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How to Compare the Authorized COVID Vaccines

May 27, 2021

It can be overwhelming deciding which COVID-19 vaccine you want to get, if you are given the choice. Below we break it down into some easy facts that will help you decide which one is the right one for you and your family.

The Vaccines

By definition, a vaccine is a substance used to start the production of antibodies and provide immunity, also known as protection, against a virus, such as COVID-19. Antibodies are blood proteins produced by your body in response to specific antigens, also known as foreign substance like viruses.

Currently available in the United States are three COVID-19 vaccines: Moderna, Pfizer and Johnson & Johnson (also known as Janssen), named after the companies that developed them. We often associate the vaccines with percentages representing how effective they are—Pfizer is 95% effective, for example. But what are the true differences between the vaccines, what do the numbers mean, and which statistics matter the most?

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There are similarities between all of them:

  • None of the vaccines will give you a COVID-19 infection.
  • None of the vaccines will change your DNA.
  • None of the vaccines contain eggs, latex or preservatives.
  • None of the vaccines are approved for children under 16 at this time.
  • No one in any of the vaccine trials has died from COVID-19.
  • All of the vaccines have been authorized by the Food and Drug Administration (FDA) for emergency use.
  • All of the vaccines could cause some injection site pain, tiredness, headache, muscle pain, chills, fever and nausea following vaccination.
  • It is not fully known how well the vaccines will prevent you from spreading COVID-19 to others, even if you do not have symptoms.
  • It is not fully known how long the COVID-19 vaccines protect people from COVID-19.
  • It is not fully known how effective the vaccines are against new variants of COVID-19.

Comparing the Available COVID-19 Vaccines


  • A mRNA vaccine - which teaches the cells in your body how to make a protein that triggers an immune response, providing protection
  • 94% effective at preventing the COVID-19 virus
  • Two doses of the shot are needed, 28 days apart
  • Some protection is provided after the first dose
  • Full protection is provided two weeks after the second dose
  • People 18 and older can receive this vaccine
  • Additional side effects include swollen lymph nodes in the arm that received the shot

Pfizer (BioNTech)

  • Also a mRNA vaccine
  • 95% effective at preventing COVID-19 virus symptoms
  • Two doses of the shot are needed, 21 days apart
  • Some protection is provided after the first dose
  • Full protection is provided two weeks after the second dose
  • People 12 and older can receive this vaccine
  • Additional side effects include swollen lymph nodes in the arm that received the shot

Johnson & Johnson (Janssen)

  • A vector vaccine – which uses a modified version of a different virus to deliver important instructions to your body to start building protection
  • 66% effective at preventing COVID-19 with symptoms
  • 85% effective at preventing COVID-19 with severe illness
  • These lower effectiveness rates can be attributed to the timing and location of the clinical trial – the trial took place during a higher peak of COVID-19 infections than the Pfizer and Moderna trials. Additionally, this trial took place not only in the United States, but also largely in South Africa and Brazil where other variants of the virus were much more common than the United States.
  • Only one dose of the shot is needed
  • Full protection is provided two weeks after vaccination
  • People 18 and older can receive this vaccine

J&J Vaccine Temporary Pause and Current Recommendation

According to the CDC, there may be an increased risk of a rare adverse event called thrombosis with thrombocytopenia syndrome (TTS) following use of the J&J/Janssen vaccine. Nearly all reports of this serious condition, which involves blood clots with low platelets, have been in adult women younger than 50 years old. The CDC says that "A review of all available data at this time shows that the J&J/Janssen COVID-19 Vaccine’s known and potential benefits outweigh its known and potential risks."

The Bottom Line

“Vaccine efficacy” is the term statisticians use to measure how much a vaccine lowers the risk of getting infected with a disease. Efficacy isn’t always the same. For example, Johnson & Johnson’s vaccine had an 85% efficacy rate against severe cases of COVID-19, which is important because that means it prevents hospitalizations and deaths.

“What is important is the herd immunity – the critical level of immune individuals that will end the pandemic by natural infection and vaccination. We do not need vaccines to be 100% effective to remove the public health measures currently in place,” says Dr. Armando Paez, Division Chief, Infectious Diseases, Department of Medicine at Baystate Health.

Each vaccine is a little bit different, but they all provide the protection we need against COVID-19. We hope that this comparison will help you make an informed decision about the COVID-19 vaccines.

Learn More

For information on Massachusetts vaccine eligibility, visit If you are a Connecticut resident, visit click here. If you are an eligible Massachusetts resident, schedule a vaccine with Baystate Health.

Still unsure? Hear Baystate Health experts share why they got the COVID-19 vaccine.

Get Your COVID Shot

You're eligible if you're age 12 + and live, work, or study in MA. Learn about how and why to make your vaccine appointment.

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