December 22, 2021

What To Know About Breakthrough COVID-19 Cases

Plus, what role delta and omicron play — and why you need your booster

coronavirus molecules

Breakthrough cases of COVID-19 seem to be more prevalent than ever. But just how common — and how dangerous — are breakthrough cases? And can they be prevented?


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To get the facts about COVID-19 breakthrough cases, we talked to infectious disease expert Steven Gordon, MD. He explains what they are, why they happen and whether vaccines are still effective (spoiler: They really, really are).

What is a breakthrough COVID-19 case?

According to the CDC, a “breakthrough” case is when a person tests positive for COVID-19 at least two weeks after becoming fully vaccinated (which includes receiving a booster or third dose, if you’re eligible).

Breakthrough cases are, as the CDC also notes, to be expected. While the COVID-19 vaccines are highly effective at delivering immunity, no vaccine is 100% effective.

But another goal of vaccination is preventing serious illness. And in that regard, the COVID-19 vaccines are successful.

“Throughout the pandemic, we’ve talked about flattening the curve,” Dr. Gordon says. “And the vaccine has effectively done that. These spikes in severe cases of COVID-19 are primarily among the unvaccinated, not among people who are fully vaccinated.”

What are the symptoms of a breakthrough case?

The symptoms of a breakthrough case are the same as with typical COVID-19 cases. But people who are fully vaccinated/boosted with cases of COVID-19 are less likely to develop serious illness than those who are unvaccinated.

“Many breakthrough cases are either asymptomatic or have symptoms that are far less severe than cases in unvaccinated patients,” Dr. Gordon says. “The vaccine and booster are absolutely key here.”

How common are breakthrough cases?

The CDC is collecting data on vaccine breakthrough infections, but because many breakthrough cases are asymptomatic or mild and aren’t reported by people, the total number of reported breakthrough cases likely represents an undercount.

The CDC is, however, keeping track of one important metric regarding breakthrough cases. “They’re primarily focusing on those breakthrough cases that result in hospitalization or death,” Dr. Gordon says, “and that number is still small.”


The CDC, which has created a database to track these cases, reports that a person who is unvaccinated is 10 times more likely to test positive for COVID-19 and 20 times more likely to die of COVID-19 than people who are vaccinated.

In short, people who are fully vaccinated and boosted can experience breakthrough cases and exhibit symptoms of the illness. But the chances of contracting a serious illness remain far lower compared to people who are unvaccinated.

Why are breakthrough cases happening?

Again, no vaccine is 100% effective, so breakthrough cases were always expected. Breakthrough cases can come from all COVID-19 variants, but right now, most seem to be from the delta and omicron variants.

“The delta variant is more transmissible than previous variants of the COVID-19 virus, and the omicron variant seems to be, too,” Dr. Gordon says. “At this point, though, cases from these two variants haven’t proven to cause more serious illness than other variants.”

Still, the data showing that people who are fully vaccinated are less likely to become seriously ill demonstrates that the vaccine is indeed doing its job. Dr. Gordon reiterates: “Right now, the definition of ‘fully vaccinated’ includes getting your third dose or booster, which is the absolute best protection against breakthrough cases from variants of concern.”

Omicron variant

First reaching the United States in December 2021, the omicron variant has since spread quickly and contributed to a rise in breakthrough cases. The CDC says it believes that anyone with an omicron infection can spread the virus to others — even if they’re vaccinated or asymptomatic.

Research is ongoing about how easily omicron spreads, what treatments are effective against it and more.

Delta variant

The CDC reports that the delta variant, which accounted for over 83% of U.S. cases in July 2021, is more transmissible than previous variants of the COVID-19 virus. “The delta variant is at least twice as contagious as previous variants,” says Dr. Gordon.

What to do if you get a breakthrough COVID-19 case

If you or someone in your home gets sick with a breakthrough COVID-19 case, the best course of action is to isolate as much as possible. This is especially true if anyone in your home is unvaccinated. If isolation isn’t possible, try to keep air circulating as much as possible and wear masks indoors.


The CDC currently says that patients who self-isolate are OK to end that isolation 10 days after the onset of symptoms and, if you have a fever, 24 hours after it breaks.

The good news, Dr. Gordon says, is that breakthrough cases for vaccinated individuals are rarely serious and usually relatively mild — underscoring why it’s so essential that every eligible person gets vaccinated and boosted.

The importance of getting vaccinated — including the booster

The highly transmissible nature of the delta and omicron variants proves how critical it is to get fully vaccinated against COVID-19. “The vaccines available in the United States are effective against COVID-19,” Dr. Gordon says, “and the two-shot mRNA vaccines, in particular, are effective against all variants of concern to date.”

That means making sure you get both doses of the two-shot mRNA vaccines (Pfizer and Moderna) and then getting your booster or third dose as soon as you’re eligible.

“The term ‘fully vaccinated,’ in the omicron era, means being boosted,” Dr. Gordon says. “Even with variants of concern, the most important tool we have for prevention of getting an infection is getting vaccinated.”

How to protect yourself against the delta and omicron variants

Even after you’ve received the vaccine, Dr. Gordon says the best course of action is to keep wearing masks and taking other precautions.

“You can cut down on risk factors by wearing masks, especially indoors with other people around,” he says.

“But the bottom line is to make sure, first and foremost, that you get fully vaccinated, including getting boosted. That’s the best protection, moving forward, for breakthroughs from variants of concern.”

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