COVID-19 vaccine booster shots and additional doses: what we currently know

According to the Centers for Disease Control and Prevention (CDC), more than half of the U.S. population is now vaccinated against COVID-19. As of Oct. 12, approximately 187 million Americans are fully vaccinated, and more than 217 million have received at least one dose.

As part of the ongoing fight against COVID-19, in August 2021 the Department of Health and Human Services (HHS) announced plans to formalize a strategy regarding COVID-19 vaccine booster shots to help provide added protection against the virus. In a press release, the agency noted that the current available data makes it clear that protection against SARS-CoV-2, the virus that can lead to COVID-19, decreases over time after the initial vaccination. In addition, the CDC recommended that patients who are immunocompromised should receive an additional dose of an mRNA COVID-19 vaccine to improve their response to their initial vaccine series. In September 2021, the CDC recommended COVID-19 vaccine booster shots for those 65 and older and for immunocompromised individuals. They also recommended individuals over 18 with underlying medical conditions or those at an increased risk for COVID-19 exposure and transmission because of occupational or institutional setting may receive a booster shot.

While studies continue that will help to inform the ongoing strategy for the use of COVID-19 vaccines, now is a good time to revisit the science behind how vaccines work and clear up some of the more confusing terminology. Is a booster shot the same thing as “an additional dose?”  

What is a booster shot?

You may already be familiar with the concept of a booster shot. Certain vaccines that are received during childhood, such as Tdap (tetanus, diphtheria and pertussis) or varicella (chickenpox), require a booster after a period of time to help the body continue to fight off the viruses they guard against.

According to Dr. Kenneth Sands, chief epidemiologist at HCA Healthcare, “A booster is for a healthy person with a healthy immune system that in response to a vaccine will make an adequate amount of antibodies, but [those] antibodies wane with time. The booster brings those antibody levels back up again.” An antibody is a protein molecule that the body’s immune system produces to fight off harmful substances such as viruses.

While the vaccine is the best protection against the virus, it is still possible to get the virus since the antibodies start to decrease over time. As Dr. Sands notes, “The chances of one having COVID-19 infection several months after having been vaccinated does appear to increase, although these infections are typically mild.”

“The main issue that infectious disease experts are grappling with is the extent that COVID booster shots will significantly reduce serious illness and hospitalization,” he added. “This is still being debated by experts, and any strategies will come into greater focus out of the data available from ongoing studies.”

What’s the difference between an additional dose and a booster shot?

With so much conversation lately focusing on booster shots, there is a second consideration that is equally important — an additional dose. An additional dose of the COVID-19 vaccine is for people who are immunocompromised, which means their ability to fight infection is not what it should be.

There are many reasons a person may be immunocompromised, including certain medications or treatments such as chemotherapy, that affect how the immune system responds, says Dr. Sands. Someone who is immunocompromised may also have a genetic condition or infection that makes it harder for their immune system to fight off the virus.

“If you have been given the standard two-dose regimen [of an mRNA vaccine] and you’re immunocompromised, there’s a possibility you did not mount an adequate immune response,” Dr. Sands says. “There’s a lot more protection from the additional dose if you are immunocompromised, so the concept here is let’s get these people to an adequate initial vaccination.”

The other difference between a COVID-19 vaccine booster shot and an additional dose is the timing. “The additional dose is to be given any time after 28 days from the second mRNA dose,” says Dr. Sands. “Whereas the booster dose is given later.” You can get an additional dose by going to a retailer and letting them know you have an immunocompromising condition, or you can talk to your healthcare provider.

When is the vaccine most effective against COVID-19?

It’s important to note that you are not fully protected as soon as you receive your COVID-19 vaccine. It takes a couple of weeks for your body to produce the white blood cells that fight off infection, so you should continue taking precautions to protect yourself against the virus, such as wearing a mask, washing your hands frequently and social distancing.

New guidance about the vaccines and booster shots is continuously emerging as experts collect and analyze more data.

Be sure to speak with your doctor if you have questions about the COVID-19 vaccine booster shot or about getting an additional dose.