The CDC stopped tracking most COVID-19 cases in vaccinated people. That makes it hard to know how dangerous Delta really is.

• The CDC stopped monitoring non-severe COVID-19 cases among vaccinated people in May.
• It's hard to assess Delta's risk without knowing what mild breakthrough cases look like - or whether they're becoming more common.
• Vaccines still seem highly effective against the variant, though.

It was great news: From January to April, just 0.01% of vaccinated Americans - around 10,000 out of 100 million people - got breakthrough infections, or cases of COVID-19 diagnosed after they were fully immunized.

That's according to a report from the Centers for Disease Control and Prevention, which also indicated that certain coronavirus variants were to blame for most of these breakthrough cases. However, the CDC only had genetic sequencing for around 5% of the post-vaccine infections, and the report didn't include data about the Delta variant. That strain, first detected in the US in March, might pose the greatest challenge to vaccine efficacy.

But before more data could be collected to answer these lingering questions, the CDC stopped tracking breakthrough infections that resulted in asymptomatic, mild, or moderate cases. Since May 1, the agency has only reported and investigated coronavirus infections among vaccinated people that resulted in hospitalization or death.

Sequencing efforts in the US haven't ramped up much, either: The country is still only sequencing about 1.4% of its coronavirus cases, according to data from GISAID, a global database that collects coronavirus genomes.

That means it's difficult to tell exactly how much of a risk the Delta variant poses to vaccinated people. Researchers still don't know whether Delta makes breakthrough cases more common, or what the typical symptoms of a breakthrough infection caused by Delta look like. As a result, vaccinated people may have a hard time weighing the risks of returning to normal social activities or knowing what to expect should they develop a rare breakthrough case.

In a recent blog post for Harvard Health Publishing, Robert Shmerling, an associate professor of medicine at Harvard Medical School, called the CDC's decision not to track all breakthrough cases "surprising" and "disappointing."

"By tracking only cases requiring hospitalization or causing death, we may miss the chance to learn how people with 'milder' disease are affected by Delta or other variant infections, such as how long their symptoms last and how the infection may disrupt their lives," Shmerling told Insider.

He added that the US could also miss important information about which vaccines are most effective against Delta, how long vaccine protection against the variant lasts, and whether the timing of a second vaccine dose might determine one's likelihood of a breakthrough case.

The CDC told Insider that in a substantial proportion of reported breakthrough cases, data on symptoms is missing, "which is one reason why CDC is publicly reporting hospitalized and fatal cases."

The agency added that its Emerging Infections Program is still working with nine states to obtain sequencing data from breakthrough cases - including asymptomatic and mild ones.

How well do vaccines protect against Delta?

So far, data suggests that vaccines hold up extremely well against Delta: Public Health England analyses have found that two doses of Pfizer's vaccine are 96% effective at preventing hospitalizations in cases involving the variant, and 88% effective at preventing symptomatic illness. Two doses of AstraZeneca's vaccine, meanwhile, are around 92% effective at preventing hospitalizations and 60% effective at preventing symptomatic COVID-19 from Delta.

Moderna announced on Tuesday that its vaccine is also highly effective against Delta based on lab studies, though the efficacy was slightly diminished compared to the original strain. And South African researchers recently found that among people who'd received the Johnson & Johnson vaccine, 94% of breakthrough infections were mild - including those caused by Delta.

However, Public Health England found that one shot of either Pfizer's or AstraZeneca's vaccines was just 33% effective at preventing symptomatic COVID-19 from Delta. Israeli health officials also reported last week that as many as half of new COVID-19 cases in Israel are among vaccinated people, with the majority of cases being driven by the Delta variant. (However, vaccinated people in Israel appeared to develop milder cases than unvaccinated people.)

Shmerling said that finding out which variants are responsible for most breakthrough cases - whether it's Delta or another strain - could help vaccine manufacturers learn whether they need to modify their current shots or roll out boosters more quickly.

"It's possible that tracking the severe cases would give us enough information about which variants are responsible for most breakthrough infections," he said. "But, again, the more we know about all breakthrough cases, the better we'll understand how they occur."

Hilary Brueck contributed reporting.