Americans can again order free rapid coronavirus tests by mail, the Biden administration announced Thursday.

People can request four free at-home tests per household through covidtests.gov. They will begin shipping Monday.

The move comes ahead of an expected winter wave of coronavirus cases.

The September revival of the free testing program is in line with the Biden administration’s strategy to respond to the coronavirus as part of a broader public health campaign to protect Americans from respiratory viruses, including influenza and respiratory syncytial virus (RSV), that surge every fall and winter. But free tests were not mailed during the summer wave, which wastewater surveillance data shows is now receding.

David Boucher, a federal health official who managers infectious diseases preparedness and response, told reporters in August that officials needed to be strategic with limited resources and that fall is a good time to deploy free tests ahead of holiday travel and gatherings.

Here are some key aspects to know about rapid coronavirus testing at this stage of the pandemic.

How reliable are at-home coronavirus tests?

Because people have developed stronger immune responses to coronavirus after repeated vaccination and infection, rapid antigen tests are no longer as reliable at detecting the virus as they were in the early days of infection.

Federal health officials, experts and test manufacturers urge you to test 48 hours apart when you are sick.

People develop symptoms as their bodies fight the virus. That battle is beginning earlier when the immune system recognizes the threat better, even with levels of virus too low to be detected by rapid tests.

“In order to be positive for an antigen test, you really have to have a lot of virus present,” said Susan Butler-Wu, an associate professor clinical pathology at the Keck School of Medicine at the University of Southern California. “If you are doing it for screening before you are meeting family members, you could be negative and you could still be infectious.”

A widely cited study showed that viral loads tended to peak about four days after symptom onset in people infected with omicron variants, later than during previous stages of the pandemic.

When is the best time to take a home test?

Because rapid tests are not as accurate in the early days of symptoms, it may make sense to isolate and wait several days to test if you can’t afford multiple tests.

However, if you are over 65 or have underlying health conditions that place you at risk of severe disease, experts say it’s prudent to still test early during an illness because antiviral treatments are best administered shortly after you develop symptoms.

PCR tests, which are usually analyzed in labs, can detect lower levels of virus and provide greater accuracy in the early days of infection.

What are the protocols when I test positive?

The Centers for Disease Control and Prevention revised its COVID isolation protocols so that it doesn’t hinge on test results.

Under the new guidelines, you should start to isolate when you experience respiratory virus symptoms regardless of what illness causes them. You should continue isolating until you have been fever-free for 24 hours without the use of fever-reducing medications and your overall symptoms are improving. Then, you should take additional precautions for five days, such as masking, social distancing or staying away from others if you are still testing positive.

Are rapid tests keeping up with new variants?

Federal officials say the tests being distributed will detect the latest variants.

The virus evolves primarily due to mutations on the spike protein which binds to cells and causes infection, but rapid coronavirus tests are designed to detect different parts of the virus, such as the N protein, that do not rapidly evolve.

“Nearly all the tests that have been developed to date can still detect the N protein, and they are still working against new viral variants,” said Nate Hafer, director of operations for the University of Massachusetts Center for Clinical and Translational Science, who has been researching coronavirus tests.

Test manufacturers must verify their products are detecting commonly circulating variants to maintain emergency authorization from the Food and Drug Administration. The false negatives that lead people to believe the tests don’t work against new variants may actually be a result of the changes in our immune system that make it easier to develop symptoms when viral loads are too low to be detected by a rapid test.

Is it better to swab my throat than my nose?

The claim that people should swab their throats for more accurate at-home tests results has circulated online, but experts caution against doing so and say people should follow test instructions to swab their nose.

While over-the-counter tests involve throat swabbing in other parts of the world, that method has not been validated for tests authorized in the United States.

“It is not easy to do with the swabs that come in these kits: They are short and hard to reach the back of the throat,” said Nira Pollock, co-director of the infectious diseases diagnostic laboratory at Boston Children’s Hospital.

She added that it’s tough for individuals to reliably swab the back of their own throats and that that method is usually more accurate when conducted by a health care professional.

Where else can you find free tests?

Free tests may be available through your local health department, workplace, doctor’s office, libraries or community health centers. Social media pages for your city, such as subreddits or Facebook communities, can also be a helpful place to ask for leads. Insurance plans are no longer required to reimburse the costs of at-home coronavirus tests, but some plans may do so voluntarily.