Why do some COVID-19 vaccines require 2 doses?

As COVID-19 vaccines continue to become available, there may be questions around how the vaccines work. One common question is why some of the vaccines require two doses in order to be effective. While this may differ from vaccinations for things like the flu, for some FDA-authorized and approved COVID-19 vaccines, a second dose is necessary.

In fact, there are several vaccines that require multiple doses to be effective. Many children’s vaccines, like Hepatitis B, Polio and Pneumococcal, require two or more doses over time to help boost immunity levels.

A senior man sits in a clinic while a nurse readies a vaccine A senior man sits in a clinic while a nurse readies a vaccine

While not all of the COVID-19 vaccines require two doses, some of them do. Dr. Anne Docimo, chief medical officer at UnitedHealthcare, breaks down the science behind these multiple-dose vaccines and how they work.

The first dose
The COVID-19 vaccines currently authorized and approved by the FDA that require two shots use messenger RNA (mRNA) to help replicate an immune response. This is not a weakened or inactive form of the virus. Instead, this vaccine teaches the cells in our bodies how to make an antibody against the “spike protein,” which is needed for the virus to enter the cells and cause infection.

The mRNA vaccine helps your immune system recognize the virus shouldn’t be there and prompts it to make antibodies that fight against the spike protein of the virus. This means the virus cannot enter the cells and the infection is prevented. Within two weeks, your body learns how to protect itself against future infection. The first shot helps build up that protection.

The second dose
The second dose strengthens the immune response to help give greater protection that lasts longer. This helps give your body another chance to develop the antibodies it needs to fight off the virus, which is especially important for those who may not have had an adequate initial immune response. While there is no difference between what is in the two doses, the second dose is needed to help boost the immune response to prevent infection.  

There is some concern that those who only get one dose may not have adequate protection. The second dose helps to remind your body to continue the production of antibodies and develops memory cells that may help your body more quickly fight off the actual virus, if you are exposed in the future.

If you receive a two-dose vaccine, the shots will be spaced a few weeks apart (typically three weeks to a month dependent upon which vaccine you receive). Clinical trials have shown this timeframe helps to best support the vaccine’s effectiveness, so make sure you receive both doses in the recommended timing.

The COVID-19 vaccines are an important step in slowing the spread of the disease. Even after you become vaccinated, you should continue to follow public health guidelines to help protect you, your family and others from COVID-19. 

The Centers for Disease Control and Prevention and state and local health departments are coordinating vaccine eligibility and availability. You can go to the UnitedHealthcare COVID-19 Vaccinate Resource Locator for more information specific to your location.

For more about the COVID-19 vaccines, click here.