Can antigen tests properly detect Omicron?

Could rapid antigen tests be missing some cases of the Omicron variant of Covid-19?


A small scale study in the US, which has not been peer-reviewed, has led to widespread media coverage suggesting that antigen tests may not be as effective at picking up the Omicron variant in the initial stages of infection.

This followed an announcement by the US Food and Drug Administration in December that while antigen tests do detect Omicron, they “may have reduced sensitivity”. Add to this anecdotal reports by people who say their infection was not initially picked up with an antigen test, and another paper from South Africa found that there was more virus present in saliva than in the nose, and the result has been a lot of confusion. So what is the situation?

Can you still trust an antigen test to detect if you have Omicron?

Yes, says Prof Mary Horgan, president of the Royal College of Physicians in Ireland and the chair of the Expert Advisory Group on Rapid Testing, who adds that antigen tests are approximately 90 per cent accurate.

“Antigen tests do work with Omicron,” she says. The scientific explanation for this is that they test a protein “called the nucleocapsid. And with Omicron there have been very few mutations in that”.

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Over the past number of weeks we have “seen how many people have been testing positive with antigen tests. That supports how good they are at picking up Omicron if you have it, whether you are symptomatic or not. That’s the important message, as opposed to [suggesting] they might not be as good if you don’t do a throat swab. There have been just two papers with very small numbers and they haven’t been peer reviewed”.

What did the study that has given rise to all these questions find?

It looked at 30 people in settings including Broadway theatres and offices in New York and San Francisco where employees were being tested daily using both antigen and PCR tests. The study – involving two tests by Abbott and Quidel, which are common in the US but not widely available here – showed there was a delay of two days between people becoming infectious on PCR and the antigen test detecting it. Viral loads peaked in saliva one to two days before peaking in nasal swabs, the data showed, leading to suggestions that throat swabs might be more effective than nasal ones.

How soon might a change in the advice come?

Prof Horgan says the Expert Advisory Group on Rapid Testing will continue to track the science on this. “If there’s any change that says we need to do the throat and the nose then we need to be able to adapt and be flexible very quickly. But it’s about tracking the science and not jumping the gun based on too small studies.”

Initial data which suggests the virus replicates in the throat rather than the nose "does seem to be holding up," says Prof Luke O'Neill, immunologist at Trinity College. He suggests "a throat swab might be better than a nasal swab".

“If the science shows that that it needs to be done, then we need to change rapidly,” says Prof Horgan. The public will adapt well to any potential changes in the advice, she believes.

Further advice could be given “in the coming weeks” if needed, says Prof Horgan. “In some countries, they already recommend swabbing throat and nose when you’re doing the antigen test.”

Here, she adds, that antigen tests are “only approved currently for nasal swabs”. However, if you wish, there is no harm “in doing a throat swab and a nasal swab with the same [test kit] with the proviso that it’s only approved at the moment for a nasal swab”.

How do you swab your throat without gagging?

With difficulty is the answer. "It's more uncomfortable," says Prof O'Neill. "You've got to get to your tonsils because it looks like the virus likes living in our tonsils." The UK has recommended this method since May 2020 for many of its tests. Its advisory video (How to take a combined throat and nose swab for COVID-19) suggests you need to wipe the soft tip of the swab around the back of the throat avoiding tongue, teeth and gums. To do this, you open your mouth and stick out your tongue. Swab behind the back of the arch at the back of your mouth and both sides. Unfortunately, it adds, "this process may make you gag but it should not be painful".

You can use the one swab to do both throat and then both nostrils, says Prof Horgan. Experts advise that you should avoid swabbing your throat after eating or drinking anything acidic, which could produce a false positive. Brushing teeth or using mouthwash before a throat swab could lead to a false negative.

If the virus is shown to replicate in the throat, could saliva tests ultimately be a more effective way of picking up the virus?

“The current rapid saliva tests are PCR based, as opposed to antigen based,” says Prof Horgan. “There are signs that it’s replicating more in the upper respiratory bronchus, and that’s why it’s really important to follow the science when it comes to saliva, throat swab and nasal swab.”

If the results of further studies support it, there’s no reason, she says, “why we couldn’t do all three with one swab.” She says the Advisory Group will be following this closely and make recommendations if necessary.

How should you treat a negative antigen test if you have symptoms?

That hasn’t changed. Follow the public health advice and regard a negative antigen test with caution, especially if you are symptomatic, say both Prof Horgan and Prof O’Neill. A positive test strongly suggests you are positive. If you test negative and “think you might be positive, the best thing to do is repeat an antigen test, either with the same product or a different product,” says Prof Horgan, while continuing to isolate if you are symptomatic.

If you’re going to visit your granny this weekend or getting together with a group of friends, should you still ask everyone to do an antigen test first?

“Absolutely,” is the advice from Prof Horgan. “I’m visiting an older relative tomorrow and I will be doing my antigen test before I go and see her.” It’s even more important to do an antigen tests if a few people are getting together collectively. Do it the day that you’ll be meeting up, she says.

Is there any reason for optimism that we’ll ever actually get out of this pandemic?

Prof O’Neill, as ever, is optimistic. “The data just gets better and better. In London the most recent data showed the number of people on ventilators has not gone up at all. So that shows you the link is broken with severe disease. Given that we’ve got everybody boosted or infected, that should stop Omicron and subsequent variants as well. That’s the hope. And then the anti-virals are going to be a huge weapon. They’ll be available in February or March. That’s another major reason for optimism.”