Michael Furia saw a friend struggle through a 10-day bout with COVID-19 earlier this year, and has no desire to share the experience. Long before that, the Haddonfield teenager had grown weary of pandemic disruptions to school and sports. Most of all, he wants to see his grandparents, aged 89, 95, and 95, without fear of infecting them.

So the 13-year-old had no objections when his mother, Janine, signed him up to get the Pfizer vaccine the first day he was eligible, May 13 at the Moorestown Mall mega-site in Burlington County.

“I was excited,” he said afterward, waiting the required 15 minutes to make sure he had no side effects.

Grandparents, brace yourselves!

“They’re all going to get big hugs,” his mother said. “Big Italian hugs.”

All great reasons for adolescents to join the millions who already have been vaccinated against the coronavirus, infectious-disease experts say. While most children who contract COVID-19 experience only mild symptoms, protecting them from the disease prevents them from spreading it to grandparents and others at greater risk of severe illness.

And despite the relatively low risk for children, thousands have nevertheless been hospitalized with COVID-19 since the start of the pandemic, and nearly 500 have died, according to the U.S. Centers for Disease Control and Prevention. Some of them had underlying health conditions such as obesity, but as with adult patients, predicting which children will get really sick remains something of a mystery, said Emily Souder, an infectious diseases specialist at St. Christopher’s Hospital for Children.

“Healthy people have gotten very sick as well,” she said.

Best for all of them to get vaccinated, both to protect themselves and others, Souder said. It all starts with that term we’ve been hearing since the beginning: herd immunity.

» READ MORE: It’s too soon to declare herd immunity against COVID-19. My blood helps explain why.

Without children and teens, it will be hard to get there.

Protecting the herd

Epidemiologists have estimated that as much as 80% of the population must achieve immunity to the coronavirus in order to curtail its spread — whether through vaccination, infection, or both. In theory, reaching that threshold would prevent new outbreaks. Even if a few people still became infected with the virus here and there, they would be unlikely to pass it on.

For a variety of reasons, it might not be that simple in practice. The virus continues to mutate, for one thing. Booster shots might be needed. And the target is not precise, leading some scientists to say herd immunity shouldn’t be the aim; getting as many people vaccinated as possible is the real objective.

But let’s say 80% is the target. Simply vaccinating adults will not get us there, as they make up just 77% of the U.S. population.

Even if every adult were to get vaccinated — and it’s no secret that millions are taking a pass — the total with immunity would fall short of 80%. Plus the vaccines, while highly effective, are not foolproof. Infectious-disease experts say children must be part of the equation.

The case seemed so clear to Keren Abraham, 17, that she signed up to be an ambassador for Philly Teen VAXX: an outreach program created by the city’s Department of Public Health, the School District of Philadelphia, the PolicyLab at the Children’s Hospital of Philadelphia, and the Black Doctors COVID-19 Consortium.

A junior at Philadelphia Academy Charter High School, Abraham has seen COVID-19’s disproportionate impact on people of color in the city. And lately, she has heard from relatives in India about what happens when vaccines are not widely available.

“I’ve seen so many people go through traumatic events in the last year,” she said. “Convincing youth to get vaccinated is so important, especially people of color.”

The obesity link

One reason the pandemic has caused such a toll in the United States is that a regular epidemic — obesity — already was well underway.

Adults with a high body-mass index are more likely to suffer severe effects from COVID-19 for a variety of reasons. One is physical, because excess pounds place more strain on the lungs. Another is chemical. Scientists say that excess adipose tissue acts as a type of “organ,” releasing hormones and other chemicals that cause the body to remain in a chronic state of inflammation. When that baseline level of stress is combined with added inflammation from COVID-19, the result can be grim.

» READ MORE: Coronavirus is revealing exactly what obesity does to the human body

Obesity does not have quite the same impact in younger people because they have not yet developed other chronic conditions that tend to come with it, such as cardiovascular disease, said Kevan Jacobson, a senior clinician scientist at BC Children’s Hospital in Vancouver, British Columbia.

Nevertheless, children with a high body-mass index tend to have more severe symptoms from COVID-19 than other children, Jacobson and colleagues found when reviewing 42 studies from around the world. Among those who are infected, children with obesity are nearly three times as likely as other children to experience severe disease, the authors wrote in the International Journal of Infectious Diseases.

“You can get that exaggerated inflammatory response as a result,” he said.

And some children without an underlying health issue can get very sick, too. Among the worst consequences is a “hyperinflammatory” syndrome called MIS-C, which tends to occur two to six weeks after the initial infection, often requiring intensive care. As of May 3, more than 3,700 children in the U.S. had been identified with the condition, 63% of whom were Black or Hispanic, the CDC said.

Asymptomatic spread

The good news is that the vast majority of children recover from COVID-19 just fine, though it is still not entirely clear why.

One possibility is that a child’s “innate” immune system — an assortment of all-purpose disease-fighting cells — is especially adept at warding off the virus before an infection gets fully underway.

Another possibility is that the numbers of virus binding sites may be lower in the nasal passages of children compared with those of adults. And the fact that children generally have fewer chronic health issues likely plays a role, said Sunanda Gaur, a professor of pediatrics at Rutgers Robert Wood Johnson Medical School.

“There is not an absolute answer yet,” she said.

» READ MORE: As COVID-19 case numbers drop and vaccinations rise, restrictions ease and officials see hope for the summer

In a new study of more than 12,000 pediatric COVID-19 patients, most did not have the symptoms typically present in adult patients. Just 16.5% had a cough or shortness of breath, and only 18.8% had a fever, the authors wrote wrote in Scientific Reports.

All the more reason for children to get vaccinated, as they may otherwise spread the virus without realizing it.

Janine Furia, the Haddonfield woman who took her son to the Moorestown Mall, needed no encouragement, making his appointment two days in advance. The vaccine he received, made by Pfizer and BioNTech, is authorized for children aged 12 to 15. Other vaccine makers are expected to seek authorization for their products to be administered to teenagers soon.

And Rutgers is among the trial sites as Pfizer seeks to expand authorization of its vaccine to the next youngest age group: children aged 6 months to 11 years.

Will everyone be as enthusiastic as the Furias?

Abraham, the teen ambassador in Philadelphia, knows that some will require more convincing. Careful explanation of the science will help, along with an appeal to the listeners’ sense of community, she said.

When Abraham got vaccinated weeks ago, the feeling was one of utter relief.

“I felt free,” she said. “I felt so empowered, and it gave me such a sense of security.”

She hopes millions more will soon feel the same.