Pfizer vs. Moderna: What You Need to Know About the New COVID Vaccines

You may be aware that both the Pfizer-BioNTech COVID-19 vaccine and Moderna's COVID-19 vaccine have been approved- but do you know how they work? Knowing how these vaccines actually work to protect you against COVID-19 can help ease any anxieties you may have about the vaccine and help you better understand why it's important to get one. Below we outline how the new COVID-19 vaccines work, the differences between the two, and details about side effects and safety.

New Approach to Vaccines

mRNA vaccines are a new type of vaccine to protect against infectious diseases. To trigger an immune response, many vaccines put a weakened or inactivated germ into our bodies. Not mRNA vaccines. Instead, they teach our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. That immune response, which produces antibodies, is what protects us from getting infected if the real virus enters our bodies.

Fact: mRNA vaccines do not use the live virus that causes COVID-19. They cannot give someone COVID-19.

Taking a Closer Look

COVID-19 mRNA vaccines give instructions for our cells to make a harmless piece of what is called the “spike protein.” The spike protein is found on the surface of the virus that causes COVID-19.

COVID-19 mRNA vaccines are given in the upper arm muscle. Once the instructions (mRNA) are inside the immune cells, the cells use them to make the protein piece. After the protein piece is made, the cell breaks down the instructions and gets rid of them. Next, the cell displays the protein piece on its surface. Our immune systems recognize that the protein doesn’t belong there and begin building an immune response and making antibodies, like what happens in natural infection against COVID-19. 

At the end of the process, our bodies have learned how to protect against future infection. Later, if a person encounters the germ again, their immune system can “recognize” it and “remember” how to fight it off. The benefit of mRNA vaccines, like all vaccines, is that those vaccinated gain this protection without ever having to risk the serious consequences of getting sick with COVID-19.

How is Moderna's vaccine different from Pfizer's?

The two vaccines are very similar, but they have a few key differences that make Moderna's vaccine "more flexible.”

  • Efficacy: Based on evidence from clinical trials, both Moderna’s and the Pfizer-BioNTech vaccine are about 95% effective at preventing COVID-19.
  • Structure: Both vaccines rely on mRNA, or messenger RNA, to work, although with slightly different structures and makeup.
  • Cold Storage: Moderna's vaccine does not need to be kept at super-cold temperatures, like Pfizer's. The Pfizer vaccine needs to be stored at about minus-75 degrees Celsius, about 50 degrees colder than any vaccine currently used in the US. The vaccine can be put in the refrigerator for only up to five days before it expires. In contrast, Moderna's vaccine can be kept at about minus-20 degrees Celsius, or about the temperature of a home freezer. Moderna's vaccine can also be kept in a refrigerator for 30 days before it expires. 
  • Dosage and Timing: Moderna requires two shots, 28 days apart. Pfizer requires two shots, 21 days apart.
  • Age: The Pfizer-BioNTech vaccine is recommended for people aged 16 years and older. The Moderna vaccine is recommended for people aged 18 years and older.

Side Effects and Safety

Both the Moderna and Pfizer-BioNTech vaccine can have common side effects such as: 

  • In the arm where you got the shot: Pain, swelling, redness
  • Throughout the rest of your body: Chills, tiredness, headache

These side effects usually start within a day or two of getting the vaccine and are a sign of your immune system kicking into gear. They should go away in a few days. They do not signal that the vaccine is unsafe. To date there are no serious, long-term side effects associated with these vaccines, which will be closely monitored as their use expands.

To recap, the Pfizer-BioNTech COVID-19 vaccine and Moderna's COVID-19 vaccine are a new type of vaccine called mRNA. This means they do not use the live virus that causes COVID-19 and they cannot give someone COVID-19. Both are also extremely effective, with an efficacy rate of 95%. While they may cause some common side effects, these are just a sign of your immune system kicking into gear. Until these vaccines become widely availalble to the general population, don't forget to social distance, wash your hands, and always wear your mask.