Federal regulators on Wednesday approved Vertex Pharmaceuticals Inc.’s cystic fibrosis drug Orkambi to treat children from 6 to 11 years old.

The decision by the Food and Drug Administration covers about 2,400 patients in the United States, expanding the domestic patient population eligible for Orkambi to nearly 11,000.

Despite that, the Boston company lowered its 2016 sales guidance for the drug to a range of $950 million to $990 million, from $1 billion to $1.1 billion. The announcement came after the close of regular trading. Vertex shares were down less than 1 percent in after-market trading.

The FDA in July 2015 approved Orkambi, a two-drug therapy, for about 8,500 patients 12 years old and over in the United States. That approval put Vertex, founded in 1989, on the path to consistent profitability for the first time in its history. It also helped drive its market value to $22 billion, second only to Biogen Inc. among Massachusetts biotech companies.

Orkambi, the second approved Vertex cystic fibrosis medicine, treats patients with two copies of the F508del mutation, the most common genetic mutation causing the life-threatening obstructive lung disease. The therapy eventually could treat close to half of the estimated 30,000 Americans who suffer from cystic fibrosis.

Vertex is testing Orkambi in clinical trials on children from 2 to 5 years old. At the same time, it is testing other experimental drug combinations on patients with different cystic fibrosis gene mutations as part of a strategy to eventually field medicines treating all cystic fibrosis patients.

“This expands the [Orkambi] population to younger patients,’’ said Vertex vice president Dawn Kalmar. “These children will now have access to a medicine that treats the underlying cause of cystic fibrosis for the first time. There’s been a lot of children and parents waiting for this news, and this is good for them.’’

Biotech analyst John Scotti, vice president for health care research at Evercore ISI, said the new approval, though “widely expected,’’ presented “Orkambi’s most significant growth potential this year.’’

Though the company had not previously lowered its sales guidance, Scotti wrote in a note to investors Wednesday night that Vertex management had “talked down’’ Orkambi’s sales prospects this year in recent conferences with brokers, telegraphing the move.

Among the factors driving the dip in projected sales are fewer-than-expected refill orders over the summer, and a slower than anticipated launch of Orkambi in Germany, the largest market in the European Union, where fewer than 20 percent of eligible patients have started treatment, according to Vertex.

The company said it plans in the first half of 2017 to file an application for European approval of Orkambi for use by children 6 to 11. It said Orkambi sales growth next year will be driven by obtaining insurance reimbursement of the drug in other European countries and by growth among eligible US patients between 6 and 11 years old.

“We are approaching peak penetration for Orkambi in the US’’ for eligible adult patients, the company said in a statement.

Robert Weisman can be reached at robert.weisman @globe.com. Follow him on Twitter @GlobeRobW.