Day nine: Mrs Keulemans (who has corona) is already feeling a lot better. Tomorrow she can according to the GGD game rules out of isolation. But if you look closely at the window of the self-test she just took: yet another faint second line. How should that be?
Jur Peppels, an editor at NPO Radio 1 with whom I sometimes have contact, decided to take a chance. ‘I called the 0800 number of the GGD to find out’, he says. ‘But my positive test and the fact that I still had a rotten voice were, according to the boy on the phone, no reason to stay inside after ten days.’ In the end, Peppels would continue to see two stripes for a fortnight – by then he had been back to work for a long time.
Don’t you infect others? ‘You never know for sure. But I wouldn’t expect it’, says virologist Richard Molenkamp (Erasmus MC), when I call him. Already after a few days the body begins to make antibodies against the coronavirus. ‘After a week you expect those virus particles to be coated with antibodies, as it were. They can no longer enter cells and are therefore not contagious.’
But the self-test still sees the virus, because the virus first falls apart in that bottle with liquid that comes with the test. The test thus still detects the so-called nucleocapsid protein of the virus, a characteristic protein in which the virus packages its genetic material and on which the test is successful. Not contagious, but a line.
Or well, that’s how it should be. In practice, some people still have some contagious virus in their throat after two weeks, Molenkamp notes. Take a study that happens to be just appeared last week† Doctors from Boston are investigating how long they could harvest a culturable virus from the throats of 66 test subjects.
After five days, more than half of them failed to do so. But after ten days there were still eight people – about 10 percent – with leftovers of alive, infectious virus in their throat. It wasn’t much of a virus, but still: in theory such a person could infect others.
Staring at the faint line on the self-test, I think of something else. Are these self-tests still to be trusted? After all, the virus has mutated. Can rapid tests still properly recognize virus particles, wonders Maria Hoet, a reader who has been faithfully sharing her corona questions and concerns with me for over a year?
The simple answer is: yes, it works reasonably well. Fortunately, the nucleocapsid protein that self-tests hit has not changed much over time. But it has not stayed the same: being in the nucleocapsid of the omikron variant three building blocks changed and is missing a bulge that was there before.
Abbott’s Panbio rapid test in particular seems to be affected by this. His sensitivity to corona has about halved, I read in an overview study† The Flowflex test my partner was using barely performs worse. Apparently it ‘looks’ at a piece of nucleocapsid that has remained unchanged. The sensitivity of the MPBio test went from 77 to 70 percent, that of the Clini test from 83 to 70 percent, Utrecht researchers have meanwhile shown†
So it differs per brand. But there’s something else: the location of the virus itself. ‘We think it makes sense to also test in the throat’, says medical microbiologist Jan Kluytmans (UMC Utrecht). Kluytmans put it to the test when he recently had corona himself. To his surprise, he noticed that the infection seemed to be moving. Initially, the self-test was ‘pop positive’ in his throat and negative in the nose, later it was mainly his nose that caught the test. ‘This shows the dynamics of the virus in the body’, he suspects.
The studies support that view. In the delta variant, the positive rapid tests mainly came from the nasal swabs; with omikron, the throat test is more likely to be positive, according to a small study that just appeared† So do both just to be sure. Sticking the cotton swab in the back of the throat first and then in the nose – not very fresh, but still a lot better than the other way around – increases the chance of hitting from about 70 to more than 80 percent, according to another analysis†
Until a new variant comes along, the virus rearranges its genetic structure and shakes up its nucleocapsid a bit again. Then the thunder starts all over again and science can start again start all over againfinding out how well their tests still work.