The third holiday season since the pandemic started is approaching, and Covid remains a persistent presence. More than 300 people die of the virus each day, on average, according to NBC News' tally.

Testing remains a crucial tool to identify cases and curb spread. But finding low-cost tests and interpreting the results isn't always simple.

Here's the latest CDC and FDA guidance, as well as expert advice, about how to get tests without paying out-of-pocket, check when an at-home test expires, and interpret a negative result if you have symptoms or known exposure.

Private insurance and Medicare cover eight at-home tests a month

Since January, the Biden administration has required private insurers and Medicare to cover up to eight at-home tests per month. People with private insurance can order the tests at in-network pharmacies or submit a claim to be reimbursed for tests they buy at other stores or out-of-network pharmacies. People with Medicare can search online for a list of providers that offer free tests.

PCR lab tests are also fully covered through private insurance and Medicare.

Free at-home tests from the government are no longer available

The federal program that distributed up to 16 free at-home tests to households through the mail ended on September 2 because of a lack of congressional funding.

How to find free testing sites near you

Some sites that provided free rapid or PCR tests earlier in the pandemic have closed, but the Department of Health and Human Services has an online search tool for finding nearby sites still offering free or low-cost tests. Most of the locations are pharmacies like CVS and Walgreens, but the site can also direct you to your state health department's website, which may offer additional options.

Once the government's public health emergency declaration expires, testing costs will probably go up

The declaration was last renewed in October, but it could end sometime in 2023. After that, Medicare beneficiaries will most likely have to pay the full cost of at-home tests but should still get clinical diagnostic testing covered, according to KFF, a nonprofit think tank focused on health. For people with private insurance, testing costs will be subject to the particulars of their plans but are unlikely to be covered in full.

Rapid tests can expire, but many shelf lives have been extended

Different tests have different shelf lives, but many of the original expiration dates have been extended since the tests were authorized.

In these cases, the manufacturer has provided evidence to the government that the tests give accurate results longer than was known when they were created. 

Abbott’s BinaxNOW at-home test, for example, says it has a shelf life of 15 months, but the expiration dates of many batches have been extended by three to six months. The government-distributed tests from iHealth Labs, meanwhile, last one year, but most of their shelf lives have been extended four to six months. Flowflex at-home tests have a shelf life of 19 months, with extensions of six months.

To check whether the expiration date of your particular test has been revised, click on the relevant link on the FDA's list and look up the lot number.

When to test if you've been exposed or feel ill

If you have symptoms, take a test immediately. If you were exposed to someone who tested positive but you feel healthy, test after five full days.

The FDA recommends that people with a known exposure who test negative take a second test 48 hours later. If negative again, test a third time after another 48 hours.

"We know antigen-based tests, you have to repeat them. They're not a one-and-done test," said Dr. Susan Butler-Wu, an associate professor of clinical pathology at the University of Southern California.

If you are exposed within 30 days after previously testing positive for Covid, you do not need to test unless you develop symptoms, according to the Centers for Disease Control and Prevention. If it has been less than 90 days since your last Covid infection, use a rapid test, since PCR results can stay positive for up to 12 weeks.

What to know about the accuracy of at-home tests

PCR tests are generally more sensitive and accurate than at-home tests, but the results can take at least 24 hours — and often several days. At-home tests (also known as antigen tests), meanwhile, rarely give a false positive but can give false negatives, even if someone is symptomatic. That's especially likely in the early days of an infection.

Scientists aren't sure why that is, but one theory is that the immune response leads to symptoms before the virus has a chance to replicate to detectable levels. That may cause someone to feel tired, achey or sniffly before they test positive.

For other people, Covid tests never come back positive even though they feel sick and were exposed to the virus. It's possible that these people have an unrelated infection, or their immune systems did a swift job of vanquishing the virus before it replicated widely enough to register on a test.

People who are elderly or immunocompromised should still seek out a PCR test if they feel sick or were exposed to Covid and get a negative result on an at-home test, according to Dr. Sheldon Campbell, an associate professor of laboratory medicine at the Yale School of Medicine. That's because these groups are eligible for treatments like Paxlovid in the days after they test positive.

"You don't want to wait two days, get a positive and realize you should have been treating for two days," he said.

Your test result could be a good indicator of contagiousness

Campbell said testing positive on an at-home test is a pretty good indicator that you're infectious. But Butler-Wu cautioned that it’s still possible to be contagious and test negative on an at-home test, or to test positive and not spread the virus to others.

"At the end of the day, there is no infectiousness test for Covid. There has never been and there still isn't," she said.

It is well established, however, that people are more likely to be infectious at the start of their illness. An August study found that 65% of people with Covid shed infectious virus five days after their symptoms started, but just 24% were still doing so after a week.

How long you're likely to test positive

For people who are mildly ill, the CDC recommends isolating for at least five days after your positive test or the start of symptoms, then ending isolation if you test negative or if symptoms have resolved or are clearing up. If your symptoms have not improved much or you still have a fever on day five, the agency advises isolating until you are fever free for 24 hours without medication.

People with moderate or severe illness — characterized by shortness of breath or hospitalization — should isolate through day 10.

"Typically, people are positive for about seven days after their symptoms" start, Campbell said. "Some people can go longer — rarely more than two weeks."

In one small study, just 25% of people with Covid were negative on a rapid test on day six of their illness, but all the participants tested negative after two weeks. In a study of college athletes with Covid, meanwhile, 27% still tested positive one week after their first positive test.

Campbell said scientists are still investigating why some people test positive for more than two weeks.

"People who are antigen test-positive beyond two weeks are in some fashion not having good immunity to the virus," he said.