To be 95 percent sure of picking up an Omicron infection, some of the accredited antigen tests needed a viral load up to a hundred times higher than with a Delta infection, the team led by virologist Oliver Keppler found.

A viral load is the amount of virus detectable in a patient’s body. 

Overall, eight of the nine commercially available antigen Schnelltests in the study didn’t pick up Omicron as well at the Delta variant, according results published in the journal Medical Microbiology and Immunology.

“There is tremendous heterogeneity in rapid antigen tests in terms of detecting Omicron,” Keppler said.

“On the one hand, this needs to be clearly communicated, and on the other hand, a list of usable tests needs to be published quickly,” he said.

The scientists looked at 166 cases of infection between October and January, 101 of which were with the Omicron variant and 65 of which were with Delta.

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They found that the least reliable tests picked up less than a third of Omicron infections with a high viral load, whereas the same type of tests picked up 70 percent of Delta infections with an equivalent viral load.

In terms of infections with a medium viral load, the tests picked up between zero and eight percent of Omicron infections and between zero and 28 percent of Delta infections.

With some 580 different antigen tests on the market, Keppler said that the Paul Ehrlich Institute – Germany’s medicines agency – should publish a list of which ones are most effective against Omicron.

“The one-eyed among the blind must now be quickly identified and published by the Paul Ehrlich Institute,” he said.

He also called on the public to exercise continued caution.

“You should never take a negative result as a free pass,” Keppler warned, adding that “asymptomatic testing with self-tests makes little sense in my view.”

He said that people should instead rely on measures such as social distancing and wearing masks, while continuing to go for testing in the case of symptoms.

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