(CNN) -- Amazon, Walmart, CVS, Walgreens are limiting the number of at-home Covid kits customers can buy due to huge demand.

The rapid spread of the Omicron coronavirus variant ahead of the holidays has sparked the surge -- and there has been anecdotal evidence over the past week of test shortages at stores across the country.

Amazon said it's limiting purchases of its own at-home Covid-19 test collection kits to 10 on its online marketplace. Amazon's third-party sellers on the site set their own quantity limits.

"We are working hard to secure additional Covid-19 test inventory" from sellers, a spokesperson said.

Walmart has high stock levels of Covid-19 tests in stores, but is more constrained online, a spokesperson said. It has not placed purchase limits on Covid-19 tests in stores nationally, but is capping testing kits at 8 for online orders.

CVS acknowledged in a statement on Tuesday that tests may be temporarily out of stock at its stores.

"To ensure equitable access to tests both in store and digitally, we've added a limit of six test kits per purchase," the company said.

CVS has more than 9,900 stores across the US, although it announced last month that nearly 10% would be shut over the next three years.

"We're committed to providing families with protection and peace of mind during the holiday season, and we continue to offer access to lab-based testing with results available in 1-2 days or rapid COVID-19 testing," the statement said.

Walgreens said it's limiting in-store and online purchases of at-home tests to four each time "due to the incredible demand for at-home rapid testing," according to a statement on Tuesday.

"We ask that our customers please show patience and understanding as together, we continue to navigate the evolving pandemic environment," Walgreens President John Standley said.

As of August 31, 2020, Walgreens has just above 9,000 stores.

On Tuesday, the Biden administration announced the purchase of a half-billion at-home rapid tests, which will be made available next month and will reach Americans through the mail.

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