Why taking an antigen test to party does not prevent the risk of Covid-19

Updated:02/17/2021 17: 55h


The pandemic of Covid-19 it is producing important restrictions in our life. For almost a year we have lived surrounded by precautions and control measures that cause us “pandemic fatigue”, and we all would like to have solutions and shortcuts so that our life resembles that of the old normality.

There is little doubt that the main solution is the vaccine, and in any case, continue to comply with protection measures. But the option of take a previous test that, if negative, could allow us to party, go to concerts, “enjoy life”, or meet someone on Tinder with “safety”. In this way, the test would be a safe conduct, a protective shield that would make it possible to compensate for relaxation in other measures: going to poorly ventilated places, approaching other people or taking off the mask – perhaps as a prior step to removing other things.

The problem is that that protective shield has a few “holes” that can pass the virus. Fundamentally, the type of test, the time it is carried out and the intention with which it is carried out. They should be taken into account to avoid false expectations that lead us to trust ourselves and contract the disease, or to transmit it to other vulnerable people in our environment.

Different tests, different diagnostic precision

Depending on the type of test used, the diagnostic accuracy is different.

A rapid antibody test, of those that are made with a small blood sample, you can tell us if our body has generated that type of immune response against the virus, but does not let us know if we have an active infection, and also its sensitivity can be quite low, generating many false negatives. Is the least recommended type of test, its use should be limited primarily to epidemiological studies and we will not mention it further in this text.

Unlike antibodies, a rapid antigen test, whose sample is usually obtained by inserting a swab through the nose, can identify an active infection by finding viral proteins, and it also does it in a few minutes. But in this case a negative result also does not rule out infection, especially in asymptomatic people.

And by the way, here there are also “brands”, as in cars, and there are manufacturers that are much more reliable than others. In Spain, for screening, it is established that, among the different rapid antigen detection tests on the market, only those that meet the WHO criteria for sensitivity ≥80% and specificity ≥97%. They must also have followed independent validation studies by clinical or reference laboratories at the national or international level. These characteristics of the test should be well verified, especially in asymptomatic patients.

Finally, the PCR, which is also done with a nasopharyngeal sample, is the “gold standard”, the best test –And also the most expensive and slowest– for the diagnostic. But in certain cases, depending for example on the skill in the way of obtaining the sample, it could also give a false negative result.

A snapshot with an expiration date

At the beginning of an infection, when the viral load is still relatively low, a test is more likely to be negative. Therefore, a person already infected but without symptoms, whether it is asymptomatic or presymptomatic, you can get a negative result. This will occur more frequently in a rapid antigen test in which viral proteins are searched without “amplifying” the sample, unlike PCR, which increases its ability to detect genetic material in successive cycles.

In any case, the result of the antigen test or PCR is still a photograph of the situation at the time it is taken. Even if the negative result is true, we could become infected just a few minutes later. And of course, if on Saturday we receive a negative PCR result done on Friday, we could already be infected even if the test would have been correct.

So, although it is not an exact simile, relying on the result of a few days before to lift the precautions makes the same sense as driving a vehicle on the road – even with less caution than usual – based on a negative breathalyzer test from the previous day .

What is the use of participating in population screening

The population screenings carried out in some territories are aimed at detecting asymptomatic infected people to interrupt chains of transmission. Therefore, the main objective of these tests – which are usually performed with rapid antigens to have an almost immediate result – is not so much individual diagnosis, but rather a population-based strategy to reduce risks in high-incidence areas.

The problem arises when the people who participate in a population screening try to take advantage of it to, in the event of a negative result, relax the protection measures. In fact, this is one of the main drawbacks of these initiatives, since all the benefit achieved with the detection of positive cases can be reversed with the change in attitude of those who obtain a negative result.

From that point of view, it is very important to make the population aware that the negative result –Which is probably the one that will probably get the vast majority, about 99% of the participants– it is not a pass to relax measurements.

No excuses to relax measures

In addition to the main ideas we’ve already discussed, there are other factors to consider. The incentive to do without a mask, to reduce interpersonal distance or to stay in places without adequate ventilation, or all of them at the same time, because of having obtained a negative result in a test, could already be a reason for not having it .

Especially if, despite our possible initial awareness on this matter, once we are in a festive atmosphere, with the consumption of alcohol or the course of the night – we leave the curfew for another occasion – let’s forgetting the precautions, protected by our confidence in the test performed and trusting that it is so also in others.

Of course, it can always be argued that the test is done for the opposite, with the aim of looking for a positive result and avoiding going to any social gathering or risky activity. It would be a motive similar to that of screening people who work in vulnerable environments, such as residences for the elderly, among others.

But it should be noted that the effectiveness of screening is based on its periodicity and repetition, not on its punctual and opportunistic performance. And of course, the negative result cannot be used as an excuse to lift protective measures.

Finally, it must be remembered that people of young and middle ages are the groups with the highest prevalence of infection and, therefore, in which it is more likely to contact an infected person. Furthermore, some of the new, more contagious forms of SARS-Cov-2, particularly the British variant, seem to have a special predilection for these ages.

And here we must make a call to responsibility: without a doubt, the risk of serious disease in these groups is lower – although not non-existent. But the infection can be transferred to other more vulnerable members family or close environment. A negative test is no guarantee of being safe. And no one is safe if we are not all safe.

Ignacio Rosell Aguilar. Associate Professor of Preventive Medicine and Public Health, University of Valladolid.

This article was originally published on The Conversation.

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