Now more than ever, antigen tests work

With the contagions of COVID-19 already fired at the gates of Christmas, the antigen self test are now more useful than ever, and well used constitute, due to their speed and ease of use, a very practical alternative to PCR. However, many people believe that they are not reliable, and those who want to use them no longer find them in the pharmacy. This report explains how to use these tests well, which increase their effectiveness with repetition, and include nuances related to vaccination and omicron variant.

How do they work?

In 15 minutes, and after introducing a 2.5-centimeter swab into the nose (it is advisable to rub the walls of both nostrils several times), detect people who are susceptible to contagion, for which it is necessary to have a sufficient viral load. When we get infected, the virus begins to multiply in our body. That viral load needs to rise to a certain level for the antigen test to be positive. The PCR they are more sensitive, detect very low viral loads, but a positive PCR is not always contagious; in addition, they require more time and their cost is about 25 times higher.

When to do them?

If we are going to meet with non-partners, just before the meeting. If we believe that we have been infected, we must wait at least 2 or 3 days after contact for the viral load to rise to a detectable level. Also, of course, if we have symptoms, although this does not always mean having a detectable viral load. If we test negative with symptoms, the test must be repeated.

Is it necessary to confirm with PCR?

Medical authorities assume that a positive antigen test must be confirmed by PCR, which is causing enormous strain on the healthcare system and delays in medical care. However, as the molecular biologist points out Maria I. Tapia, one of the greatest experts on these tests in Spain, the positive value of antigen tests on the market, also those we do at home, has the same reliability as the positive value of a PCR. For this reason, he argues that "it would be much more efficient, faster and cheaper to confirm or discard the positive with another rapid test." Unfortunately, in Spain, Without official confirmation by PCR, no sick leave is given and the positive does not count in the statistics.

Do you detect the omicron variant?

Yes. The fact that the spike protein (S) has mutated has no influence, since the tests recognize the protein (N) of the virus nucleocapsid as antigen.

Does the vaccine influence?

That some people test negative for symptoms reflects that vaccines work, as the American epidemiologist has explained Michael Mina, expert in antigen testing. Thanks to the immunity acquired by vaccines, we can have symptoms before being contagious, as occurs with colds: Being vaccinated, our system already recognizes this coronavirus and therefore develops symptoms, and if the virus overcomes this first barrier, it replicates faster later.. Therefore, "if you are symptomatic and negative, although it means that you are probably not contagious at the time, you must be very cautious," warns Mina. Quarantine yourself if possible and get tested again the next morning or that night. "

Can we trust ourselves if it tests negative?

Let's imagine that we get negative just before a weekend with non-partners in a rural house. Rapid tests "catch" most infectious people at any given time. Therefore, perhaps we have done the test when the virus has not yet reached a "contagious" viral level in our body and we can reach it on that weekend or catch it later. The test should be repeated.

Why does it matter that they are cheap?

In Spain - only in pharmacies - they cost between 5 and 8 euros, which makes it difficult for many people to test themselves several times. In the UK they are free and in Portugal they cost about 2 euros in supermarkets. A study in the "Journal of the American Medical Association" demonstrated a very high sensitivity of antigen tests when performed repeatedly: doing them on an isolated day is not as effective, since the result can change in a matter of hours or the next day.

In short, should we use them?

Yes, if we use them well and being aware that they are not perfect. Nothing is. They are one more layer of protection, along with vaccines, masks, ventilation, distance ... Like Swiss cheeses, all these layers have “holes”. As María I. Tapia says, “with rapid tests, the same thing that happened with masks is happening: first they say they are not worth it. When they realize that they are, and when we need them most, there is not for everyone. Bad foresight, bad management ”.


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