Starting Thursday, you can once again order free Covid tests from the government.

Every household in the United States will be able to order four tests from Covidtests.gov between now and the end of the year. People can also get tests by calling 1-800-232-0233.

Dawn O’Connell, the assistant secretary for preparedness and response at the Department of Health and Human Services, said it was important to make sure Americans had access to free tests as they spend more time indoors and gather for the holidays.

“If past is prologue, we have always seen increasing cases as we head into fall and winter,” Ms. O’Connell said.

Here’s a refresher on how the tests work, and answers to common questions about them.

Do the tests work against the newest variants?

Yes, they do, Ms. O’Connell said. “We check every time.”

The tests still pick up on a protein in the virus, no matter the variant, said Nathaniel Hafer, an associate professor of molecular medicine at the UMass Chan Medical School.

The Food and Drug Administration estimates that at-home tests will detect the virus at least 80 percent of the time when someone is infected.