Booster vs. third dose: What's the difference, and which do you need?

When it comes to COVID-19 vaccines, it'll be a third dose for some, booster for others. Here's the difference between the two.

Jessica Rendall Wellness Writer
Jessica is a writer on the Wellness team with a focus on health news. Before CNET, she worked in local journalism covering public health issues, business and music.
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Jessica Rendall
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Between the rise of the delta coronavirus variant, fear of waning immunity due to breakthrough COVID-19 cases and the White House announcement that all adults will need a COVID-19 booster starting as early as Sept. 20, news has been flooded with unsatisfying advice and confusing timelines about if, and when, you'll need a coronavirus booster. (If you're a person with an immunocompromising condition who received an mRNA COVID-19 vaccine, you're eligible for an extra shot right now.)

But what exactly is a booster vaccine? Is it different from a third or extra shot? Does the meaning change if we're talking about a booster instead of a third dose?

As scientists and public health experts have been communicating with the media and the public about doses of the COVID-19 vaccine, the two words have sometimes been used synonymously. But we're here to get technical, so there's a difference in whether the third shot you'll likely eventually receive is called a "booster" or an "additional dose," depending on your circumstances. Here's what we know.

Read more: COVID-19 booster shot timing still in flux for most. Everything to know today

A third or extra dose is different from a booster

According to the US Department of Veteran Affairs, an "additional dose" of COVID-19 vaccine is for people who haven't mounted a sufficient immune response to the first two doses (or one, in the case of Johnson & Johnson, though the lack of data on Johnson & Johnson in immunocompromised people further complicates things and leaves them out of the Centers for Disease Control and Prevention's recommendation for immunocompromised people). A booster shot, according to the VA, is for people whose immune response likely weakened over time. 

Expanding on this logic, according to a Cleveland Clinic Q&A with Dr. Michelle Medina, third doses are what's being given to folks whose immune systems likely won't react as well to the current COVID-19 regimen (one or two shots), and boosters are given to everyone else when our immunity gradually wanes, due to new variants like delta, for example. 

When the Food and Drug Administration authorized another COVID-19 shot for some immunocompromised people in August, it did so with an "additional vaccine dose" of Pfizer.

Dr. Reynold Panettieri, director and vice chancellor of the Rutgers Institute for Translational Medicine and Science Professor of Medicine, says that deciphering a booster dose "depends on the vaccine and what is required to achieve maximum immunity." And with all we don't know about the coronavirus, and the way our bodies build (and lose) immunity to it, it might not be appropriate to call any COVID-19 vaccine a "booster." In fact, it might be too early to use the term "booster" at all, at least scientifically.

"We just don't know with COVID if the term 'booster' is appropriate," he says, adding that he prefers the use of "third dose." 

"Some of this is semantics," Panettieri says.

Shots that definitely are (and aren't) boosters

"The reason the flu vaccine is not a booster is because every year, the vaccine changes," Panettieri says. He says that a good example of a booster is the vaccine for whooping cough, tetanus and diphtheria, because you get the booster "when you know you don't have any more immunity that can be measured." Another example of a booster is that for the polio vaccine, he says, which some adults may receive, according to the CDC. 

Without getting totally lost in the weeds and wording, and whether or not your bonus COVID-19 shot is referred to by your healthcare provider as a third dose or a booster, the most important thing to know is your own health history. That's what will dictate your eligibility to get another shot when one is available to you. Also, there's no "extra," "additional" or "boost" without first receiving the standard coronavirus vaccine regimen. 

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.