WASHINGTON (AP) — The top developers of U.S. COVID-19 vaccines are set to face questions from Congress about limited supplies of the shots needed to end the pandemic.

Ahead of the hearing, Johnson & Johnson announced initial supplies of their one-shot vaccine will be limited to 20 million doses by the end of March. J&J plans to tell lawmakers it faces “significant challenges” in scaling up production.

The pace of vaccinations is picking up nationwide, but demand for the shots continues to outpace limited supplies distributed by the U.S. government.

The Energy and Commerce Committee panel is questioning executives from five companies about what could be done to boost manufacturing. Those coming before lawmakers include representatives for Pfizer, Moderna, J&J, AstraZeneca and Novavax.

“The most pressing challenge now is the lack of supply of vaccine doses,” Rep. Diana DeGette said as she opened the hearing. “Some of the companies here today are still short of the number of doses they promised to initially deliver when they last testified before this subcommittee in July.”

DeGette says Tuesday's hearing is a "straightforward assessment" of the manufacturing process.