After the fear of getting sick with COVID-19, the biggest fear is getting infected again. What’s the likelihood of becoming infected within a short period of time and why?

Several cases of SARS-CoV-2 reinfection have occurred around the world during 2020. The main question researchers have been (and are still) asking: is the reinfection caused by the same strain of the new coronavirus or is it a different strain (i.e. variant)? Before answering this question, it’s important to know how variants of SARS-CoV-2 can be distinguished.

Sequencing of SARS-CoV-2

After taking a molecular swab, if the result is positive, to know for sure which variant of SARS-CoV-2 you’re dealing with, you need to perform molecular sequencing of the virus. Since sequencing is a technique with a certain cost and must be performed with special instruments that are not available in all hospitals, sequencing of the coronavirus is rarely performed, as compared to the performance of molecular swabs. However, there are specialized labs around the world that perform hundreds of sequences every day, hunting for variants. One of the best laboratories is the Sanger Institute of Cambridge (UK) that has sequenced almost 300,000 genomes of SARS-CoV-2. In Italy, unfortunately, there is no center so advanced, and with such a concentration of DNA sequencers, but there are small or medium-sized laboratories that provide a sequencing service.

Sequencing the virus, however, is only the first part of the work; it’s like publishing a book, and then you have to read it, and it’s written in a very special language that only specialized researchers can understand.

A recent article published in an important international scientific journal by a group of Italian researchers from the University of Verona, has demonstrated the importance of how virus sequencing is performed, along with its reading and interpretation. In fact, in this work, the researchers clearly state that nasopharyngeal samples (i.e. those used for molecular swabs) from patients with a low viral load, to which mild symptoms are usually associated, produce sequences that are not very reliable and difficult to interpret, especially in cases of reinfection.

When is a case of COVID-19 reinfection attributed?

COVID-19 reinfection occurs when a person is diagnosed with SARS-CoV-2 infection twice. The diagnosis must be made by molecular swab, but the real question is whether the reinfection is caused by the same viral strain or “just” a variant of it.

The answer can be provided only if, we sequence both a nasopharyngeal sample of the person, taken during the first infection, and during the second infection.

Researchers at the University of Verona had such samples, and saw that in some cases, it was the same strain of SARS-CoV-2 and in others, different strains.

Moreover, a Danish study, performed with over 11,000 Danes infected during the first wave (from March to May 2020) demonstrated that only 0.65% had been reinfected in 2020. Thus, giving a clear indication that re-infection with SARS-CoV-2 (at least within 6-to-8 months) is quite unlikely. However, focusing the analysis within a specific sub-group of the Danish population, the over-65s, the protection induced by the infection appeared far weaker. That is, only about 50% of the elderly group was found to be protected from the risk of re-infection during the second wave.

This can be explained by so-called immunosenescence, the process by which the immune system gradually loses its effectiveness over time.

This Danish study doesn’t clarify whether the reinfection is borne by the new variants, which is important information in understanding whether the currently available vaccines are also effective against the new forms of coronavirus or if it’s necessary to develop new vaccines and then have to receive periodic booster shots.

So, the question of whether we can reinfect ourselves after becoming sick with COVID-19 or having been vaccinated, now has some answers, but still many studies need to be performed to actually confirm what is now known.

This post is also available in: Italiano


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