If you’re testing for COVID before visiting vulnerable loved ones this holiday season, be warned: At-home rapid tests may give you a false sense of security, according to a new study.

Researchers in the Netherlands had medical professionals swab the noses of thousands of individuals, then sent them home with a rapid test to complete themselves. They found that rapid tests provided false negatives about 75% of the time, according to the study, published Saturday in Clinical Microbiology and Infection.

When researchers eliminated infected individuals with the lowest viral loads, rapid tests still gave false negatives more than half the time. Such tests perform “adequately,” but only after individuals have developed symptoms, researchers wrote.

The holidays are prime time for family get-togethers—and for illnesses like COVID, flu, and RSV to spread. While new variants have prevented COVID from becoming a seasonal illness so far, spikes are common during cold weather, when individuals tend to stay inside and gather for holiday celebrations.

Since early in the pandemic, public health officials have urged those visiting elderly or immunocompromised friends and relatives—both vaccinated and unvaccinated—to test and quarantine ahead of their visit. The same goes for those planning to gather in large groups.

But in the era of Omicron, rapid tests “may only detect the minority of infections,” researchers wrote. This is likely because individuals sick with COVID who don’t have symptoms usually have low levels of the virus circulating—and don’t have a runny nose that would make testing positive much easier.

Self-testing has “limited value for asymptomatic individuals wishing to protect vulnerable persons and may even lead to a false sense of security,” the authors wrote. “One should be aware of this, and better informed about other prevention options such as physical distancing or mask use.”

Researchers at the University of Washington School of Medicine’s Institute of Health Metrics and Evaluation project a continued gradual rise in COVID cases through January, the end of their forecasting period. Weekly U.S. deaths from the virus are hovering at around 2,500, according to the U.S. Centers for Disease Control and Prevention.