The Centers for Disease Control and Prevention, 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 CDC 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.

Not only is the delta variant more contagious, health officials now believe it more readily causes symptomatic — and contagious — illness in vaccinated people.

That meant that unlike with previous variants, vaccinated people are more likely to spread the disease more easily to unvaccinated individuals or those who are vaccinated but vulnerable because of their age or health.

There’s also evidence that suggests breakthrough cases can happen at a higher rate. On Friday, CDC released a study of Covid cases in Massachusetts in July that found 274 symptomatic breakthrough cases, almost all related to the delta variant, among people on Cape Cod. 

It’s a new dynamic that punches a hole in the wall of immunity the U.S. had been building to cordon off and stamp out the virus.

Anyone else feel like asking why the CDC would move the goalposts in a way that would limit the data about vaccinated peopled getting infected but still collect all data about the unvaccinated?

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