SARS-CoV-2, the virus that causes COVID-19, took the world by storm when it suddenly surfaced in Wuhan, China, at the end of 2019. SARS-CoV-2 belongs to the family of coronaviruses, which owe their name to a crown-like protein that these viruses use to attach to specific cell receptors in order to infect them.

Obviously, SARS-CoV-2 isn’t the first coronavirus known to science. In fact, some of them are known to cause the common cold — and getting sick with the cold might offer some protection against the new coronavirus, scientists report in a new study published this week.

Alessandro Sette, a Professor at the La Jolla Institute for Immunology, has dedicated more than 35 years of his career to understanding the immune response and measuring immune activity. Sette and colleagues were initially stunned to see an immune response to SARS-CoV-2 in blood samples collected before the COVID-19 pandemic. Either the novel coronavirus landed in the San Francisco area much earlier than believed, which is almost impossible, or something different was at play.

The only viable explanation the researchers could think of was that some people must have been exposed to some older cousins of SARS-CoV-2, which primed their immune system against it, despite having never been sick with COVID-19.

There are four coronaviruses that are known to cause the common cold. These are 229E, NL63, OC43, and HKU1, which usually cause mild to moderate upper-respiratory tract illnesses.

To put their theory to the test, Sette and colleagues sampled blood from San Diego residents that were collected well before the pandemic began. After the researchers matched the viral regions, they noticed that immune cells revved up, suggesting that some people’s immune systems could respond both to the previous coronaviruses and to the novel virus.

These findings have been confirmed by previous studies performed on patients from the United States, United Kingdom, Netherlands, Germany, and Singapore. According to their results, these studies suggest that between a fifth and half of those who’ve never been exposed to SARS-CoV-2 have already had some immune response to it.

This seems to be true for other coronaviruses, such as SARS-CoV-1, which caused a small epidemic in 2003. Francois Balloux, the director of the Genetics Institute at the University College of London, highlighted a recent study published in pre-print, which found all 24 participants from Singapore infected with SARS-CoV-1 in 2003 also have immune cells against SARS-CoV-2. That’s despite more than half of them never having been exposed to the novel coronavirus.

In light of these findings, it may be possible that a proportion of the population may have pre-existing immunity to SARS-CoV-2, perhaps due to prior exposure to coronaviruses that cause the common cold.

However, it’s important to note that no study thus far has proven that exposure to previous coronaviruses offers protection against COVID-19. But that may very well be the case. It would also explain why the impact of the virus seems so unpredictable across the same demographics — some recover with only mild symptoms while others are hit hard as if by a train.

Also, some readers should bear in mind that there are only four coronaviruses responsible for the common cold — a respiratory illness that is caused by over 200 different viruses. The large number of viruses that cause the common cold is one of the reasons why there is no vaccine for it. Meanwhile, there are only a handful of strains of influenza that regularly infect humans, a manageable number that has enabled scientists to produce vaccines for the flu.

So just because you were sick with the cold, that doesn’t mean you actually picked up coronaviruses to prime the immune system against COVID-19 — if such protection genuinely exists in the first place.

Much of this confusion may be dispelled after ongoing COVID-19 vaccine trials measure the T cells and antibodies required to actually prevent illness.

The new study’s findings appeared in the journal Science.