The latest wave of people hospitalized for the coronavirus are increasing younger adults infected with the b.1.1.7 variant that was originally identified in the United Kingdom, a more infectious and potentially more dangerous strain.

According to CNBC, Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said that more hospitalizations are required for people in their 30s and 40s, as opposed to earlier waves which predominantly hit the elderly the hardest.

“Data suggests this is all happening as we are seeing increasing prevalence of variants, with 52 jurisdictions now reporting cases of variants of concern,” Walensky said at a press briefing Wednesday.  More than 16,000 infections from the variants have been in those jurisdictions.

The causes of this swing are various.  The b.1.1.7 variant is believed to be more easily transmitted among younger people than early strains, and the elderly are also the largest demographic in the American population to have been vaccinated.

Another cause may be that as states open up their economies and lowering regulations–in some states, eliminating them entirely–people in their 30s and 40s are quick to leave their homes without taking protective measures.  Public health officials are pleading with the public to adhere to masking and social distancing guidelines, even if they’ve been lifted by their state.

“I’m pleading with you, for the sake of our nation’s health,” Walensky said at a briefing last month. “Cases climbed last spring, they climbed again in the summer, they will climb now if we stop taking precautions when we continue to get more and more people vaccinated.”