ANYONE who has ever seen a graph of the Spanish flu pandemic knows it came in three waves – and the second was the deadliest.

The virus which first emerged around March 1918 seemed much like a seasonal flu, albeit a highly contagious and virulent strain.

But when the second wave hit in autumn of that year, it was capable of killing previously health young men and women within 24 hours of the first signs of infection: the virus had mutated.

That goes a long way to explaining why the second wave was worse than the first, but a failure to impose civilian lockdowns in Britain amid a war effort that encouraged citizens to “carry on” and prioritised keeping workers in weapons factories helped to accelerate its spread.

The third wave in 1919 had the same high mortality rate and claimed millions of additional lives, but the end of the war and a degree of herd immunity meant it paled in comparison to the second wave.

Putting aside the possibility of a deadlier, mutant strain of Covid-19 (its mutation rate is about a third of influenza and according to scientists it does not appear to be evolving rapidly), the prospect of a second wave in a global population with limited herd immunity is one that alarms the World Health Organisation as an increasing number of countries move to ease their lockdowns.

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It warned on Monday that a “slow, steady, lifting” of restrictions was key, and stressed that governments must first be confident of three things: that the epidemic is under control; that its healthcare system could cope with a resurgence; and that it has effective surveillance in place to detect new cases and trace their contacts.

Even when the answer to all these is “yes”, the WHO stressed that the situation could still escalate quickly.

It pointed to South Korea, which is scrambling to contain a new outbreak linked to Seoul’s nightlife. Nearly 70 new cases were identified over the weekend – the highest number in a month – and authorities had embarked on a mission to trace and test some 7000 revellers potentially exposed to the virus.

All residents in Wuhan in China – a city of 11 million people where the new coronavirus strain originated in 2019 – are now being tested for Covid after it also reported six new cases for the first time since lockdown was eased on April 8.

This is important because, as the saying almost goes, what happens in Wuhan will not necessarily stay in Wuhan. A Lancet study previously predicted that if social distancing was gradually relaxed in the Chinese metropolis in March, a second wave of Covid cases could occur across the northern hemisphere by mid-summer.

Meanwhile, Germany – widely praised for its handling of the first wave – saw its ‘R’ number surge from 0.65 last Wednesday to 1.1 by Saturday. Anything above one indicates that the reproductive rate of the virus is on an upward curve.

Germany had begun reopening some small shops and non-essential businesses on April 20, and on May 4 schools and hairdressers also began opening their doors.

By Tuesday of this week, Germany’s public health experts said ‘R’ appeared to have dipped back to 0.94.

In countries such as China, South Korea and Germany, which are primed to respond with some of the world’s best test, trace and isolate regimes, the prospects of containment are good.

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But the WHO is rightly concerned that less prepared nations with their economies chafing against lockdown may move prematurely.

It warned that serious thought must be given in particular to schools, namely their geographical location and “the capacity of the schools to maintain infection, prevention and control measures”.

Children and teenagers might be at virtually no risk themselves, but the fact that the WHO is citing education premises as a potential incubator for a second wave is something we should take very seriously indeed as talk of resuming classes before summer is stepped up.

We know a robust system of testing, tracing and isolating (TTI) will be vital, but our capacity lags far behind the likes of South Korea and Germany and only a fraction of the capacity we currently have is actually being used even though we know the demand is high. Whatever blockages and bottlenecks are preventing people who want to be tested from being tested must be urgently cleared.

Another major obstacle is the slow turnaround time for test results: other countries are managing it in four hours while the UK averages 30. That could spell catastrophic delays for the tracing element of TTI that relies on rapidly tracking down and quarantining the potentially infected.

As we watch Italy, Spain and France easing up on lockdown we must remember that they were ahead of us on the infection curve, and their tentative exits are not a signal of our own readiness to leave. Our R number remains barely below one.

If unconvinced, take this sobering statistic from public health professor KK Cheng in this week’s British Medical Journal: the R number in Wuhan seven weeks into their lockdown was below 0.2.