NEW YORK (NYTIMES) - On the July evening that he was diagnosed with Covid-19, 7-year-old Ethan Chandra lay in bed with a high fever next to his mother, holding her hand and whispering, "I'm scared I'm going to die."

His mother, Alison Chandra, 38, didn't know what to say. Although a vast majority of children his age who test positive for the coronavirus either don't have symptoms or fully recover, Ethan has heart and lung defects that make him especially vulnerable.

His family in Lehi, Utah, has spent much of the past year and a half at home to keep him safe. He and his sister studied at home and went outside only to play with a few trusted friends, wearing masks and staying distanced. In November, Ethan's parents tested positive for the virus after his father briefly returned to work in person, but they managed to avoid infecting their children. Since then, both parents have both been working from home.

But recently, Ethan got Covid-19 anyway. On social media, Chandra posted photos of him in a special pink medical vest and nebuliser, and blamed low vaccination rates in Utah, where about 45 per cent of the eligible population is fully vaccinated, for his illness.

"There literally are not words for the frustration but also the fury that I feel that this has gone on as long as it has," she said in an interview. "It didn't have to be this way. It didn't."

The Delta variant has led to a surge in coronavirus cases and hospitalisations across the country, leaving families with high-risk children who cannot be vaccinated especially concerned. Like Chandra, a growing number have shared their stories online, accompanied by desperate pleas for people to become inoculated, for the sake of their children.

Many parents say they are angry and exhausted from trying to keep their children safe while balancing the emotional trauma of more than a year of isolation.

"We were seeing the light at the end of the tunnel," Chandra said, "and we were telling our kids it was worth it, you did the right thing and you stayed safe, and now it just feels like it was for nothing."

Elena Hung, 43, of Silver Spring, Maryland, knows Covid-19 could be deadly for her daughter Xiomara, 7, who has heart issues and chronic lung and kidney disease and breathes through a tracheotomy. Hung is the executive director and co-founder of Little Lobbyists, a national nonprofit advocacy group of families with disabled and medically complex children.

"We're just pleading, begging people to get vaccinated and wear their mask for the sake of our children," Hung said.

Her family worked and went to school virtually in 2020, forgoing visits with relatives and friends. After she and many others in her community were vaccinated, she finally felt comfortable allowing her two children to play at the park or playground again.

Now, because of the Delta variant, Hung said she planned to keep them home.

Case counts among US children have been steadily increasing over the summer. In one week at the end of July, the number of new cases doubled to 71,726 from 38,654 the previous week, according to data from the American Academy of Pediatrics.

There is no estimate for how many American children are at higher risk from Covid-19 because of medical conditions, said Dr Dennis Z. Kuo, a pediatrics specialist in Buffalo, New York, and former chairman of the American Academy of Pediatrics Council on Children with Disabilities. But he estimates about 19 per cent of children have special health care needs, and 1 per cent have severe medical complications.

Within those groups, he said, children with heart, lung or immune system disorders are especially at risk from the coronavirus, as well as those with intellectual or developmental disabilities.

Jay Berry, the chief of complex care at Boston Children's Hospital, said that children with underlying medical conditions in his hospital had taken months or longer to recover from Covid-19, and that there was no clear data on the long-term risks that the virus poses to them.

"Those are the children, in my mind, that we worry about the most," he said.

Ethan Chandra has a rare condition called heterotaxy syndrome; he was born with half a working heart and other heart defects. The syndrome also causes dysfunction in his lungs, which makes it hard for him to clear his airways, which, in turn, makes him vulnerable to a serious respiratory illness.

"We worry about Covid in any kid because even healthy kids can be affected," said Dr Nelangi M. Pinto, Ethan's cardiologist. "But we do worry more when there's some underlying conditions."

Chandra, a former pediatric intensive care nurse, now performs checkups on her son every morning, surrounded by Legos and stuffed animals. While listening through a stethoscope, she said his breathing was fast and that he sounded "junky," as if his lungs were clogged. His fever has been as high as 103 degrees since his Covid-19 diagnosis, and oxygen levels in his blood have hovered in the low 90s; a normal reading for children his age is 98 per cent.

If they dip below 90 per cent, he will need to immediately go to the hospital, Chandra said - just as he did when he was a baby and he had consistent high fevers and low oxygen levels from his medical complications.