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The U.S. plays catch-up

American officials are scrambling to ramp up testing for the coronavirus, after weeks of setbacks and missteps that may have allowed the virus to gain a foothold.

The Trump administration has promised a rapid expansion, saying as many as a million diagnostic tests could be available — though not necessarily administered — by the end of this week.

Many scientists wonder if the moves came too late, and a surge of positive test results has raised critical questions: Why weren’t more Americans tested sooner? How many may be carrying the virus now?

The Centers for Disease Control and Prevention botched its first attempt to mass-produce a diagnostic test kit. A replacement test took several weeks, and the C.D.C. imposed stringent, narrow criteria and still did not permit state and local laboratories to make final diagnoses. The result: A relatively tiny number of Americans have been tested, with consequences that are now becoming clear.