It would be hard to overstate the importance of developing a vaccine to Sars-CoV-2 – it’s seen as the fast track to a return to normal life. That’s why the health secretary, Matt Hancock, said the UK was “throwing everything at it”.

But while trials have been launched and manufacturing deals already signed – Oxford University is now recruiting 10,000 volunteers for the next phase of its research – ministers and their advisers have become noticeably more cautious in recent days.

This is why.

Why might a vaccine fail?

Earlier this week, England’s deputy chief medical officer Jonathan Van-Tam said the…