The agency said in May that it would stop routinely tracking so-called breakthrough infections that didn’t lead to hospitalization or death. Several states then stopped tracking mild breakthrough cases, and at least two states told POLITICO they are having trouble reliably tracking infections in vaccinated people.

Now some public health experts and lawmakers are pressuring the CDC to reverse its decision and collect comprehensive data on infections in vaccinated people — from mild to severe. Such cases are very rare, but monitoring them can help reveal how likely a vaccinated person is to infect others, and how well vaccines work over time and against emerging Covid-19 variants. State and local officials also need information on breakthrough cases to make better, more targeted, decisions about when masks are warranted or whether to limit the size of gatherings.

“Given what we know about Delta now — and more what we don’t know about Delta — having a fuller picture of breakthrough cases no matter what the level of symptoms is important to be able to decide what rules and mitigation strategies you need to put in place in your community,” said Adriane Casalotti, chief of public and government affairs at the National Association of County and City Health Officials.