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What a Booster Shot Really Does To Your Body

Does it really defend against COVID?

This fall and winter, many Americans have gotten booster shots to protect against COVID-19. (But not enough of us, health officials say—only about 40% of the U.S. has gotten a booster to date.) Several new studies have just been released about how effective those booster shots have been against COVID, particularly the super-contagious Omicron variant. But what is a booster shot, exactly? What does it do to the body, and does it really defend against COVID? Read on to find out more—and to ensure your health and the health of others, don't miss these Sure Signs You've Already Had COVID.


What a Booster Does In Your Body

Doctor holding syringe, medical injection in hand with glove.

A booster shot basically provides your immune system with a reminder on how to prepare a response to a disease-causing pathogen, preventing serious illness and clearing it from the body.

Some vaccines provide lifelong protection after the initial dose or doses (known as the prime series). The polio vaccine is one example; it's given in four doses to children between infancy and age 6, and protection lasts a lifetime. (Although at-risk adult travelers are encouraged to get a booster when traveling to countries where polio infection is high.)

But the protective effect of some vaccines fades, requiring "booster" doses after the initial doses are given. Children, pregnant women and healthcare workers get various booster shots. Experts say everyone should get a Tdap booster (against tetanus, diphtheria and pertussis) every 10 years throughout their lifetime.

Some vaccines become rapidly less effective. They might even become obsolete. That's what happens with the flu vaccine—new mutated strains of flu emerge constantly, so a new vaccine tailored to those strains is given each year. 

Last summer, studies found that the COVID-19 vaccines became significantly less protective against symptomatic illness after six months. Booster doses—a third shot after the two-dose Moderna or Pfizer vaccines, or a second shot after the single-dose Johnson & Johnson vaccine—were granted emergency authorization, first in the elderly, then in most age groups.

Several new studies have shown how those booster doses have affected our bodies' ability to fight off COVID and prevent serious complications. Read on to find out more.


COVID Boosters Make You Less Likely to Contract The Virus

Woman coughing in her elbow in grocery store.

Although "breakthrough" cases of COVID have been common in people who've been fully vaccinated—even in people who've received a booster shot—new data indicates that the booster significantly protects you from contracting the virus. 

Two studies released in late January found that people who'd been vaccinated and boosted were much less likely to contract COVID. One study that analyzed more than 10 million infections found that during the highly contagious Omicron surge:

  • Unvaccinated people were nearly five times more likely to contract COVID
  • People who gotten only two vaccine doses were 1.7 times more likely to catch COVID compared to people who'd had three.

Another study found that COVID boosters reduced the risk of symptomatic illness by 66 percent lower compared to two doses alone.


COVID Boosters Make You Less Likely to Get Seriously Ill or Die From The Virus

Elderly woman wearing oxygen mask sleeping in hospital bed

A newly released CDC study found that during the Omicron wave, a booster dose was 90 percent effective at preventing hospitalization from COVID. By comparison, the initial two-shot regimen was only 57 percent effective six months after the second dose. Booster shots were also 82 percent effective at preventing visits to the emergency room or urgent care, compared to 38 percent for people who'd been vaccinated but hadn't received a booster.


Is a COVID Booster Really a Booster?

Close up shot of hands checking Covid-19 vaccine report card and ticking 3rd or booster dose after vaccination.

The effectiveness of COVID boosters has led several health experts to question whether they're really booster shots at all—maybe the COVID vaccine should have been considered a three-injection prime series from the beginning. (Dr. Anthony Fauci, the nation's top infectious-disease expert, has been raising that possibility for months.) 

Where do we go from here? It's unclear. We may need regular COVID boosters going forward. An annual coronavirus vaccine—much like a flu shot—may be developed instead. Additional variants, when and if they emerge, may affect those outcomes.  

But the science is pretty clear: So far, COVID boosters help the body's immune system fight off the virus, preventing infection, serious illness and death.


How to Stay Safe Out There

A mid adult woman protects herself by placing an N95 face mask over her nose and mouth.

Follow the fundamentals and help end this pandemic, no matter where you live—get vaccinated ASAP; if you live in an area with low vaccination rates, wear an N95 face mask, don't travel, social distance, avoid large crowds, don't go indoors with people you're not sheltering with (especially in bars), practice good hand hygiene, and to protect your life and the lives of others, don't visit any of these 35 Places You're Most Likely to Catch COVID.

Michael Martin
Michael Martin is a New York City-based writer and editor. Read more about Michael
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