COVID: Hidden long-term harm from undetected cases | Science | In-depth reporting on science and technology | DW | 16.07.2021

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Science

COVID: Hidden long-term harm from undetected cases

A German study says 40% of people infected with COVID don't know it. And long COVID symptoms may go unidentified. DW spoke to study author, Philipp Wild.

A medic takes a swab from a man for a COVID-19 test

Got to get tested!

DW's Ben Fajzullin: As principal investigator of the Gutenberg COVID-19 study, how did you and your team determine that over 40% of Germans don't realize they have COVID-19?

Philipp Wild: We drew a population-based sample from local registry offices, and we asked study participants whether they had had an infection twice, four months apart. We asked them to provide a PCR test, which we did with them, and we measured their anti-bodies. Those anti-bodies indicate whether you have had an infection, no matter whether you are vaccinated or not.

And you've looked at over 10,000 people over a six-month period.

Yes. Among the elderly, especially, there are more unknown infections — about two-thirds (63%) of the people in our study at the ages of 75 and over. Whereas, among 25 to 30-year-olds, it is only a third [of the people we asked, who had had undetected infections]. 

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So, is it time to get self-tests out there — not only so people can see whether they are infected but also to work out whether they have had the virus in the past?

It's certainly a good idea to start testing people better, especially now when we're pulling back on preventative measures. We've got major public events again, like the recent European Football Championship, so we need early warning systems. And vaccinated people should also get tests because they can still transmit an infection to people who aren't vaccinated.

But we've also learnt that we still have to work out which antibodies we need to measure in individuals. The majority of people only show certain antibodies [Ed.: And there are different types]. So, we need to learn which anti-bodies to look for, depending on each individual, before we can start screening for long COVID.

How many of these unknown cases could be long COVID?

That's a matter of debate. We're still investigating this in our study, and it will take a little more time, because the definition of long COVID is six months after the [original] infection.

But what we do know is that among the known cases, 10% are thought to develop long COVID, and then 10% of them, which is 1% of all known cases, develop a severe form of long COVID. And what we must learn now is how many of the asymptomatic infections can get [long COVID].

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Long COVID: A new challenge for society

Could there be a concern for younger people that they could have COVID, but they may not know it, and yet their organs may be damaged but that that may only come out years later?

Indeed, that's what we fear. We're asking: Is there a molecular signature, something you can measure in the blood, that indicates that you're on the way towards developing long COVID syndrome. But that is still a matter of research. Everything we're doing there right now is still speculative. We need to gather more data to be sure about how to tackle these [cases] and how to identify them. But, yes, we fear that it is possible, even in asymptomatic infections or mild infections, that a few people are at risk of developing long COVID.               

Dr. Philipp Wild is a professor of clinical epidemiology at the University of Mainz and principal investigator for the Gutenberg COVID-19 study.

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