Here's what you need to know about COVID-19 breakthrough infections

The COVID-19 vaccines that are used in the U.S. are highly effective. But sometimes, the coronavirus can still infect people who are vaccinated, causing so-called breakthrough infections. So what are breakthrough infections, what are the symptoms and how common are they?

If a person tests positive for COVID-19 at least two weeks after receiving all recommended doses of vaccines authorized by the Food and Drug Administration, the Centers for Disease Control and Prevention (CDC) considers them to have a breakthrough infection. 

That means that anyone who tests positive at least two weeks after receiving the second dose of the Pfizer-BioNTech vaccine or the Moderna vaccine or two weeks after receiving a single dose of the Johnson & Johnson vaccine is considered a breakthrough infection case. Vaccine breakthrough cases "are expected," because no vaccine is 100% effective, according to the CDC. 

Related: Coronavirus variants: Here's how the SARS-CoV-2 mutants stack up

What are the symptoms?

While the symptoms of COVID-19 are similar in vaccinated and unvaccinated people, there are some subtle differences. 

Crucially, vaccination decreases the severity of COVID-19. That means the vast majority of breakthrough cases are "mild" cases. What's more, two ongoing CDC-funded studies suggest that vaccinated people are likely to have both milder cases and shorter illnesses than unvaccinated people, according to the journal JAMA.

In one small study, researchers found that five fully vaccinated and 11 partially vaccinated people who developed COVID-19 between Dec. 14 and April 10 had a 58% lower risk of developing fever. The symptoms of fully vaccinated people subsided six days earlier, and vaccinated people spent an average of 2.3 fewer days sick in bed, compared with unvaccinated people.

Still, the highly transmissible delta variant started spreading after this study period ended, so the findings may be less applicable now. 

There are also some subtle differences in the most common symptoms between cases in vaccinated and unvaccinated people. Breakthrough cases tend to cause symptoms similar to those of a head cold, Live Science previously reported. According to the ZOE COVID study, the top five symptoms of COVID-19 in a person experiencing a breakthrough infection are headache, runny nose, sneezing, sore throat and loss of smell — but not fever, persistent cough or shortness of breath, as is common in unvaccinated people. 

It felt like "a bad cold," JR Miller, a 32-year-old who experienced a breakthrough case, told the San Francisco Chronicle. And 40-year-old Justin Robinson, who also had a breakthrough case, felt like his allergies were acting up and one morning woke up feeling like "I'd swam on my side for a while, and my head had filled with water." His breakthrough infection, like most others, ended up feeling like a head cold.

Others across the country had similar experiences. For instance, 20-year-old JD Moore told The Boston Globe his breakthrough case was mild and that he had "a little bit more symptoms than the common cold, but nothing too crazy." 

Occasionally, however, even a breakthrough case can feel worse than a cold. "I was completely maxed out. I had to take, like, multiple naps. I had all the symptoms: loss of appetite, headache, congestion, fever, sore throat," 40-year-old Jeff Davis told the Globe. "It was a bit surprising how intensely it hit me, to the point where I was like, I certainly can't imagine — don't want to imagine — having this without a vaccine."

Not common, but likely not rare

It's still unclear how common mild or asymptomatic breakthrough infections are. But with the highly transmissible delta variant surging, they're more common than they were earlier in the year, and they are not "rare."

An internal CDC presentation that was leaked estimated the rate of symptomatic breakthrough infections to be about 35,000 per week among 162 million fully vaccinated people in the U.S., as of July 24, Live Science previously reported. In the week leading up to July 24, around 384,000 people tested positive for COVID-19, which would suggest that at that time, less than 10% of new cases were breakthrough infections. 

Counting the true number of breakthrough cases will not be easy. The CDC stopped tracking mild breakthrough infections in May. And until recently, the agency didn't recommend vaccinated people get tested for the virus if they were exposed to a case and did not experience symptoms. What's more, people who take at-home COVID-19 tests may not always report their results to their state department. Finally, only a handful of states or localities are tracking all asymptomatic, mild and severe breakthrough cases. All of those factors suggest that breakthrough cases are being undercounted.

It's not clear what factors put someone at higher risk of a breakthrough infection, though the internal CDC slides suggest that breakthroughs are likely to occur more frequently in congregate settings and in groups for which vaccines work less robustly, such as those who are immunocompromised or elderly, Live Science previously reported.

One thing that is clear is that severe breakthrough infections are very rare: As of Aug. 2, a little less than 0.005%, or 1 in 20,000 fully vaccinated people in the U.S., have gotten breakthrough infections severe enough to lead to hospitalization or death. Moreover, just 7 in 1 million fully vaccinated people have died as a result of COVID-19, according to data the CDC has collected from state and local health departments. Those data show unequivocally that the best way to protect yourself from the coronavirus is to get vaccinated.

What's more, the unvaccinated population can serve as an incubator for potential new variants to evolve, and those future variants could be even more dangerous than delta.

"If another one comes along that has an equally high capability of transmitting but is also much more severe, then we could really be in trouble," Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, told McClatchy Wednesday (Aug. 4). "People who are not getting vaccinated mistakenly think it's only about them. But it isn't. It's about everybody else, also."

Originally published on Live Science.