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Rapid tests less sensitive to Omicron variant, but Ontario science table says new swab method can help

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Ontario’s COVID-19 Science Advisory Table is urging people to change the way they use rapid antigen tests after determining the regular nasal swabs are less sensitive to the Omicron variant.

In a new brief released Thursday, the science table warned that a single negative rapid test “cannot reliably rule out infection” and shouldn’t be used as a “green light for abandoning or reducing precautions.”

A regular nasal sample, especially those taken in the first one or two days after infection, are less sensitive to the Omicron variant compared to the Delta strain, officials said.

“What we see based on early results, but very consistently, is that if you just use a swab of your nose, that especially early on during the first perhaps 48 to 72 hours after infection, the sensitivity of the test is relatively low. You won't detect Omicron infection that easily,” Dr. Peter Juni, scientific director of the advisory table, told CTV News Toronto in an interview.

“One of the reasons might be that the distribution of the virus is different than what we saw for previous variants.”

However, the science table says the tests can be more accurate if both cheeks are swabbed, in addition to the back of the tongue or throat, prior to going up the nose.

According to the brief, a nasal sample alone was about 68 per cent effective in detecting Omicron while a combined nasal and throat sample was about 82 per cent.

An instructional video released by the science table on Feb. 7 says that rapid test users should swab the inside of their cheeks, between the cheek and gums while rotating the swab for five seconds.

The video then instructs the user to swab the arch at the back of the mouth for another five seconds “in a circular fashion.”

A gag reflex is normal, officials said, but the process should not be painful.

Finally, users should insert the swab about two centimetres into the nose and gently wipe around the inside of the nostril about three to four times. Then, do the same in the other nostril.

“Stop advancing the swab when slight resistance is felt, even if it is less than two centimetres. No force is needed,” the video says.

The same swab should be used for the cheek, throat and nose.

Juni added that rapid test users should remember to start with the mouth, “otherwise it’s just a bit disgusting.”

If someone does the swab properly and still gets a negative test, the science table warns that it doesn’t necessarily mean the person is clear of infection.

“You still should not rely on a single negative test,” Juni said. “You should repeat tests and have two tests, perhaps 48 hours or more apart.”

A positive result using a rapid test can be considered accurate.

The province is providing free rapid antigen tests to Ontario residents at select grocery stores and pharmacies starting this week.

About 5.5 million tests will be distributed to just over 2,400 sites each week.

Witih files from CTV News Toronto's Colin D'Mello

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