In a brief interview on Thursday, Dr. Birx stressed that there had been no pressure to alter data. But concerns about official statistics are not limited to the death toll, or to administration officials.

Epidemiologists said they were stunned to learn that the C.D.C. is combining tests that detect active infection with those that detect recovery from Covid-19 — a system that muddies the picture of the pandemic but raises the percentage of Americans tested as President Trump boasts about testing.

Experts said that data from antibody tests and active virus tests should never be mixed.

“It just doesn’t make any sense,” said Natalie Dean, a biostatistician at the University of Florida. “All of us are really baffled.”