That’s because some people who are infected are asymptomatic, COVID vaccines aren’t 100% effective at preventing infection, and they also appear to be particularly vulnerable against the new Omicron variant.

“A test is better than no test,” said Dr. Riccardo Valdez, director of the Division of Clinical Pathology at University of Michigan Health. “Use an at-home test as directed by the test insert and know that the more frequently you use them if you are feeling sick, the more likely you are to detect the infection.”

Roughly one in four Americans is already planning on asking family and friends to get a COVID test before holiday visits. About 63% of Americans say that if asked, they would take a COVID test before visiting family, according to a recent poll by Rapid Test & Trace USA published last week. 

But on Thursday, Dr. Anthony Fauci said not all COVID tests are picking up Omicron infections. “We’re getting preliminary information that not all of the diagnostic tests will be accurate with Omicron,” said the director of the National Institute of Allergy and Infectious Diseases, and the chief medical adviser to President Joe Biden.

“Some, and many of the commonly used ones, appear to pick up and detect Omicron quite well,” Fauci said during a U.S. Chamber of Commerce Foundation webinar. “We’re in the process of doing large screenings to determine which of these antigen, point-of-care rapid tests still maintain their accuracy of diagnosis. But clearly there are some that do. We’re trying to find out those that don’t reflect an accurate result, and if we do, then to make sure that those tests are not used to diagnose Omicron.”

Fauci did not name specific tests that did or did not detect the Omicron variant. A spokesperson for the Food and Drug Administration told Fortune Thursday that the agency has been monitoring tests authorized to detect SARS-CoV-2 (the virus that causes COVID-19) for potential effects of genetic variation on test performance. “The agency is currently collecting data for the Omicron variant and will continue to communicate information about whether and how it affects authorized tests as it becomes available,” the spokesperson said.

The FDA has already identified three COVID tests it believes will not detect the Omicron variant: Applied DNA Sciences’ Linea COVID-19 Assay Kit, Tide Laboratories’ DTPM COVID-19 RT-PCR test, and Meridian Bioscience’s Revogene SARS-CoV-2 test (which has not been made available for distribution). 

Meanwhile, New York City Mayor Bill de Blasio announced a number of measures his administration is implementing Thursday to reduce the spread of COVID, including distributing 500,000 at-home, rapid COVID tests through community organizations for free. 

“We want to use at-home testing a lot more. We want to make it more available to New Yorkers,” de Blasio said Thursday. New York City joins several states—including Massachusetts, Minnesota, New Hampshire, New Jersey, and Washington—in now working to distribute free, at-home tests.

At-home testing provider Vault, which has partnered directly with Minnesota, New Hampshire, and New Jersey, says its tests are detecting Omicron cases. “Vault Health’s tests can detect the presence of Omicron without additional sequencing,” a company spokesperson told Fortune. Vault, which uses a PCR test, helped detect the second known case through the company’s partnership with Minnesota.

While some COVID tests may not be as effective at detecting Omicron, Dr. Fauci is still recommending Americans get fully vaccinated, and get a booster. He acknowledged Thursday that the current vaccines have reduced protection against contracting an Omicron infection. However, research shows there is still less of a chance of severe hospitalization or death from Omicron among vaccinated individuals, and getting a booster shot does help, Fauci said, adding that means it’s more important than ever before to be not only vaccinated, but to get a booster.

“The antibodies that are induced by [COVID] vaccination lose much of their potency. That’s the sobering news. The somewhat encouraging news is that when you boost someone who’s been doubly vaccinated, you can reconstitute a lot of that diminished protection that occurs because of Omicron,” Fauci said. “It is essential for people, particularly the elderly and the vulnerable, but quite frankly everyone who has been vaccinated to get a booster shot.”