Why do Some Vaccines Require Two Doses?

Vaccines prevent diseases that can be deadly. Vaccines work with your body’s natural defenses to create immunity by imitating an infection. Vaccines use small parts of the organism that causes the infection to introduce your body to it. The body’s cells produce an immune response that remembers it. If you encounter the organism, then the body can remember and fight off the illness.

In some cases, you need more than one dose of a vaccine to help your body fight off the infection. For example, some of the COVID-19 vaccines will require two doses, while others just require one. Why is this? Does that mean one is less effective than the other?

The series and dosing for each vaccine is based on the initial results from the clinical trial data. Just because some vaccines may have one shot versus two, doesn’t mean that one is less effective than the other. Some vaccines may require more than one dose to provide more opportunity for your immune system to recognize the virus and build up more antibodies.

The Pfizer and Moderna vaccine are two shots – one shot followed by a booster either 21 or 28 days later depending on the vaccine. Both use mRNA, or messenger RNA, which gives cells instructions to create a protein like the SAR-CoV2 (COVID-19) protein. Once the protein is created, it triggers the immune response leading to the production of antibodies.

The Janssen and AstraZeneca/University of Oxford vaccines both utilize the adenoviruses, which is the same viruses that can cause the common cold. These inactivated/non-infective adenoviruses can be used to deliver DNA, which are instructions for proteins. For the COVID-19 vaccine, researchers swap in a gene from SARS-CoV-2 so that the modified cold virus makes the SARS-CoV2 protein, stimulating production of antibodies and additional immune responses to fight off the virus. The process creates an immune memory, which ultimately teaches the body to both recognize and attack the virus. The Janssen vaccine will be one dose, and the AstraZeneca/University of Oxford will require two doses.

If you are receiving one of the two-dose vaccines, to best maximize the efficacy of the vaccine and to ensure an optimal effectiveness, it is highly recommended that you receive the second dose within the specified timeframe as directed by the manufacturer. If you don’t receive the second shot, you will greatly decrease the chance of the vaccine working.

Vaccines are proven to protect your health and the health of those around you. Natural infections can cause severe complications, and it is essential to stay up to date with all vaccine schedules. For more information on the COVID-19 vaccine, visit www.ochsner.org/vaccine or call the COVID-19 hotline at 1-844-888-2772 or 504-842-0201.

The information in this blog post is accurate at the time of publication. However, as the situation surrounding COVID-19 continues to change, it's possible that information has changed since being published. While Ochsner Health is trying to keep our blog posts as up-to-date as possible, we also encourage readers to stay informed on news and recommendations by using the CDC website.