The purchase of at-home coronavirus tests will be capped at Walgreens and CVS locations nationwide as demand for kits continues to surge amid the rapid spread of the omicron variant.

CVS announced in a statement Tuesday that the pharmacy had added a limit of six test kits per purchase both in stores and online. The company, which has more than 4,800 locations, said that kits sold online might become temporarily out of stock as it tries to make them available in stores to meet demand.

Walgreens also announced Tuesday that the company would limit purchases of at-home test kits to four. The pharmacy chain said some stores might experience a temporary shortage in rapid over-the-counter testing kits.

“We ask that our customers please show patience and understanding,” John Standley, the president of Walgreens, said in a statement.

Amazon also limited the purchase of at-home test kits made by the company, which customers must mail in to a lab to be processed, to 10 per person. The cap does not apply to third-party sellers, who can set their own product quantity limits on the website.

Walmart took similar measures Tuesday, limiting the purchase of at-home testing kits to eight per customer online. In-store purchases are not being capped, though the retail giant said that each store could set its own limits based on its inventory.

The caps come as omicron’s rapid spread — the fastest of any variant yet — has customers flocking to testing sites and pharmacies. At-home tests are flying off shelves, and Americans are sometimes waiting in line for hours at testing sites.

Virus cases in the United States have risen 27% in the past 14 days, and hospitalizations have climbed 13% in the same period, according to a New York Times coronavirus tracker. President Joe Biden said Tuesday that the government would buy 500 million rapid coronavirus tests and distribute them free to Americans, as well as create new vaccination and testing sites and send 1,000 military medical professionals to help hospitals nationwide.

Walgreens and CVS offer several brands of coronavirus tests, including Abbott BinaxNOW, Acon FlowFlex, Quidel Quickvue, Ellume and Pixel by LabCorp. The pharmacies also offer lab-based testing with a one- to two-day turnaround.

This article originally appeared in The New York Times.