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. The pace of vaccinations is picking up nationwide, but demand for the shots continues to outpace limited supplies distributed by the U.S. government. A House panel will question executives from five companies about what could be done to boost manufacturing. Drugmaker Johnson & Johnson revealed ahead of the hearing that initial supplies of its one-shot vaccine will be limited to 20 million doses by the end of March. The company plans to tell lawmakers it faces “significant challenges” in scaling up production.