Despite being in a high-risk environment, not everyone in the study came down with Covid. The results, published in the journal Nature, showed some people just managed to avoid the virus.
However, around one-in-10 had signs of being exposed, but never had symptoms, never tested positive and never developed Covid-fighting antibodies in their blood.
Part of their immune system was able to get on top of the virus before it managed to take hold - what's known as an "abortive infection".
Learning how to fight one virus may leave protection against another
Blood samples showed these people already had (as in before the pandemic) protective T-cells, which recognise and kill cells infected with Covid.
Dr Leo Swadling, one of the researchers, said their immune systems were already "poised" to fight the new disease.
These T-cells were able to spot a different part of the virus than the bit most of the current vaccines train the immune system to find.
Vaccines are largely aimed at the spike protein, which covers the outer surface of the Covid virus. However, these rare T-cells were able to look inside the virus and find the proteins that are necessary for it to replicate.
"The healthcare workers that were able to control the virus before it was detectable were more likely to have these T-cells that recognise the internal machinery before the start of the pandemic," Dr Swadling added.
These internal proteins are very similar in all related species of coronavirus, including the ones that are widespread and cause common cold symptoms.
It means targeting these proteins with a vaccine could give some protection against all coronaviruses and new Covid variants.
The team said the current vaccines were doing an excellent job of preventing people from becoming severely ill, but were not as good at stopping them catching Covid.
Prof Mala Maini told me: "I think we could all see that they could do better.
"What we're hoping, by including these T-cells, is that they might be able to protect against infection as well as disease, and we hope they would be better at recognising new variants that arise."
While nearly everyone will have caught these common cold coronaviruses, not everyone will have developed the right kind of protective T-cells. It may be that healthcare workers are more regularly exposed to the viruses through their work and that is why some of them had protection.
Dr Alexander Edwards, from the University of Reading, said: "Insights from this study could be critical in the design of a different type of vaccine.
"Hopefully this study will lead to further advances in vaccine development, as we need all the types of vaccine we can get."
Source: www.bbc.com/news/health
