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.

At the time, the agency had identified only 10,262 cases across the U.S. where a fully vaccinated person had tested positive for Covid. Most people who got infected after vaccination showed few symptoms, and appeared to be at low risk of infecting others.

But in the months since, the number of vaccine breakthrough cases has grown, as has the risk that they present. And while the CDC has stopped tracking such cases, many states have not. Bloomberg gathered data from 35 states and identified 111,748 vaccine breakthrough cases through the end of July, more than 10 times the CDC’s end-of-April tally.

At the same time the CDC stopped tracking those cases, the delta variant began to spread in the U.S. Small numbers of delta-variant cases were identified in mid-April. The strain began to take over in some parts of the country in June, then exploded nationally in July.

It now makes up the vast majority of cases, in part because it is more contagious than prior strains. New waves of Covid cases have caused a surge in hospitalizations in the South among unvaccinated people, and led the Biden administration and states to push for vaccine mandates.

“When I saw CDC was going to stop tracking vaccinated people who get infected, my heart sank,” said Charity Dean, who helped lead California’s response to Covid as the state health department’s assistant director. “We lost our shot at being able to characterize how this variant is moving through the population and how new variants might emerge.”

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