BOSTON — Allergy shots, a mainstay of asthma treatment since the turn of the century, appear to be worthless for many youngsters.

A major new study found the shots do nothing for children with moderate to severe year-round asthma who are already taking standard medicines.

Asthma is caused by allergic reactions to a variety of common substances, such as grass pollen, dust mites and cockroaches.

To fight it, doctors often give gradually increasing injections of the allergy-causing substances that trigger the attack. The goal is to prompt the body make antibodies that will then block future allergic reactions.

Researchers began rigorously testing this approach — known as immunotherapy — in the 1960s. They found that in the laboratory, at least, the shots could reduce reactions to common allergy-causing materials.

The new study was intended to see how this translates into the everyday lives of children who are often sick with asthma attacks.

“We didn’t find any statistically significant benefits,” said Dr. N. Franklin Adkinson Jr., who directed the study at Johns Hopkins University. “This doesn’t mean the treatment is ineffective. But it means that in children who are getting adequate medical care, including avoidance of allergenic substances in the home, adding immunotherapy doesn’t do what we had hoped.”

The study dealt only with the use of allergy shots for asthma, not for the other allergy problems for which they are given. For instance, doctors often administer allergy shots to relieve the itchy eyes and runny nose of chronic hay fever as well as to prevent life-threatening reactions to bee stings.

Dr. Daniel Rotrosen of the National Institute of Allergy and Infectious Diseases said the study’s message seems clear: “Immunotherapy should not be the first line of therapy” for children with asthma.

The results, published in Thursday’s issue of the New England Journal of Medicine, are likely to be controversial among allergists who give the shots routinely.

Dr. Betty Wray, an allergist at the Medical College of Georgia, said the study may have failed to show a benefit because the researchers did not give the youngsters the right allergy-blocking shots. For instance, she noted that they did not treat them for cockroach allergy, which experts have come to realize is a major cause of asthma in poor neighborhoods.

“I would expect at least 80 percent of patients to take less medication and make fewer emergency room visits on appropriate immunotherapy,” Wray said.