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There's another theory about the Covid-19 pandemic going around, and while it is pretty cool, it's not exactly the solution some are suggesting it might be.

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Once again, there's another theory about the COVID-19 pandemic going around. This time, some are claiming that even though only a very small percentage of the population has had the illness, many more are immune already because they've had colds. Some are even saying that because of this, we are approaching herd immunity, that point where enough people are immune that the virus can't really continue circulating.

And we didn't realize this earlier because this immunity is different than usual. It's conferred by something called T-cells rather than by antibodies.  But as awesome as that would be, unfortunately, it's a big misunderstanding; if you actually look at the research, it tells a very different story.  This whole idea comes from a bunch of studies published over the last few months, and they do show that many people have what immunologists call T-cell immunity.  T-cells are immune cells, and they work with cells that produce antibodies to identify and fight diseases.

There are different types of T-cells that do different things, but one of their main jobs is to recognize and destroy things that cause infections. Like when certain T-cells recognize a pathogen you've fought before, they stimulate other immune cells to make antibodies to fight off the invader. And some T-cells attack and destroy infected cells all on their own, so they can play a big role in quelling a viral infection.

That's why people have gotten pretty excited about these recent studies. See, several studies have found that lots of people have T-cells in their blood that respond to the virus that causes COVID-19, even if they've never been exposed to it.  We're talking 20 to 50 percent of unexposed people, depending on the study.

And we're not totally sure why but immunologists think it has to do with cross-reactivity. That's the idea that once T-cells have been trained to respond to one virus they can respond to similar but not identical viruses. And more than 90 percent of us have been exposed to at least one of the coronaviruses that caused the common cold. So scientists think that when T-cells trained to spot those other coronaviruses see the new one some of them go, "hey, that looks like that cold I had a while back", and then they jump into action.

It's not hard to see why people would get excited by this. On the surface it sounds like these studies are saying that up to half of the population might already be immune to COVID-19 even if they've never had it. The studies even call these reactions quote "T-cell immunity". Except, the thing is, when immunologists say that they don't necessarily mean T-cells are protecting people from a disease.

All T-cell immunity means is that T-cells are mounting some form of immune response to the pathogen, which, yes, is massively confusing, but it is a quirk of scientific lingo. The fact is while we know these T-cells are reacting to the virus, we're not sure what that means, we don't even know if it's good or bad. And if you look at the studies themselves the authors are pretty open about that.

Now researchers have suggested that T-cell reactions could help explain why COVID-19 looks so different in different people. Maybe the T-cells help people launch a quicker, more effective, defense so they don't end up as sick, and maybe that contributes to herd immunity, but we don't know if that's the case or to what extent. At the end of the day it's very hard to predict how a person's immune system actually behaves when infected with something, no matter how much information you have in advance.

For example a July study published in Nature pointed out that immunity to a closely related virus can make you less susceptible to a disease, or it can make the disease more severe.  And a study published in Science in June noted that cross-reactivity could contribute to what's known as a cytokine storm, where the immune system overreacts to an invader and causes more problems than it solves.

More recently a study published in Nature on August 26th looked at the differences between male and female COVID-19 patients. Among other things, the study found that female patients had less severe cases and mounted a stronger T-cell response, but also in male patients a stronger T-cell response correlated to more severe disease. So again are T-cells good or bad or both somehow? 

The most obvious way to test this kind of thing would be to find some people who've never been exposed to the virus and check out their T-cells. Then, if they get COVID-19, scientists can see if anything changes, or if differences in T-cells before they get sick were tied to different degrees of illness afterwards. And we might actually have an opportunity to do this kind of research.

In a comment article published in Nature Reviews Immunology last month, a pair of immunologists from California pointed out researchers could measure the T-cell responses of subjects in vaccine trials before they're given the vaccine or placebo. Among other things that would help us figure out whether a pre-existing T-cell response is tied to vaccine effectiveness or case severity. But what none of these researchers are saying is that we can or should relax social distancing or masking or any other measures in place to slow the spread of COVID-19.

Scientists and immunologists the world over are working extremely hard to figure out the answers to these questions, but we still have a lot of them about T-cells, including really basic ones. With all of the new research and information about the pandemic coming out every day, it's not surprising that misconceptions can spread, but in cases like this taking those misconceptions at face value without looking into the actual research can be dangerous.

If you have any burning questions about COVID-19, or anything else that you'd like us to answer in a future episode, feel free to leave those questions in the comments below. In the meantime, thank you for watching Scishow news, and thank you to fasthosts for sponsoring today's episode.

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