In September, a man boarded a flight from Dubai to New Zealand. Not 48 hours before he had tested negative for Covid-19. Days after disembarking, while quarantining in compliance with national containment policy, he and six other passengers of the 86 on board were confirmed to be infected with the virus SARS-CoV-2.

According to a preprint case study released by New Zealand health authorities, at least four of the infections occurred in-flight—all tracing back to one man who, at least by the time he stowed his belongings and took his seat, was presymptomatic but shedding active virus. The chain of transmission was confirmed afterwards via genomic analysis.

Incoming travelers to New Zealand are required by law to quarantine for two weeks in designated hotels upon arrival, where they will be monitored and tested at least twice before being released. The samples of those who test positive are isolated, genomically sequenced, and compared to determine their exact origins. This was how health authorities were able to identify the four passengers who contracted the virus in-flight, as well as the man who gave it to them.

While not the first piece of evidence to affirm the possibility of in-flight transmission—just last month another study traced nearly 60 Covid-19 cases back to a single flight—reports of the incident make for a cautionary tale, especially with Thanksgiving fast approaching in the United States. The first takeaway is that air travel is dangerous, even when everyone, including the crew, is instructed to wear masks and, as far as we can tell, observes those instructions. The second takeaway is that a negative test isn’t foolproof. Most likely, the man on the New Zealand flight was infected shortly before or after completing a test and didn’t develop symptoms until a day or two after arriving at his destination.

Many Americans traveling by plane this Thanksgiving and for the winter holidays soon to follow will be heading straight from the airport into the arms of relatives and loved ones, rather than supervised quarantine facilities. Surveys show that in general, most recognize the risks this entails and will take necessary precautions. But they also show that two in five Americans plan on attending a gathering of more than ten people—and as for the hosts of these large gatherings, one in three won’t require guests to wear masks.

Now New Zealand, unlike the United States, has come incredibly close to eliminating Covid-19. So has China. Perhaps in anticipation of the travel surges to come, the Chinese government has tightened restrictions on points of entry, now requiring all airline passengers entering mainland China to submit not just a negative PCR test, but a negative IgM antibody test. My interpretation of this is that health authorities picked up on enough  “re-positives”—that is, people who initially test negative only to later test positive for SARS-CoV-2—to take action to prevent more.

While confirmed cases of “re-positives,” just like those of in-flight transmission, may be rare, many more likely elude detection and pose risks that cannot be ignored. Even if all guests at a given Thanksgiving dinner test negative prior to their commutes or the event itself, that doesn’t guarantee safety. Far from it. Between the frozen food that might be on the menu to the relative who flew in too late to quarantine, too many variables will be in play for those test results to be definitive.

Anyone traveling by plane this Thanksgiving should gear up as if their lives—and their loved ones’—depended on it. That means an N95 mask, face shield, gloves (enough pairs to change after every handwashing), and preferably a hazmat suit. But the wiser and safer decision is to stay home. Painful though it may be to forego the creature comforts of gathering in person for a decidedly less cozy virtual affair, the protection will be well worth it.