From Thursday you can again order free Covid tests from the government.

Every household in the United States can order four tests from Covidtests.gov through the end of the year. People can also get tested by calling 1-800-232-0233.

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

“If the past is any prologue, we have always seen an increase in cases towards the fall and winter,” Ms O’Connell said.

Here’s a refresher on how the tests work and answers to frequently asked questions.

Are the tests also effective against the latest variants?

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

The tests always detect a protein in the virus, regardless of the variant, said Nathaniel Hafer, associate professor of molecular medicine at UMass Chan Medical School.

The US Food and Drug Administration estimates that home tests will detect the virus in at least 80 percent of cases in which a person is infected.

I have had contact with Covid. When do I need to get tested?

If you’ve been exposed to the virus but feel fine, wait a few days to test — it can take time for the virus to build up in your body. The FDA recommends that people without symptoms take at least three tests total. Wait 48 hours each time before taking another test.

Get tested immediately if you experience symptoms such as sneezing, coughing, sore throat, fatigue, headache, body aches, nausea and vomiting. If this test is negative, get another one two days later.

A negative test can mean two things: you do not have Covid, or you are infected but do not have a high enough concentration of the virus in your nose to register a positive result.

Many of us have built up immunity through vaccinations and repeated infections, says Dr. Michael Mina, a former Harvard epidemiologist who now works as chief scientific officer at eMed, a health technology company. That means our bodies have become accustomed to recognizing the virus, and our immune systems can quickly trigger symptoms before we’ve built up enough virus to trigger a positive test.

False positives are very rare. If a test says you have Covid, you can be sure you have it, Dr. Mina said. But if you have a feeling you might catch something, or if someone close to you just told you they have Covid, you shouldn’t treat a negative test like a license.

I spend time with a person at risk and don’t want to infect that person. When should I get tested?

Try to get tested right before your appointment. “Don’t worry if you get tested two days early or two days late, because that information is not nearly as useful as it is two minutes early,” Dr. Mina said.

If you have symptoms, you should stay home, even if your test is negative, to avoid transmitting other infections.

I got a nosebleed in the middle of the swab. Will my test still work?

Unless it’s a severe nosebleed, a little blood won’t affect the quality of the test, Dr. Hafer said.

And you should always blow your nose before the test, he added, to remove any mucus or particles that could contaminate the test.

My trial has expired. Can I still use it?

The FDA has extended the expiration dates on many home tests, so you may still be able to use them. You can check the lot number on your test kit against the FDA list, but if your test has actually expired, it may no longer be effective.

Where can I get more free tests?

Long-term care facilities, nursing homes, food banks, schools, libraries and local health departments may offer additional free Covid testing. Some private insurance plans also cover the cost of testing. You can check your insurance company’s website or ask a pharmacist to help you determine if your plan will reimburse you.