By Camille Caldera

At first, it seems like a cold. Then, it feels worse. You’re fully vaccinated, so it couldn’t be COVID-19 — or could it?

Unfortunately, as we all now know, it could. In the most recent tally, 6,373 Massachusetts residents have contracted so-called breakthrough cases of COVID-19 — an infinitesimal percentage of those who have been vaccinated but still a source of anxiety for many and heartbreak for the families of 91 who have died.

Though the vast majority of cases are mild, the uptick — sparked by the highly contagious Delta variant — has fueled new fears about a pandemic that some thought was behind them.

So, what is it actually like to have a breakthrough case of COVID-19? We spoke with numerous individuals in the region who got the shot and got sick to varying degrees. Here are their stories.

JD Moore, 20, West Roxbury

Before he was vaccinated, JD Moore, 20, had a few close calls with COVID-19. His boyfriend tested positive in February just after they spent time together. But Moore, now a rising sophomore at Northeastern University, didn’t catch the virus at that time.

“I never tested positive, ever, so I was shocked about that,” he said. “To get COVID after being vaccinated was obviously even more shocking.”

Stranger still, Moore is clueless as to how he contracted the virus in June. None of his friends or family were sick, and he had been careful in public because his mother is a cancer patient and immunocompromised.

When he started to feel sick, he thought it was just a sinus infection. Out of an abundance of caution, his doctor tested him for COVID-19, but told him not to worry; they had never had a patient with a breakthrough infection.

“Sure enough, the next day, I got a positive test,” Moore said.

He said his case was mild, and described it as “a little bit more symptoms than the common cold, but nothing too crazy.”

The hardest part was the fear of transmitting the virus to his mom, whom he lives with. She’s vaccinated, but her chemotherapy renders the shots less effective. Anytime he went upstairs, Moore wore a mask and stayed at a distance.

“I was super stressed to spread it to her,” he said. “Thankfully, she never got it.”

Moore said he was grateful for the vaccine — both because it kept his case mild and because it is believed to reduce the likelihood of passing on the virus.

“If I didn’t have the vaccine, it’s more likely that my mom could have caught it,” he said. Besides that, “if I wasn’t vaccinated, the symptoms I got could have been really bad.”

Cathy Mertz, 60, Needham

Cathy Mertz, 60, was careful throughout the pandemic. She’s on medication that renders her immunocompromised, which made her vulnerable to serious sickness from COVID-19.

Even after she received her shots of Moderna in February and March, Mertz continued to take precautions, including wearing a mask while shopping.

A few weeks ago, her sister came to visit from Portland, Maine. She had a cold, but Mertz didn’t think anything of it. After she left, Mertz started to feel like she had caught the cold. Her husband, who had started to feel sick too, suggested she take a test for COVID-19.

“The fact that my sister had a little cold — it never even occurred to me that was COVID-related,” Mertz said. “I really thought that we were out of the woods.”

Back in Portland, much of her sister’s family — all vaccinated — also tested positive, creating a “whole family cluster in two different states.”

While sick, Mertz said, she has had “virtually every symptom”: congestion, fatigue, headaches, back and chest aches, a slight fever, a cough, loss of smell, and loss of taste. That ruined her appetite, and she said it’s been difficult to stay nourished.

“I’ve gotten hit pretty hard,” she said. She was still sick after 10 days, with a worsened cough, slight fever, and a “very tired, very foggy” mind.

Still, Mertz considers herself a moderate case. “The only reason I don’t call it serious is that I don’t have any breathing problems,” she said.

She encouraged others to get tested, so that they don’t pass the virus to their friends and family.

“I think there are a whole lot of people walking around with what they think is a summer cold who actually have COVID,” she said.

Jeff Davis, 40, Cambridge

While most vaccinated individuals are shocked when they get a breakthrough infection, that wasn’t the case for Jeff Davis, 40, who was vaccinated in April.

He’s Black, a population that has been disproportionately affected by COVID-19, and lost relatives earlier in the pandemic. His children — ages 4 and 6 — are not able to be vaccinated, and they attend summer camp at a neighborhood center in Cambridge.

“It’s in a majority African American and immigrant and lower-income community within Cambridge, and statistically, at least, there are less vaccinated people there,” Davis said. “It’s a known risk that we’re aware of.”

