On Monday, Dr. Peter Juni, director of the Ontario COVID1-9 Science Table, said people should be extra careful as the virus is “very dynamic.”

“We know from Omicron now, a rapid test [result] that is negative, three or four hours later could become positive but nobody knows,” he told Canadian news outlet CTV.

This discrepancy can come into account when planning travel or gatherings as while a rapid test might appear negative the morning of, during the flight — or event — another test might read otherwise.

“The rapid tests used wisely doesn’t mean, ‘Okay I do a rapid test today, tomorrow evening, I can go to a party with 50 people,’” Juni continued. “Forget it. It doesn’t work that way.” He also stressed the importance of keeping gatherings as small as possible

Dr. Lorna Thorpe, director of the division of epidemiology in the department of population health at NYU Langone, explained to Newsweek that rapid tests work best when the person is symptomatic.

“If you are symptomatic, a rapid antigen test is as good as a PCR,” she said. “If you are not symptomatic, and you really want to be sure, a PCR is best.”

Thorpe explains that both tests are “important and good,” but that the main difference between the two is that the PCR test is more sensitive and works best over a longer range of days.

Dr. Wafaa El-Sadr, professor of epidemiology and medicine for Columbia University Mailman School of Public Health, told Newsweek that if a person is asymptomatic, a negative result on a rapid test “should not give you total reassurance of peace of mind.”

A rapid test’s accuracy depends on the amount of viral pieces present in a person’s body. El-Sadr explained that people who are totally asymptomatic likely have a smaller amount of virus shedding.

Dr. Eric Topol, the founder and director of the Scripps Research Translational Institute, said on Monday that rapid antigen-based COVID tests are best when used frequently so as to establish confidence in the result.

“If we’re going to use rapid tests, they’ve got to be as close proximal to the time of the event as possible, and that would include travel,” Topol told host Jim Cramer on CNBC’s Mad Money.

Thorpe echoed this sentiment saying “take it today, take it tomorrow—reduce the chance of a false negative” when talking about rapid tests, and highlighted that these test kits often come with two tests for that reason.

El-Sadr told Newsweek that these tests should be used “strategically and in the appropriate context.”

She said for someone who does have symptoms and wants to know their COVID-19 status quickly the rapid test can be “valuable.” While not, “100 percent,” she said the performance is “quite good.”

El-Sadr said for those planning on gathering with family or friends in the coming days, testing beforehand is a good idea to be “double sure” they are not infected and not transmitting the virus to others.

“Most of us think that repeated testing improves the performance of the test,” she explained. “Test the day before a gathering and the morning of the gathering. Having sequential negatives is better than one alone.”

While the at-home tests are not “fool proof” and there is a learning curve that exists, Thorpe said that they are “quite simple” but says it’s important to read the instructions carefully.

Thorpe explained that the importance of testing cannot be overstated as the holidays approach, and said the most important thing is having as many tests available to individuals as possible.

It’s important that people reduce large gatherings and to know the vaccine status of the people gathering, she told Newsweek. Beyond that, wearing masks and keeping windows cracked when possible will also help stop the spread of the virus.

“The number one thing, if you are already vaccinated—get boosted,” Thorpe advised. From the beginning of the wave of Omicron cases in the U.S., she said the uptick in people receiving vaccine boosters has not been as significant as she had hoped.

Dr. Anthony Fauci, chief medical advisor to President Joe Biden, said last week the latest variant can evade the protection provided by the initial vaccines alone though getting a booster shot increases efficacy and provides better protection, per ABC News.

“No amount of testing can replace the value of vaccination and boosting,” El-Sadr said.

Newsweek contacted Gigi Gronvall from John Hopkins University for comment.