A group of Senate Democrats are demanding answers about a shortage of a new drug that prevents respiratory syncytial virus (RSV) in infants.

In a letter led by Sen. Tammy Duckworth (D-Ill.) sent Friday, the lawmakers asked manufacturers AstraZeneca and Sanofi for more information about the current supply of nirsevimab, when the companies first became aware of the shortage, and why they were so unprepared for the demand.

“As our nation braces for the 2023-2024 RSV season, we are concerned that health care providers and families are having difficulty accessing this new immunization product that can be used to prevent severe RSV infections in infants,” the senators wrote.

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The letter was co-signed by Sens. Kirsten Gillibrand (D-N.Y.), Ron Wyden (D-Ore.), Ed Markey (D-Mass.), Elizabeth Warren (D-Mass.). Richard Blumenthal (D-Conn.), and Jon Ossoff (D-Georgia).

Nirsevimab, marketed as Beyfortus, is a monoclonal antibody rather than a traditional vaccine, meaning babies will be able to directly receive antibodies to prevent severe RSV disease, rather than prompting the immune system to develop them.

The drug is one of two available treatments in the U.S. that can protect infants from RSV, which is the leading cause of hospitalization among babies nationwide. It cuts the risk of hospitalizations in infants by about 80 percent and was hailed as a game changer.

Unlike the older treatment called palivizumab, Beyfortus was approved for use in all infants up to 8 months old, not just those at highest risk. And it’s given as a single shot, whereas palivizumab is administered once a month during RSV season.

But shortages of Beyfortus have been plaguing the country since it was approved in August. Pediatricians and hospitals have been struggling to stock the immunization due to what Sanofi said was “unprecedented” demand.

A representative from Sanofi told the American Academy of Pediatrics earlier this month that demand for the 100-milligram (mg) doses, for infants who weigh more than 11 lbs, exceeded the supply meant for the entire season within weeks of shipping.

Existing doses will continue to be allocated and shipped to providers throughout the season, but there won’t be any new products manufactured.

As a result, the CDC last month called for providers to ration doses.

CDC recommended prioritizing available 100mg doses for infants at the highest risk for severe RSV disease: young infants under six months old, and infants with underlying conditions that place them at highest risk for severe RSV disease.

In addition, the CDC’s Vaccines for Children (VFC) program, which covers the cost of the shots for uninsured and underinsured kids, instituted a temporary pause on ordering. Soon after, Sanofi said it was no longer taking orders for the 100 mg doses and could only fulfill orders that had already been placed.

The VFC program subsequently reopened orders for nirsevimab but had to do so with an allocation system to ensure an equitable distribution between state VFC programs.

“This supply shortage is impacting large hospitals and small health clinics alike, which highlights a concern for equitable care and access. It leaves parents searching for this immunization and desperate for an option to reduce risk of illness and save children’s lives. We must prioritize safeguarding our most vulnerable neonates, infants and children this RSV season,” the lawmakers wrote.