The Pfizer Vaccine Explained

  • Kelly Shen ’23, photo from the CDC on

There are several vaccines against the Sars-Cov2 virus which are being studied, one is made by Pfizer, and is an mRNA based vaccine. It was found to be 95% effective in their phase 3 study of at least 8,000 patients. As is standard for clinical trials, the trial was “double blinded,” meaning that no one except the independent board — not the volunteers, doctors, or the company’s top executives — knows how many of the 94 people sickened by the virus got the vaccine or the placebo. Here’s how it works (credit to the New York Times):

  1. The vaccine uses messenger RNA, genetic material that our cells read to make proteins. The molecule — called mRNA for short — is fragile and would be chopped to pieces by our natural enzymes if it were injected directly into the body. To protect their vaccine, Pfizer and BioNTech wrap mRNA in oily bubbles made of lipid nanoparticles.
  2. After injection, the vaccine particles bump into cells and fuse to them, releasing mRNA. The cell’s molecules read its sequence and build spike proteins. The mRNA from the vaccine is eventually destroyed by the cell, leaving no permanent trace.
  3. Some of the spike proteins form spikes that migrate to the surface of the cell. These protruding spikes and spike protein fragments can then be recognized by the immune system.
  4. The cell presents fragments of the spike protein on its surface. When other cells called helper T-cells detect these fragments, the helper T-cells can raise the alarm and help marshal other immune cells to fight the infection.

Timeline of the creation of the vaccine: (also credit to NY Times)

January, 2020 

BioNTech begins work on a vaccine after Dr. Ugur Sahin, one of the company’s founders, becomes convinced that the coronavirus will spread from China into a pandemic.


BioNTech and Pfizer agree to collaborate.


The companies launch a Phase 1/2 trial on two versions of a mRNA vaccine. One version, known as BNT162b2, had fewer side effects.

July 22 

The Trump administration awards a $1.9 billion contract for 100 million doses to be delivered by December, with an option to acquire 500 million more doses, if the vaccine is authorized by the Food and Drug Administration.

July 27 

The companies launch a Phase 2/3 trial with 30,000 volunteers in the United States and other countries, including Argentina, Brazil and Germany.

Sept. 12 

Pfizer and BioNTech announce they will seek to expand their U.S. trial to 44,000 participants.

Nov. 9 

Preliminary data indicates the Pfizer vaccine is over 90 percent effective, with no serious side effects. The final data from the trial shows the efficacy rate is 95 percent.

Nov. 20 

Pfizer requests an emergency use authorization from the F.D.A.

Dec. 2 

Britain gives emergency authorization to Pfizer and BioNTech’s vaccine, becoming the first Western country to give such an approval to a coronavirus vaccine.

Dec. 8 

Margaret Keenan and William Shakespeare are among the first people to receive shots of the vaccine in Britain, on the first day of vaccinations for at-risk health care workers and people over 80.

Dec. 10 

The F.D.A. will meet in an open session to discuss emergency authorization of the Pfizer-BioNTech vaccine.

Dec. 31 

Pfizer expects to produce up to 50 million doses by the end of the year, and up to 1.3 billion doses in 2021. Each vaccinated person will require two doses.

Spring 2021 

Vaccines by Pfizer and Moderna are expected to reach large-scale distribution in the spring.

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