COVID-19: Vaccine Boosters and Breakthrough Cases Explained

Published on: Dec 3, 2021fb-roundtwitter-roundemail-round
A nurse administers a dose of the Pfizer Covid-19 vaccine to a college student at the California State University Long Beach (CSULB) campus on August 11, 2021.Getty Images

As of mid-November, nearly 60% of the US population has been fully vaccinated against COVID-19. And some people have been vaccinated for about a year – receiving their first shot as far back as December 2020. Now the country has administered more than 464 million doses. But research shows that vaccine efficacy can decrease over time

Here’s What To Know About Booster Shots… 

In the fall, the FDA and CDC gave the green light to Pfizer, Moderna, and Johnson & Johnson booster shots for...pretty much every fully vaxxed adult in the US. Here’s what you need to know…

  • Any adult can get a Pfizer or Moderna booster shot six months after their last shot. The agencies initially approved boosters for those 65 and older, or those who are more high-risk for COVID-19 because of medical conditions (think: diabetes, heart conditions) or exposure at work (think: health care workers and teachers). And they expanded their guidance in mid-November.

  • Those who got a J&J shot are eligible to get a booster two months after their initial dose.

  • You can also mix and match your booster shot. Meaning: if you got a two-dose vaccine regimen, you can get J&J as your third shot. Or, if you got a one-dose regimen, you can get either a Pfizer or Moderna booster.

So far, more than 44 million Americans have gotten a booster shot. And the decision to expand eligibility for all adults allows millions more to get an extra dose.

Btw, if you’re planning to get a flu shot and a COVID-19 booster this year, the CDC says you can get both – even on the same day

But the US isn’t the only one hopping on the booster bandwagon: A number of countries, including Israel, Germany, and France have offered booster shots to certain people. The World Health Organization has previously asked countries to ‘hold the boosters.’ That’s because many living in poorer countries haven’t had a chance to get even one dose yet. Our World in Data shows that only 6.2% of people in low-income countries have gotten at least one shot.

Delta and Omicron Variants’ Impact on Boosters

The delta variant accounts for more than 99% of new coronavirus infections in the US. The CDC says that it’s more contagious than other strains – and that fully vaccinated people who experience breakthrough cases (more on that below) can spread the virus. But, the agency also noted that the vaccines authorized in the US are highly effective, including against this variant.

Meanwhile, we’re still waiting for more data on how omicron can affect current boosters, vaccines, and treatments on the market. While we won’t know more about the latest COVID-19 “variant of concern” until mid-December, here’s what we do know.

Here’s What To Know About Breakthrough Cases… 

Breakthrough cases (aka when someone gets the virus after being fully vaxxed) are rare, but can still happen. The CDC is tracking how many breakthrough cases have led to hospitalizations, and how many have led to deaths.

Reminder: Vaccines help prevent the spread of COVID-19 and prevent severe symptoms. But they are not a cure for the disease. And do not prevent illness 100% of the time. Other factors outside of variants can also cause breakthrough cases, including timing. It takes a couple of weeks for your body to build up immunity, so you could contract the virus right before or right after you get your shot. And if you are exposed to or test positive for COVID-19 after getting vaccinated, here’s what you need to know to keep yourself and others safe.


COVID-19 vaccines are essential in protecting people against the virus. But the virus’s variants are slowing those plans, causing new spikes and breakthrough cases around the world. Officials hope that booster shots will give tens of millions of Americans another layer of protection. Meanwhile, as the data rolls in, it’s important to stick to the basics — even if you’re fully vaxxed.

Updated on Dec. 3 to include details about the omicron variant.

Updated on Nov. 19 to reflect the FDA and CDC's decision to approve booster shots for all adults.

Skimm'd by Maria McCallen and Kamini Ramdeen

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