For months, public health experts and federal officials have said that significantly expanding the number of coronavirus tests administered in the United States is essential to reining in the pandemic. By some estimates, several million people might need to be tested each day, including many people who don’t feel sick.

But the country remains far short of that benchmark and, for the first time, the number of known tests conducted each day has fallen.

As the coronavirus pandemic continues to wreak havoc in the United States and treatments are needed more than ever, clinical trials for some of the most promising experimental drugs are taking longer than expected.

Researchers at a dozen clinical trial sites said that testing delays, staffing shortages, space constraints and reluctant patients were complicating their efforts to test monoclonal antibodies, man-made drugs that mimic the molecular soldiers made by the human immune system.