Airbnb saw something of a surge as the health crisis turned travelers away from hotels toward smaller rentals. But now comes a warning.

The company behind the popular home sharing app said Thursday the slowing pace of vaccinations in the U.S. and the Delta variant could hit its business. Airbnb now expects bookings will slow from the prior quarter and remain below 2019 levels. Its CFO said it has fewer bookings for the fall season.

That warning sent its shares lower at the market open.

Like all travel and leisure, AirBnB was initially hit hard by the health crisis. It bounced back strongly by shifting focus toward longer rentals for remote work.

That paid off again handsomely in the latest quarter, has bookings quadrupled to over $13 billion. The company said listings grew especially in non-urban areas across Europe and North America.

Another property-sharing company has already gotten hit by the Delta variant. Office-sharing startup WeWork said the emergence of the variant resulted in a slower-than-expected recovery for the first half of the year.