MedImmune Inc. said yesterday that it is altering the warning label on its biggest product to quantify the incidence of allergic reactions among infants who take the medication to prevent respiratory infections.

The label on Synagis, which generated 83 percent of the Gaithersburg biotechnology company's revenue last year, will be changed to reflect that two patients out of 400,000 that have received treatment experienced an allergic reaction, said Lori Weiman, a company spokeswoman. Both patients fully recovered, she said.

"When we first licensed Synagis, there was the theoretical potential for anaphylactic reaction," she said. With the change in language on the label, the company is informing doctors and patients exactly how many cases have occurred in the four years that the product has been available, she said.

Additionally, MedImmune is changing the label to state that there were no adverse reactions among patients who received more than five doses of Synagis, she said. In clinical trials the effect of taking more than five doses was not observed, she said.

Synagis treats premature infants who are at high risk of lung infections caused by respiratory syncytial virus, most common during the winter months. Synagis generated $516 million of MedImmune's $619 million in revenue last year.