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Vaccines train the immune system to spot invaders and stop cells being hijacked. The coronavirus vaccines under development are all designed slightly differently, but here’s how the Oxford/AstraZeneca vaccine works

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AZD1222 uses a modified chimpanzee “adenovirus”, a harmless virus that causes the common cold in chimps, as a delivery vehicle. The precious cargo is the genetic code for the outer “spike” protein of the coronavirus: this distinctive spikiness is the “key” that the virus uses to enter cells.

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When injected, the trial volunteers’ own cells use the genetic code to begin assembling the spike protein.

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Over a period of weeks, this triggers the immune system to start pumping out antibodies, which swarm around the spike protein and latch onto it, meaning the spike cannot attach to cells to “unlock” them. T-cells, which help to clear infected cells, are also produced.

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Vaccination is a practice run. Should the spike protein ever be encountered again in a real coronavirus infection, the immune system should be able to respond immediately.