitate a more normal environment. Whatever happens they can learn from it, and frankly they have left viruses alone too long without good treatments. If you need a shot every 6 months so what.

  • But do the antibodies dissipate after a few months? Whether they come from previously contracting the disease itself or from a vaccine. That’s the problem with these types of viruses.

  • ” Go Take a Hike” was meant for the naysayers. Good work posted below was for all the researchers who are working day and night to pull us out of this mess. I will gladly take your vaccine. It is a lot better than Covid! Thanks!

  • Oh go take a hike….. but wear a mask please. If you won’t get the vaccine, at least you can do that to protect us from your viral particles that you may not know you are spewing out. No side effects to a mask.

  • Keep up the good work all you researchers. You are true heroes. We worship your efforts.

    • I’ll be happy with the side affects, given they were not serious, had no lasting effects AND it protects me and family from Covid. Normally I say the “no vaccine” phobia-mites don’t bother me but in Covid’s case, those people don’t just endanger themselves and family, they endanger any and all of us. When vaccine is developed the no takers should have to sign a no treatment/no hospital action commitment.

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