How does the COVID-19 Vaccine Work?

COVID-19 vaccines are dominating the headlines, mostly focused on when they’ll be approved and who will get one first. But how do the vaccines work? And how do they compare to common vaccines like the flu and measles? We’re breaking down the basics for you.

There are two main types of COVID-19 vaccines: ones that use live viruses and ones called mRNA vaccines. Let’s look at the mRNA vaccines first.

The Pfizer and Moderna vaccines are mRNA vaccines. They use a genetic molecule called RNA to cause your own cells to make a viral protein, as opposed to a live virus working to get the protein in your cells.

Your immune system then makes antibodies and immune cells that recognize the protein quickly and jump into action. Think of it like this: If DNA is the gene, then the RNA gives instructions for certain proteins. So, an mRNA vaccine is the instructions for the SARS-CoV-2 protein.

Pfizer has estimated that the vaccine is over 90 percent effective. But how do these results stack up against other vaccines you typically receive? On the low end, the flu vaccines are 40-60 percent effective at best, as the influenza virus keeps evolving into new forms each year. On the flip side, two doses of the measles vaccine are 97 percent effective.

On the other hand, the Johnson & Johnson/AstraZeneca vaccine, for example, uses a live virus. But, the virus used in these type of vaccines is not the virus that causes COVID-19 (SARS-CoV-2). This is a common myth. If you do happen to get sick with COVID-19 soon after receiving the vaccine, it’s likely you were already infected with it at the time. It takes a few weeks for the body to build immunity, so not enough time would have passed to provide you with any protection.

This vaccine actually uses something called an adenovirus, which causes the common cold -- a method that’s been used for decades. The version of this “living” virus has been weakened so that it doesn’t cause serious issues in people with healthy immune systems.

Live vaccines are the closest thing to an actual infection, so they act as teachers for our immune system, teaching them how to recognize and fight the virus. The live viruses deliver DNA, which are instructions for proteins. For the COVID-19 vaccine, researchers swap in a gene from the SARS-CoV-2 virus. When the vaccine is given to someone, the modified cold virus makes the SARS-CoV-2 protein, which builds the immunity.

A common concern for many people when it comes to vaccines is feeling side effects. It’s important to note that most vaccine side effects — which could include a sore arm or muscle aches — are a sign of the immune response, not a mild form of the illness. They are a positive sign that your body is gearing up to the fight the real infection.