President Joe Biden joined a growing club of Americans who, despite being fully vaccinated and boosted, contracted a breakthrough case of COVID-19.

Since his July 21 diagnosis, Biden has been isolating in the White House and appearing virtually in meetings rather than in person. His physician, Dr. Kevin O’Connor, wrote on July 25 that Biden had mild symptoms that “have almost completely resolved.” ​​On July, 27, 2022, Biden tested negative and returned to working in person, CNN reported.

Breakthrough infections used to be unusual. But Biden’s case, and likely millions of others in the U.S. this summer, show the limits of old assumptions about an evolving disease.

What we know about breakthrough infections has changed over time. Last year, reports from the Centers for Disease Control and Prevention showed that only a small fraction of COVID cases were breakthrough infections — less than 1%. Now, these infections among vaccinated people are more common.

But that doesn’t mean vaccines are ineffective. COVID-19 vaccines continue to keep people out of hospitals and prevent severe illness.

Here’s what we know about breakthrough cases in 2022.

Breakthrough cases are a lot more common

We reported last year that, based on CDC data, the chances of getting a breakthrough case were astronomically low — in May 2021, the number of cases made up 0.01% of fully vaccinated people. In September 2021, data and health experts showed that the odds of catching a breakthrough case of COVID-19 was about 1 in 35,000 per day.

Today, that breakthrough case number seems higher. The CDC reported that as of June 12, for every 100,000 Americans, there were about 133 vaccinated people who caught a breakthrough case of COVID. On the same scale, for every 100,000 Americans, 440 unvaccinated people were infected with COVID.

Cindy Prins, clinical associate professor at the University of Florida, said breakthrough cases are increasing, but understanding why requires context.

“We are seeing plenty of breakthrough cases, but you have to look at those in the context of the total number of cases of COVID, which is high right now. So, even though people can get breakthrough infections, they are still much less likely to get infected if they are fully vaccinated and boosted,” Prins said.

Unvaccinated people still face the most risk from COVID-19

Epidemiologists told PolitiFact that unvaccinated people still face a higher risk of catching COVID-19. Prins said unvaccinated people are nearly three times more likely than those who have at least had the primary doses of the vaccine to catch COVID.

Timothy Brewer, a professor of medicine and epidemiology at the University of California, Los Angeles, told PolitiFact that unvaccinated people still have the highest risk — but people with only primary vaccinations have less protection than those who get boosted.

Omicron subvariants can infect the vaccinated, but vaccines still protect against serious risk

We can blame omicron subvariants for the rising breakthrough cases, Prins and Brewer said. Time magazine reported in June that the newest omicron variants, BA.4 and BA.5, accounted for 21% of new COVID cases. That jumped swiftly in just a month — on July 20, USA Today reported that BA.5 made up 80% of new cases.

The subvariants can reinfect people who previously had COVID, and people who have been vaccinated could still catch the virus.

“The latest omicron variants are different enough from the COVID-19 variant the vaccine was designed on that the vaccinated immune system may not recognize the new variants as well as it recognized the earlier variants,” Prins said.

But Brewer said there’s good news — the vaccines do help people avoid serious illness, hospitalization or death.

Breakthrough case symptoms are usually mild to moderate

Symptoms of a breakthrough case can include cough, fever, loss of smell or taste, fatigue, runny nose, or sore throat. Symptoms are typically more likely to be mild to moderate, according to Johns Hopkins Medicine. And some people may experience no symptoms at all, said Prins.

The CDC recommends that people who test positive should isolate for at least five full days (with the first day of symptoms counting as day 0) and wear a mask if they have to be around other people at home. After day 5, people can end isolation, the CDC said, if they have been fever-free for 24 hours and have improving symptoms.

COVID is risky for people with pre-existing conditions, but vaccines mitigate the risk 

Breakthrough COVID cases for people with preexisting health problems or who are older can be risky —  perhaps leading to severe illness or death, said Prins. The vaccine helps mitigate that risk.

Biden got his first COVID-19 vaccine dose on Dec. 21, 2020, when he was the President-Elect, and received his second dose on Jan. 11, 2021, several days shy of being sworn into office. The U.S. Food and Drug Administration authorized booster doses of the Pfizer shot on Sept. 22, 2021, and Biden received his first booster five days later.

Biden got his fourth shot in March 2022, joining a number of others in the 50-and-older category allowed to get two booster doses. People aged 12 or older with pre-existing health conditions can also get a second booster — but that eligibility hasn’t been extended yet to the rest of the general public.

“The vaccine is definitely protective in these older populations,” Prins said. “In the 65-and-older population, people who are not vaccinated have a nine times higher risk of dying from COVID than people who are vaccinated and have had at least one booster. This is why booster shots are so important.”

Overall, it’s safest to get vaccinated and stay masked in crowds

Everyone 6 months or older should get vaccinated, Brewer said, and people who are eligible for more than one booster shot should get them. (In June, the CDC recommended that children from 6 months to 5 years old get the Pfizer or Moderna vaccine). Brewer added that people should consider wearing masks while in indoor public settings such as stores, and wash their hands after going out or before eating.

Well-fitting masks such as N95s or KN95s work best to protect against the spread.

“(People) can ask themselves, ‘Am I going to a crowded place, will I be inside or outside, will I be in close contact with other people for a long period of time?’ Those are all situations where they should wear a mask as much as possible to try to lower their risk of getting COVID,” said Prins.

RELATED: ‘Reinfections are going to continue to occur’: COVID-19 immunity and the risks of reinfection 

This article was originally published by PolitiFact, which is part of the Poynter Institute. It is republished here with permission. See the sources for this story here and more of their fact checks here.