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data on the vaccine were disclosed Wednesday in a paper released on medRXiv, a preprint server, meaning it has not yet been peer-reviewed or published in a journal.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

  • I am curious to know what is being used as the placebo for the phase 3 trials as I read that Moderna is using the meningococcal vaccine as it’s control placebo. Are you using a pure saline injection or something else?

  • A fever is no big deal. I experienced a low grade fever for three months after getting a Yellow Fever Vaccine. I could live with the fever.

  • As long as no serious side effects with the smaller dose, that needed a booster dose, I suggest this result is good enough to publish. And build upon it. Many patients are waiting, thank you guys

  • There needs to be a disclosure that the RNA Moderna vaccine uses the genetic makeup of a fetus aborted about 50 years ago. I am wanting to know if the Pfizer/ BioNTech vaccines also use aborted fetal genetic material. Some people strongly object to explotation of an aborted baby, especially when fetal cells are not the ideal method to make the vaccine- there are other means that are more effective.

  • Great job, you guys know your stuff, keep pushing. You will figure it out. I am in rural Alabama. I beleive !

  • For those folks bothered by fever after vaccination — it means its working and generating a robust immune response!! Yeah, baby!

  • Very encouraging progress in a short period of time! I feel help is on the way to help us get through this awful pandemic. Keep up the excellent efforts. We all await the benefits of your efforts.

  • Great going ladies and gentlemen, keep up the good work, let’s annihilate this damn virus. And if need be I’ll volunteer for the trials. I do however have epilepsy.

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