The Centers for Disease Control and Prevention (CDC) has strongly advised people not to travel or gather for the winter holidays. COVID-19 cases and deaths are holding at alarming rates across the country, and it’s thought that Thanksgiving-related travel helped fuel surges in some states. With the pandemic hitting a dangerous new peak, the most responsible thing to do is to spend the holidays with your immediate household—especially if you haven’t been isolating yourself from others for the past two weeks.

It’s clear that many aren’t heeding this warning, however. Air travel saw more than 1 million US passengers for three consecutive days over the past weekend, marking the busiest travel days since the pandemic began in March.

If you’re currently en route to a holiday destination—or even just planning to meet friends and family for Christmas dinner—you may think that getting tested for COVID this week is enough of a precaution to make your plans safe. But getting tested before an event won’t keep everyone safe.

As PopSci reported last month, a negative COVID test isn’t a guarantee that you’re not carrying the virus—it represents how much of the virus was in your body during a single snapshot of time. It can take several days after exposure for the pathogen to replicate enough that it shows up on a test.

“If you were someone who did not have a high viral load on Thursday, that viral load may have reached a point where the test will pick it up on Friday,” Neha Nanda, medical director of infection prevention and antimicrobial stewardship at Keck Medicine of USC, told PopSci.

And it’s important to remember that rapid tests—which are probably your only option for getting a pre-Christmas result, at this point—are much better at catching a highly infectious patient than an asymptomatic one. That makes it even more likely that you’ll get a negative result, even if COVID is brewing inside you. In a few more hours or days, you might be dangerously contagious.

If you haven’t spent the past two weeks isolating yourself from potential COVID-19 sources, you shouldn’t travel or gather with people outside your household during the holiday. The likelihood of catching or transmitting the virus is simply too high, and we must all stay vigilant to keep case counts as low as possible while we wait for vaccines.

Should you choose to celebrate the holiday with people outside your household—whether or not you’ve spent two weeks isolating—follow the CDC’s tips on making the event as low-risk as possible. Wear masks, avoid spending time inside together, practice social distancing, and wash your hands. Once you’re back home, follow the CDC’s quarantine guidelines to ensure you won’t expose others to anything you might have picked up on your travels.