vaccine comparison

Enlarge image

Learn about each COVID-19 vaccine and how they are similar:


The Pfizer BioNTech COVID-19 vaccine was the first vaccine approved for emergency use by the Food and Drug Administration, with the first doses going to frontline healthcare workers and those living in long-term care facilities. Moderna’s COVID-19 vaccine was the second one approved for emergency use in the United States, and is currently being distributed across the country. The FDA is also reviewing several other vaccines for safety and efficacy. Find out the differences between them.

Pfizer Vaccine

The Pfizer vaccine is a 2-shot series given three weeks apart. It consists of genetic material called mRNA that stimulates the immune system to make antibodies that protect against COVID-19.


Why 2 Shots? More than one dose helps your immune system figure out how to stop a future COVID-19 infection.

The vaccine material breaks down shortly after it’s taken into our cells and does not impact our genes. This vaccine is new but RNA vaccines are not. They have been studied for several years and used against influenza, Ebola and the Zika virus.

This vaccine needs to be stored in extremely cold temperatures, between -112 degrees Fahrenheit and -76 degrees Fahrenheit, until its ready for use. This ensures the vaccine remains stable. The first vaccine acquired by Yale New Haven Health for its frontline workers is the Pfizer vaccine and we have the necessary storage to keep it at the appropriate temperatures.

The Pfizer vaccine is about 53% effective after the first dose and 95% effective after the second dose, which is why it’s so important for recipients to take both doses.

Common side effects include pain at the injection site, fever, fatigue, muscle pain and joint pain. Some of those mild side effects can occur after getting the flu shot.

Moderna Vaccine

The second COVID-19 vaccine approved by the FDA for emergency use was developed by Moderna. Like the Pfizer vaccine, it is a mRNA vaccine and consists of 2 injections. Unlike the Pfizer vaccine, the Moderna vaccine injections given four weeks apart. Neither vaccine contains any live or dead virus, so you can’t get COVID-19 from them.

There are, however, some slight differences. The Moderna vaccine requires storage at -4 degrees Fahrenheit, but is stable at room temperature for a slightly longer period of time than the Pfizer vaccine.

Common side effects can include pain at the injection site, fever and fatigue. The Moderna vaccine is around 94% effective after both doses.


Synthetic Messenger RNA (mRNA) tricks your body into producing some of the vital proteins so it can produce a defensive response to them. Once injected with Double-Stranded DNA vaccines, our bodies make antibodies to defend against infection.

Find out how vaccines work

AstraZeneca Vaccine

The AstraZeneca vaccine, which is not yet approved for emergency use in the United States, is based on a different technology than the Moderna and Pfizer vaccines. It is made from a weakened version of a common cold virus (known as an adenovirus) from chimpanzees. It has been modified to look more like coronavirus - although it can't cause illness in humans.

When the vaccine is injected into a patient, it prompts the immune system to start making antibodies and primes it to attack any coronavirus infection.

This vaccine appears to be very stable at relatively normal, refrigerated temperatures, potentially making it a better candidate for the general public. However, it still needs to be reviewed by the FDA.

There are open questions about the AstraZeneca vaccine’s efficacy. There was some confusion about the implementation of the clinical trial, so additional research is necessary to determine it’s true efficacy. It is effective enough to clear the bar set by the FDA, but its approval is still further off.

How are these vaccines similar?

All of the vaccines developed in response to the pandemic were studied using adult participants. The Pfizer vaccine is approved for recipients 16 and older, and the Moderna vaccine for those 18 and older. The AstraZeneca vaccine most likely will be approved for those 18 and older. Researchers are now turning their attention to younger participants to see if these vaccines are safe for patients as young as 12.

Any time a vaccine is approved for emergency use, they will continue to be studied and monitored for safety by the FDA. It is unlikely that patients will be able to choose between the Pfizer and Moderna vaccines since they are so similar. Additionally, it is not clear if patients will be able to choose between the AstraZeneca vaccine and the two mRNA vaccines, depending on the availability of vaccine supply.

Learn more about COVID-19 vaccines