Two families of children with a rare and debilitating disease celebrated Thursday after UnitedHealthcare reversed previous denials and approved coverage for a $2.1 million gene therapy.

The giant insurance company said it has now approved all six coverage requests it received for the new gene drug, four on the basis of initial claims and now two more after the families appealed.

Zolgensma, which is marketed by Novartis, is intended as a one-time treatment for spinal muscular atrophy. The families’ fight for coverage highlights the coming insurance battles over advanced gene treatments.

Lauren Sullivan, whose 21-month-old daughter, Daryn, was running out of time to receive the drug before her second birthday in October, said she was deeply relieved when a UnitedHealthcare representative called to relay the decision Wednesday.

“I was sobbing, calling everyone, and thanking everyone and sending emails. We couldn’t believe it,’’ said Sullivan, of McLean, Va. “It was totally overwhelming.’’

A similar scene played out in Columbus, Ohio, where Rajdeep Patgiri learned Thursday morning that his 10-month-old daughter, Tora, would also be covered. The news arrived during one of Tora’s routine physical therapy sessions.

“There were a few high fives,’’ Patgiri said.

The Food and Drug Administration approved Zolgensma in May and said it could be given to most children under 2 with spinal muscular atrophy, or SMA. The approval covered most types of the disease: type 1, the most severe and also the most common form, which causes infants to die or be reliant on respirators and feeding tubes; type 2, which renders children unable to walk without assistance; and type 3, which allows walking but with some impairment. Another category of the disease, type 4, affects adults.

The stressful appeals the Sullivan and Patgiri families undertook were described by The Washington Post last week. UnitedHealth Group’s chief medical officer, Richard Migliori, said in an interview Thursday that public attention on the families’ plight played no role in the reversal. He said the decisions were based on clinical evidence.

In Daryn’s case, the initial denial was based on an apparent lack of symptoms, said Migliori, who discussed details of the case with written permission from Daryn’s mother. During the exhaustive appeal, Daryn’s doctors at Children’s National Medical Center documented symptoms resulting from her defective genes, he said.

“If a child is symptomatic, we don’t care what type they have. If they have the genetic mutation, they are getting covered up until age 2,’’ Migliori said. The major exception would be if a child already has so much nerve loss that the drug has no chance of working, he added.

UnitedHealthcare developed its complex policy in consultation with physicians who have deep expertise treating SMA, Migliori said.

Other insurance firms have more restrictive policies, according to a recent survey conducted by financial analysts at Bernstein. Anthem cuts off coverage after 6 months of age. Anthem told The Post that it based its criteria on published clinical trial data, a position that leaves the door open for broader coverage after more trial results are published.

The Bernstein analysis said restrictive coverage policies and denials were a signal that insurance companies were unhappy with Zolgensma’s high price. But Migliori said UnitedHealthcare did not consider price when it made its coverage determinations.

“Let’s just take the money off the table,’’ Migliori said. “The decisions we make are on clinical impact. On the price of a drug, you should talk to the manufacturer who decided to put that kind of a price on it.’’

Novartis bought the company that developed Zolgensma, AveXis, for $8.7 billion in 2018. It has said the $2.1 million price of the therapy reflects the long-term value of a one-time treatment that should permanently halt SMA’s devastating nerve loss. The only other SMA treatment on the market, Biogen’s Spinraza, costs $375,000 a year for a patient’s entire life and requires three spinal injections annually.

In the case of his daughter Tora, Patgiri said the initial insurance denial apparently was based on a low score on a motor function test. The low score meant Tora had the opposite problem to Daryn, who was deemed too healthy. Tora was deemed too sick.

But Patgiri said that on the day in May that she took the motor function test, Tora, whose family moved to Ohio from London so that she could be treated, was tired from a battery of evaluations by her new doctors. A new motor test Wednesday produced a better score, above the coverage threshold, Patgiri said.

Within three hours of that test’s completion, Patgiri had sent three copies of the report to UnitedHealthcare. The denial was reversed by Thursday morning.

Tora is expected to get her Zolgensma infusion within two weeks. Daryn Sullivan is expected to receive hers by early August.

Lauren Sullivan said she is coaching other families denied coverage on how to mount successful appeals. In addition to telling her story to The Post, Sullivan, with her husband, Kevin, contacted multiple public officials, including U.S. Sen. Mark R. Warner (D-Va.). Warner’s office confirmed that he contacted UnitedHealthcare on the Sullivans’ behalf.

“This treatment has the potential to be life-changing for Daryn and her family,’’ his office said in an email, “and Senator Warner is glad for whatever role he may have played in pushing the insurance company to cover it after all.’’