When he was notified that his kids had been exposed to COVID at camp, “it wasn’t surprising but it’s no less disappointing and frustrating,” he said. “The racial disparities in health are pretty telling.”

Just a day prior to the notification, Davis also received a call from his doctor: a routine test had revealed his white blood cell count was low, weakening his immune defenses.

“So I’m like, cool. Now I’m going to be in a house for two weeks with kids who have COVID and my immune system is compromised. Awesome,” said Davis, a human resources professional.

At first, Davis and his wife were both negative for COVID-19. They stayed at home and took care of their children. It wasn’t until the weekend that he started to feel sick.

“I was exhausted, I was achy, I had a little bit of a sniffle,” he said. “It’s kind of like the early onset of a flu — something feels off.”

He went to bed. The next day, Davis woke up debilitated.

“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,” he said. “It just kind of hit me like a wall.”

“It was like the longest day I can remember in the last year and a half,” he added. “At that point, I was hoping I could make it to Monday and get a COVID test just to have that certainty.”

As he suspected, his test came back positive. By Wednesday, Davis was feeling better, with some lingering soreness and congestion.

“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,” he reflected.

Within his household, they had the “full gamut” of experiences. One of his sons was sick with a mild fever and fatigue. His other son “hasn’t missed a beat.” His wife, who was also vaccinated, stayed healthy.

The takeaway, Davis said, is that “anyone can get it. The vaccine is not a carte blanche to go back to the way things were. . . . And we still have to be responsible to ourselves and others.”

Jojo Jacobson, 36, Boston

When her toddlers started to sniffle and develop fevers earlier this month, Jojo Jacobson, 36, assumed they had caught a cold. Then she started to feel exhausted and feverish, too.

“I kept telling myself, this isn’t possible. I shouldn’t have this,” Jacobson said. She was vaccinated in April, and had continued to be “extra, extra cautious.”

Soon, Jacobson found out that her nanny, who was also vaccinated, had an asymptomatic case of COVID-19. Still, Jacobson’s doctor reassured her that under the circumstances, it was unlikely she had contracted the virus.

“When the tests came back positive, I couldn’t be in denial anymore that what I was experiencing was real,” Jacobson said. “I took my temperature and sure enough, I had a fever. I was pretty devastated and just collapsed into bed.”

Jacobson had a fever and significant fatigue for the next two days, and experienced regular cold symptoms — including congestion and a cough — for four or five more.

“I wasn’t totally debilitated. I could get out of bed, I could make myself a cup of tea and some soup, and I was grateful for that,” she said. “It’s not the worst cold or flu I’ve ever had in my life, but I was definitely, definitely sick.”

She also lost her sense of smell, and hasn’t regained it. To not even be able to smell garlic, she said, has been one the most disorienting things she’s ever experienced.

On top of the physical symptoms, Jacobson said the experience “created a certain amount of chaos” for her family, with the logistics of quarantining from her spouse and coordinating care for their children, ages 2 and 3.

“I never in a million years would have thought that our household would have gotten COVID after vaccination,” she said. “It seemed so far-fetched.”

For now, Jacobson plans to continue to wear a mask — indoors and outdoors — and avoid crowds.

She compared the pandemic to an arms race. With the vaccine, she said, “we made ourselves a little bit safer.” But with the variants, “the virus gets a little bit stronger.”

Still, Jacobson said the vaccine afforded her “a sense of reassurance” throughout her sickness.

“I knew that I wouldn’t have to go to the hospital. I knew that it was likely to be just like a bad cold and be over in a few days,” she said. “That was really reassuring.”

Anna Silvi, 98, West Yarmouth

Anna Silvi, 98, was a mother of five, grandmother of 10, and great-grandmother of nine. She was a resident of Maplewood at Mayflower Place, a nursing home in West Yarmouth where a cluster of infections occurred in July — most in vaccinated residents.

In the weeks before her infection, she was “fine and good,” said her son, Domenic Silvi. On a recent visit, he brought her a prosciutto sandwich. “We were talking to her, and she was lucid,” he said.

But after she caught COVID-19, “she rapidly failed in like two days,” he said. Though she had other health issues, he said, he attributes her decline to COVID-19.

Her final hours were difficult, he said. Nonetheless, “You mother brings you into this world, and it’s hard to see her go.”

She died on July 22. “She was an amazing woman,” Domenic Silvi said.