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After months of good news, including falling Covid cases and a hugely successful vaccination campaign, the tone has shifted.

England's full relaxation of rules in June is in jeopardy and there is the looming spectre of greater pressure on the NHS.

The thorn in the plans is the variant of coronavirus that emerged in India - B.1.617.2 - and has started spreading around the world.

Concerns have been mounting over the past week and, for the first time, the scientists advising the UK government are now confident it does spread more easily.

The expectation is the variant will edge out the one that emerged in Kent and eventually "dominate" cases in the UK.

We are in a race between the virus and the vaccine. Easing restrictions on Monday and a more transmissible variant means the virus will move even faster.
 
But there is still huge uncertainty about exactly how much better the B.1.617.2 variant is at spreading.

"That's a really critical question to which we do not yet have the answer," said Prof Chris Whitty.

If it is just a smidge faster than the Kent variant, which still makes up the overwhelming majority of cases in the UK, then there's not much to worry about.

However, the government's scientific advisory group for emergencies (Sage) says there is a "realistic possibility" it could spread 50% faster, which is a lot.

There are some higher estimates of 60% coming out of India, but these are based on published genetic sequences from viral samples, and it's not clear how representative of the overall picture they are.

Sage estimates the problems would really start with a variant that was 40% more transmissible as it "would lead to a substantial resurgence of hospitalizations" and put pressure on the NHS.
 
The uncertainty is reminiscent of last year when the new variant in Kent was first detected and raised questions about the approach to controlling the virus.

But it is important to remember we are not in the same situation as last year.