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CDC Scaled Back Hunt for Breakthrough Cases Just as the Delta Variant Grew​​​​​​

Bloomberg identified more than 100,000 vaccine breakthroughs

A medical assistant administers a Covid-19 test in Los Angeles, California.

A medical assistant administers a Covid-19 test in Los Angeles, California.

Photographer: Mario Tama/Getty Images North America

The U.S. agency leading the fight against Covid-19 gave up a crucial surveillance tool tracking the effectiveness of vaccines just as a troublesome new variant of the virus was emerging. 

While the Centers for Disease Control and Prevention stopped comprehensively tracking what are known as vaccine breakthrough cases in May, the consequences of that choice are only now beginning to show.