[kazinator]

They wanted "red alert".

https://en.wikipedia.org/wiki/Red_Alert

This is just an idiom for denoting a high alert state.

[RupertSalt]

No, "Red Alert" is called when there is an attack imminent, incoming weapons detected, enemies sighted.

So a macrophage on "red alert" would be reacting to an active infection or disease.

[kazinator]

Sure, and this vaccine goads the macrophage into that state, putting it on "red alert", without there actually being an infection.

[RupertSalt]

I suppose it is not worth debating a metaphor written by BBC reporters for the general public, without even reading the research.

But I was edified to learn that Dr Edward Jenner was a vaccine pioneer even without the honors of knighthood.