On Thursday, the agency released data finding that there have been 226 cases that meet the CDC’s criteria for myocarditis and pericarditis (swelling in or around the heart) in patients who received either Pfizer’s or Moderna’s mRNA-based vaccines. Myocarditis specifically can lead to conditions such as heart failure or cause sudden death, and this particular spate of cases appears to affect younger people who have received mRNA vaccines, although it’s a condition that is usually temporary.

The critical caveat: There are just a few hundred of these reported heart inflammation cases in the United States, where nearly 142 million Americans, or close to 43% of the population, have been fully vaccinated. The overwhelming majority of patients who had this side effect and were discharged from a hospital due to it in potential vaccine-related cases, 81%, had full recovery from their symptoms. And heart-related complications are prevalent in actual COVID-19 patients themselves to a far greater extent.

But the CDC believes it’s important enough to keep tabs on these adverse events as the U.S. attempts to get closer to 70% to 80% of the population vaccinated in order to achieve wide-scale immunity. Building trust along those lines will be critical for holdouts, especially given how new mRNA vaccine technology is and the rapid timeline for Pfizer’s and Moderna’s FDA emergency authorizations during the pandemic.

“We recognize that some adverse reactions, for example myocarditis or pericarditis as discussed earlier today, may be too infrequent to detect in a safety database of typical size for pre-licensure clinical trials,” said the FDA’s Doran Fink, a vaccine specialist, in a statement on Thursday.

It’s unclear exactly why (or whether) COVID mRNA vaccines are directly linked to this side effect and will be the central area of focus, particularly when it comes to effects on younger people, in next week’s CDC meeting. Pfizer’s and Moderna’s vaccines were the first treatments ever to receive authorization for an mRNA product, which works very differently from conventional vaccines and could theoretically be used to treat diseases ranging from cancer to other infectious disorders by turning the body’s own cells into mini-drug or antibody-making factories of sorts. That can be a big advantage since it’s a more malleable process for tackling coronavirus variants or adjusting for a disease’s unique genetic quirks.

It’s a technology that’s been in the making for more than a decade and the COVID pandemic has allowed it to shine. But while the Pfizer and Moderna vaccine trials have firmly established their products’ safety, they are still brand new in a real-world setting.

It’s far too early to tell, especially since the CDC meeting has yet to occur, but a definitive link between mRNA technology and inflammation issues, however rare or unlikely, could throw a wrench into hopes for latching onto this platform in other therapeutic spaces. Moderna, for instance, has a pipeline of experimental mRNA products ranging from cancer to flu vaccines. And this is a market that could be worth more than $15 billion by 2026, making scrutiny of the products all that more important for industry trust even if serious side effects appear extremely rare.