Coronavirus is far from over. Some countries are still dealing with large epidemics, but even those currently controlling the virus fear “the second wave”.

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The second phase of Spanish flu a century ago was deadlier than the first.

So, is a second wave inevitable? And how bad could it be?

Firstly, what is a second wave ?

You can think of it like waves on the sea. The number of infections goes up and then comes back down again – each cycle is one “wave” of coronavirus.

Yet there is no formal definition.

“It’s not particularly scientific: how you define a wave is arbitrary,” Dr Mike Tildesley, from the University of Warwick.

Some describe any rise as a second wave, but it is often a bumpy first wave. This is happening in some US states.

In order to say one wave has ended, the virus would have been brought under control and cases fallen substantially.

For a second wave to start you would need a sustained rise in infections. New Zealand, which has its first cases after 24 days without coronavirus , and Beijing, which is facing an outbreak after 50 virus-free days , are not in this position.

But some scientists argue Iran may be starting to meet the criteria for a second wave.

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Will a second wave come to the UK?

The answer lies almost entirely with the decisions we make, so it could go either way.

“I really think at the moment there’s huge uncertainty… but to be honest it’s something I’m very concerned about,” says Dr Tildesley.

The potential is clearly there – the virus is still around and it is no less deadly or infectious than at the start of 2020.

Only around 5% of people in the UK are thought to have been infected and there is no guarantee they are all immune.

“The evidence is the vast majority of people are still susceptible. In essence, if we lift all measures, we’re back to where we were in February,” says Dr Adam Kucharski from the London School of Hygiene and Tropical Medicine.

“It’s almost like starting from scratch again.”

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