(Bloomberg)—CVS Health Corp. is limiting customers’ purchases of rapid, over-the-counter COVID-19 tests, with a maximum of six packages available online and four in its pharmacies, as the spread of the delta variant spurs demand.

Put in place this week, the limits apply to Abbott Laboratories’s BinaxNOW along with a test from the startup Ellume, according to an email from a CVS spokesperson. Both tests are available without a prescription.

Surging interest in rapid virus tests has made the products a scarce commodity at some online retailers and in certain stores. The renewed demand for testing has arisen just as the highly contagious delta variant threatens many people’s plans to return to work and school this fall.

Online availability of rapid COVID-19 tests can shift based on demand, and the company is working with its suppliers to meet testing needs, the CVS spokesperson said.

The change was made this week “in order to serve our customers’ OTC testing needs, and due to high demand,” according to the email.

Supplies of the tests will be constrained in the coming weeks as manufacturing ramps back up, Abbott said in a statement

“Just as we have done throughout the pandemic, Abbott is deploying our resources and expertise from all over the company to help quickly meet rising demand,” the company said.

Ellume is scaling production and working with retailers to ensure consumer access to its tests, which are in stock at CVS, Walmart, Target and Everlywell stores, a spokesperson for the Australia-based company said in an email.

CVS is No. 116 in the 2021 Digital Commerce 360 Top 1000.