vaccine comparison

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


The Pfizer, Moderna and Johnson & Johnson COVID-19 vaccines are being distributed across the country. But the FDA is also reviewing 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 was 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 and this vaccine has been distributed to Yale New Haven Health as well. 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

Johnson & Johnson

Johnson & Johnson has received the FDA emergency use authorization for its vaccine. It works by using a modified adenovirus to enter cells, allowing the body to create antibodies. This technology has been used in the past to fight Ebola, HIV and Zika. Like the other COVID-19 vaccines, it does not alter your DNA or give you the virus.

One big difference between this vaccine and the other well-known vaccines is that it only requires one dose. Johnson & Johnson found it was 72% effective in the U.S. and 66% effective overall in preventing moderate to severe cases, 28 days after vaccination. It also does not require the ultra-cold temperatures needed for the mRNA vaccines. It is stable for at least three months at 2-8 degrees Celsius, or around 36 to 46 degrees Fahrenheit.

AstraZeneca Vaccine

The AstraZeneca vaccine 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. Something that sets this vaccine apart from the mRNA vaccines is that it appears to be very stable at relatively normal, refrigerated temperatures and it is about 63% effective against infection.

The World Health Organization granted emergency authorization for the AstraZeneca vaccine in February. However, it has not yet been approved for emergency use by the FDA in the U.S. 

How are these vaccines similar?

All of the vaccines developed in response to the pandemic were initially studied using adult participants, although studies have since expanded. The Pfizer vaccine is approved for recipients 16 and older, and the Moderna vaccine for those 18 and older. Researchers are looking 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 two mRNA vaccines and any other vaccines that get approved, depending on the availability of vaccine supply. 

It's important to note that all of the vaccines approved by the FDA help to protect against severe cases of COVID-19 and death. If patients are eligible to get a vaccine appointment, Yale New Haven Health does not recommend waiting for access to one particular vaccine based on their efficacy. The best way to protect yourself is to get vaccinated, regardless of the manufacturer. 

Learn more about COVID-19 vaccines