There is also a possibility that different immune systems respond differently to the virus. For Sars-CoV-2 to infect, the spike protein on the surface of the virus needs to stick to specific proteins on the target cells, like the ACE2 protein. Is it possible that those resistant to infection have different levels of ACE2 than others? Age-related ACE2 expression in the lungs of children compared with adults may partly explain why children often show milder infection.

It is also possible that some of us may have rare types of ACE2 that the coronavirus spike cannot stick to. Differences in protein expression between people are known as polymorphisms, and they are valuable to discover. People that have a rare genetic polymorphism for CCR5 protein have been immune to HIV infection. To support this theory, recent genetic analyses have revealed that rare types of ACE2 may influence susceptibility to Covid.

Additionally, studies in healthcare workers who consistently remained negative for Covid showed the presence of pre-existing T-cells that recognise peptides – the chain of molecules that make up a protein – from less variable parts of the virus than the spike protein (which, under pressure from our immune response, mutates frequently to evade our antibodies). This work suggests that it would be wise to not rely on spike-targeting vaccines if we want to induce immunity to new variants, and we should think about incorporating more parts of the virus that don’t change over time (“evolutionarily conserved proteins”) into our vaccine design.