CMAAO Coronavirus Facts and Myth Buster - Antigen Testing |
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CMAAO Coronavirus Facts and Myth Buster - Antigen Testing

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With input from Dr Monica Vasudev

1168:   In Thailand, among the 10% of individuals who did not wear a mask or wear one correctly, nearly three-fifths did not wear the masks correctly, such as under their chin, while two-fifths did not wear a mask at all, reported Richard Maude of Mahidol-Oxford Tropical Medicine Research Unit in Bangkok.

Not wearing masks correctly was found to be more frequent among adults 19-60 years of age, as compared to young children or older adults. Additionally, men were more likely to wear masks incorrectly compared to women, stated the researchers in a late-breaking presentation at the virtual annual meeting of the American Society of Tropical Medicine & Hygiene. [Medpage Today]

1169: Children in Hamburg, Germany, were found to be four times more likely to contract coronavirus during private gatherings compared to when at school, suggested an analysis of infections between August and October. About 78% of the 372 children infected with the virus between the summer and autumn holidays contracted the infection outside school, with children below 12 only half as likely to become infected as older ones. [Reuters]

1170:  Rapid antigen testing is a mess: Unlike lab-based, molecular polymerase chain reaction tests, which detect particles of the virus’s genetic material, antigen tests are less sensitive as they only detect samples with a higher viral load. The tests were likely to give more false negatives and false positives.

The antigen tests should have been released primarily to communities with outbreaks instead of expecting them to work just as well in large groups of asymptomatic people.

Healthcare workers in Nevada and Vermont reported false positives. It took another few weeks for the US Food and Drug Administration to issue an alert on November 3 confirming what was experienced in Nevada: Antigen tests were prone to false positives, warned the FDA.

Some studies suggest that antigen tests reliably detect high viral loads. People are most infectious when they have high viral loads, therefore, these tests will flag those most likely to infect others.

About 40% of infections are spread by asymptomatic people with high viral loads. Therefore antigen tests, despite their imperfection, should not be dismissed.

The tests are authorized for the most straightforward cases: people with COVID-19 symptoms in the first week of symptoms. A recent study, not yet peer-reviewed, noted that the Quidel test detected more than 80% of cases when used on symptomatic individuals and those with known exposure to the virus, but only 32% among people without symptoms, reported The New York Times.

As antigen tests started giving false-positive results in nursing homes, state public health officials in Vermont and Nevada held off. Health and Human Services officials; however, dismissed their concerns and insisted them to keep using the tests.

In July, an urgent care clinic in Manchester, Vermont, noted that among 64 patients (mostly asymptomatic) who were positive as per the Quidel test, only four, all symptomatic, got a positive PCR result.

A paper in August noted that if a quarter of American school kids were tested three times a week with an antigen test that is 98% specific, it would yield 800,000 false positives a week that need to be confirmed by PCR tests. (The US is processing an average of 1.4 million tests a day, nearly all of them PCR). []

Dr KK Aggarwal

President CMAAO, HCFI and Past National President IMA

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