(The Hill) – The monoclonal antibody for preventing the respiratory syncytial virus, known as RSV, in infants is in short supply, and federal officials are advising doses be prioritized for those at the highest risk for severe illness, with the drug’s manufacturer saying demand has outpaced expectations.

The Centers for Disease Control and Prevention (CDC) on Monday issued a health advisory recommending prioritizing available 100-milligram doses of the RSV monoclonal antibody nirsevimab — known commercially as Beyfortus — for “infants at the highest risk for severe RSV disease.” These include infants younger than 6 months and those with underlying conditions.

There are two forms of Beyfortus, a 100 mg dose for infants weighing more than 11 pounds and a 50 mg dose for infants weighing less than that. Citing “manufacturing capacity and currently available stock,” the CDC said there would not be enough doses of the 100 mg Beyfortus to protect all infants this season.

The recommendation for 50 mg doses remains unchanged, and the agency advised against using two doses of the 50 mg shots in infants weighing more than 11 pounds.

The CDC further recommended that the use of Beyfortus be suspended among infants who are also eligible for palivizumab, another monoclonal antibody for RSV that is reserved for premature infants younger than 6 months, babies with lung disease and babies with heart conditions.

Earlier this month, drugmaker Sanofi said there had been “unprecedented demand” for Beyfortus which was approved in August.

“Despite an aggressive supply plan built to outperform past pediatric vaccine launches, demand for this product, especially for the 100 mg doses used primarily for babies born before the RSV season, has been higher than anticipated,” the company said, adding it was working with the CDC to ensure an “equitable distribution” of doses through the federal Vaccines For Children Program.

The drug is indicated for preventing serious lung disease caused by RSV in infants younger than a year who are entering their first RSV season and children up to 24 months old who are entering their second RSV season and remain at high risk of severe illness.

Beyfortus is the first approved preventive treatment for RSV in infants. Uptake was expected to be high following last year’s severe RSV season, which saw many hospitals reach capacity with infants sick with the common “day care disease.”

But the rollout of the drug has been shaky, with many parents struggling to find locations that have the shot. At $495 per dose in the private sector, doctors are unsure if they would be reimbursed by insurers for Beyfortus.