Public health experts have noted a disturbing decline in COVID-19 testing in recent weeks, threatening to leave them blind as dangerous variants make their way to the U.S. The trend has diverse causes, but officials warn that vaccination effectiveness could be blunted if testing can’t keep up, The New York Times reports.

On average, testing has fallen 30%. Some areas have seen even sharper declines, like the 50% drop in Michigan since November. Delaware’s testing has fallen by two-thirds.

“It’s shocking how quickly we’ve gone from moving at 100 miles an hour to about 25,” Dr. Clemens Hong, leader of Los Angeles County’s testing efforts, told The Associated Press.

The cause hasn’t been conclusively found, and experts speculate that a multitude of forces could be at work. With the holiday travel season over, people aren’t getting exposed as often. Extreme weather and the resource drain of vaccination efforts have forced testing sites to close down. Fatigue could also be a factor: as the pandemic stretches on, people have proven less willing to follow safety guidelines.

“My sense is that it’s probably that there are fewer options for testing, fewer communications about it, people may be perceiving that it’s less necessary — maybe they just don’t see the point anymore,” Dr. Jennifer Nuzzo, with the Johns Hopkins Bloomberg School of Public Health, told The New York Times.

The fall comes just as health officials are desperate for a clear picture. The threat of virus variants, many more infectious and dangerous with some boasting resistance to current vaccines, threatens to undermine the already struggling effort to innoculate the public.

“There’s nothing about the current situation that has made testing any less necessary,” Nuzzo said.

Many institutions, however, are dealing with limited resources. Given the choice between allocating them to testing or setting up vaccine distribution, the latter often seems more attractive. Health officials have been emphasizing how important testing will be to reopen schools.

“Schools have asked themselves, justifiably, ‘Is the juice worth the squeeze to set up a big testing effort?’” Mike Magee, CEO of the nonprofit advisory Chiefs for Change, told the Associated Press. “Our message to the school systems we work with is: ‘Yes, you need to stand up comprehensive testing because you’re going to need it.’”

Manufacturers are bolstering those efforts, aiming to produce 110 million fast-acting home tests in March.

A health professional poses with a test cassette used for an antigen rapid test for Covid-19 at a testing center in Berlin
A health professional poses with a test cassette used for an antigen rapid test for Covid-19 at a testing center in Berlin AFP / John MACDOUGALL