Finding an at-home COVID-19 rapid test has become almost as difficult as buying the most sought-after toy this Christmas shopping season.

While some metro area pharmacies had dozens of tests on their shelves this week, others posted signs telling customers their supplies were depleted. Pharmacies report high demand for the tests, with some national chains, including CVS and Walgreens, placing limits on the number of kits each customer can purchase.

The rapid tests, which can produce results in about 15 minutes, are becoming increasingly sought after at a time when people want to know their COVID-19 status before travel, visiting family or attending public events as the highly infectious omicron variant becomes the dominant virus strain.

In Western Europe, rapid tests are plentiful and some countries provide them for free. But in the United States, they have not been a key element of the national COVID-19 prevention strategy until this week.

On Tuesday, President Joe Biden announced that 500 million free rapid tests would be available sometime in January.

"There's a lot of experts in lab testing who have been calling for greater access to testing on multiple fronts," said Dr. Amy Karger, a clinical pathologist with the University of Minnesota Medical School and M Health Fairview.

"Many of us feel that expanding testing beyond the walls of the lab to help provide greater accessibility is another piece of the puzzle to help fight the virus and this pandemic," she said. "It would have been nice if these had been available before the holidays."

In the meantime, people might have to visit several pharmacies or retail stores before finding an at-home test, which can cost $10 for a single test and up to $30 for two.

Often, supplies quickly dry up.

An hour after 180 COVID-19 rapid home tests showed up on store shelves at a Walgreens on France Avenue in Edina, Rosanna Faber snapped up the last boxes Wednesday afternoon.

Using home tests, she recently discovered her children had become infected with COVID-19, a diagnosis that was later confirmed by tests taken at the Minneapolis-St. Paul International Airport testing site.

She wanted a supply of tests at home as she continues to monitor her children's health. Had she arrived at the Walgreens, which has a limit of four boxes per purchase, 10 minutes later, she might have left empty-handed.

"I just got lucky today," said Faber, 51, who lives in Minneapolis.

John Eidem also picked up some tests during the rush at Walgreens. The 33-year-old from northeast Minneapolis plans to test before and after he and his girlfriend attend a wedding in California.

"Everyone is scared of the variant, but I just want to try to be safe," he said.

Biden has said that he would invoke the Defense Production Act, a Korean War-era law that allows the federal government to marshal resources, in an effort to help test manufacturers produce more at-home kits.

The uneven supply chain, as well as the unpredictability of foot traffic at any one retail location, means that test kit availability can change daily or even by the hour.

For instance, a Cub Foods store in Robbinsdale had plenty of rapid tests on Wednesday, while a Hy-Vee in the same city had none.

Cub Foods reports that rapid tests are generally available as of midweek, including at stores in Edina, New Hope and West St. Paul.

The independent St. Paul Corner Drug had the tests, but its inventory vanished in the run-up to Christmas.

"We did have a lot of them really early, but we sold out this morning, unfortunately, around 11. Right before lunch," pharmacy rep Margie Morrison said Thursday. "It's a hot commodity right now."

A Target Corp. spokeswoman said the company is working with vendors to ensure supply and regularly restock at-home tests in stores but encourages customers to buy them at Target.com. Target stores in Edina and Inver Grove Heights were among those that had no tests available midweek.

Some home tests available at Amazon are sold only in bulk and are not available to be shipped until after Christmas or next month.

At-home rapid tests are not as sensitive as the tests typically processed by a laboratory, known as a PCR or polymerase chain reaction test. Those tests look for the genetic material of the coronavirus.

The home tests, known as antigen tests, look for proteins on the surface of the virus. They do not always detect early infection but still can be a good indicator of infectiousness.

The big advantage of at-home tests is speed. It can take days to get results from a PCR test, some of which are processed by diagnostic labs outside of Minnesota.

"That is how the at-home test is the most useful. You are testing yourself right before you interact with others," Karger said. "That is something that the PCR can't offer because of the turnaround time."

Rapid tests should be taken as close as possible to attending a party, visiting a loved one or attending holiday services.

"The timing of when the testing is done is extremely important," said Matthew Binnicker, vice chair of practice for Mayo Clinic's Department of Laboratory Medicine and Pathology. "The amount of the virus in my respiratory tract may change. It may go from low levels to high levels in 24 hours."

Binnicker said it is important for anyone who tests positive for COVID-19 using a rapid test to immediately notify a doctor, who most likely will order a PCR test for confirmation. Some COVID-19 treatments, including monoclonal antibodies, are most effective in the early stages of infection.

Although the at-home tests are in high demand, Minnesotans are not going to testing sites in the same numbers as one month ago. For the week that ended Saturday, there were an average of 38,000 tests daily, compared with 45,500 before Thanksgiving.

The state's testing positivity rate has fallen to 8.8%, and COVID-19 hospitalizations were at 1,415. Both numbers are down from peaks in early December.

Another 3,378 new COVID-19 cases and 52 deaths were announced by state health officials Thursday, bringing the pandemic totals to 996,224 infections and 10,306 fatalities.

Although Minnesota appears to have peaked in its most recent wave, public health officials are concerned that the newly arrived omicron variant could reverse those gains. They hope that providing more rapid tests, and offering them at no cost, can help prevent new infections.

"There's a lot of European countries that have been providing free tests, so there has been frustration," Karger said. "Many of us are happy that we are finally to this point. It is not clear to me why it has taken this long."

Staff writers Evan Ramstad, Patrick Kennedy and Dee DePass contributed to this report.