You will know, or possibly be, someone who gets COVID-19 after a vaccine. Here's what to expect, according to 2 people with breakthrough cases.

• Breakthrough cases will become more common as the more transmissible Delta variant keeps spreading.
• But most disease experts still expect COVID-19 to be milder in vaccinated people.
• Two young people who got sick after they were vaccinated described how their symptoms progressed.

Breakthrough infections, or COVID-19 cases diagnosed after someone is fully immunized, were supposed to be rare - at least that's what a report from the Centers for Disease Control and Prevention suggested in May. At the time, data indicated that just 0.01% of vaccinated Americans got a breakthrough case from January to April.

But that was before the more transmissible Delta variant became the dominant strain in the US.

The share of breakthrough infections also inevitably rises as more people get vaccinated - though it's hard to know how many of these cases occurred since the CDC report came out. The agency stopped tracking asymptomatic, mild, or moderate breakthrough cases on May 1.

Though the current vaccines still protect against Delta, the chances that you'll know someone with a breakthrough case, or develop one yourself, are higher than they've ever been.

But most disease experts still expect COVID-19 to be milder in vaccinated people, regardless of the variant.

"The variants may cause you to still maybe get an infection, but not serious," Peter Gulick, an associate professor of medicine at Michigan State University, told Insider in June. "It would be almost like getting an infection with the common cold or one of those nagging things that gives you sniffles and a cough and makes you feel a little tired, but nothing serious enough to put you in the hospital or put you on a ventilator."

Indeed, the COVID Symptom Study - a project that tracks self-reported COVID-19 symptoms via an app - suggests that a headache and runny nose are two leading indicators of a COVID-19 infection among vaccinated people in the UK, followed by sneezing, sore throat, and loss of smell. Vaccinated people in the study also reported fewer overall symptoms over a shorter period of time than those who hadn't received a shot.

Insider spoke with two young people who recently recovered from mild breakthrough infections to see how their symptoms progressed.

A 32-year-old man had fatigue and shortness of breath

Sam Reider, a musician who lives in San Francisco, was fully vaccinated with Pfizer's shot in mid-April. (New research suggests that Pfizer's two-dose vaccine is 88% effective against symptomatic COVID-19 from Delta - down from 95% against the original strain.)

In mid-June, Reider taught a music camp a few hours outside the city. Of the roughly 50 people in attendance, he said, most were older children who had been vaccinated. A handful of unvaccinated kids under 12 wore masks.

But Reider got sick anyway. His first symptoms - exhaustion and a headache - resembled "that feeling of being super jet-lagged after an international flight," he said. Soon after, he started to feel congested and short of breath - symptoms that lasted for about two weeks after he tested positive for COVID-19.

"Basically any sort of exercise or even talking on the phone was hard for the first week," Reider said. "At the ends of my sentences, I would need to slow down and stop, take a bunch of deep breaths."

Reider also lost his sense of taste and smell, which is just starting to return more than a month after his symptoms started. His most severe symptom, fatigue, came in spells.

"Out of the blue, I would need to just lie down," Reider said.

At the beginning of July, though, he started to feel better. He has now returned to normal exercise, including bike rides and a six-mile hike.

He said getting sick after being fully vaccinated is a still bit traumatic, though.

"After a year and a half of all of this, to go out feeling confident with the vaccine and then get it - it was just upsetting," Reider said. Still, he added, "it was nowhere near as bad as the times that I've had pneumonia or a severe flu or cold."

A 30-year-old man had an overnight fever and lost his sense of smell

Ryan Forrest, who lives in Midland Park, New Jersey, received Johnson & Johnson's single-dose shot at the end of March, then attended a roughly 150-person indoor wedding on July 1.

He woke up on July 5 "feeling a little foggy," he said. That night, Forrest developed a fever and woke up sweating. The following day, his rapid COVID-19 test came back positive - around the same time that body aches and a runny nose set in.

"I never had more than two symptoms at the same time," Forrest said, adding, "The runny nose turned into a stuffy nose, which then turned into a headache and then my body aches went away and I got that dry cough."

The most persistent symptom, he said, was loss of smell (he never lost his sense of taste). But none of the symptoms was unbearable, he added, and all of them dissipated within about two weeks.

Breakthrough infections could be more common with Johnson & Johnson's shot than Pfizer's or Moderna's, since the efficacy of that vaccine is lower: J&J's shot was found to cut the risk of moderate and severe COVID-19 by 66% globally. But South African researchers recently found that, among people who'd received the J&J vaccine, 94% of breakthrough infections were mild - including those caused by Delta.

Even though he got sick, Forrest said, he's thankful that the vaccine seemingly protected his parents, who rode in the car with him for an hour and a half the day before he tested positive.

"That's the important part for me," he said. "I don't really care how sick I got as long as I wasn't giving it to my dad, who's 67."