One of the biggest mysteries of Covid-19 has been why some people become very ill, while a large number of people who are infected show no symptoms at all. It turns out that this protection may be linked to the most mundane of illnesses…the common cold. A thread.

That’s right – the week you spent cursing your sore throat and runny nose because you caught a cold may end up helping your body fight off a Covid infection. It all has to do with the shape and structure of viruses.

As we dive into the science, we have to start with a quick primer on the vocabulary of viruses. While the virus responsible for this current pandemic is commonly called “the coronavirus” it is not the first or only coronavirus.

Coronavirus just refers to a virus that has protein spikes on its surface, as this makes it look a bit like a crown when viewed under a microscope. The viruses that caused the SARS and MERS outbreaks were also coronaviruses. And, so are the viruses that cause the common cold.

The outside surface of a coronavirus is made up of several different protein molecules, which together form a sphere to encapsulate the RNA molecule that holds all of the instructions for making more virus. There are S, M, and E proteins.

Each coronavirus has a slightly different set of instructions written in its RNA, and thus makes slightly different S, M, and E proteins to be displayed on the surface. But, while each combination is unique, some viruses are related and have similar versions of specific proteins.

As a child of the 1980s, this mix-and-match strategy of viruses reminds me of a Mr. Potato Head toy – using a small number of pieces, you can build a seemingly infinite number of variations. And, which each is unique, many are related because they share common pieces.

So, what does this all have to do with asymptomatic Covid-19 infections? That comes down to how your body recognizes and deals with viruses. When you’re infected, your immune system makes antibodies to the proteins it sees on the virus.

And, your immune system doesn’t always stop with just one virus protein. You can make antibodies for different proteins, or even different parts of a protein (these are called “epitopes”).

Your body uses these antibodies to fight the infection and then remembers how to make them in case you see that virus again in the future. In fact, that’s how vaccines work – they teach your body to make antibodies, but without you having to be infected.

The interesting discovery that scientists have made is that human blood samples from before Covid-19 ever showed up actually contain antibodies that are able to bind to the SARS-CoV-2 virus that causes Covid-19. https://www.nature.com/articles/s41586-020-2550-z?flip=true

This is likely because these individuals had been infected with a different virus that had similar proteins – either SARS, MERS, or just the common cold – and they generated antibodies that are also able to bind to the proteins on the Covid-19 virus.

The hypothesis is that when these individuals encounter Covid-19, they are able to produce these antibodies and fight off the infection without showing symptoms. In fact, the high exposure of children to cold viruses may explain their high rate of asymptomatic Covid-19 infection.

Scientists are just starting to test this hypothesis, but if it does turn out to be correct, that would mean that nature has already given some of us a Covid-19 vaccine…in the form of that seemingly pesky cold that we had last winter.

Editing to add a correction: the researchers did not find antibodies, but rather the ability to activate T-cells, which is indicative of immune response, and can be linked to antibody production. Either way, this remains a hypothesis, but one that could explain immunity to COVID.

And, in response to some questions:
-No, this does not mean flu shot offers protection. Flu is a different type of virus
-Not all cold viruses are coronaviruses, and even if this hypothesis is correct, not all will cross react with SARS-CoV-2
-Yes, you should still wear a mask!