Paris: Like a giant tidal wave, the COVID-19 pandemic is battering the health systems of several European countries, leaving experts scrambling to know when it will reach its peak.

What will the aftermath of this "tsunami", as Italian health workers have called it, be like? A general receding and a return to normality, or regular recurrences that will overwhelm hospitals?

Calm before the storm?

The wave seems to have already subsided in China, where the novel coronavirus first broke out late last year: over recent days no new local cases have been recorded. But French public health specialist and epidemiologist Antoine Flahault in the Lancet medical journal wonders if the worse is not yet to come.

China could so far have "experienced a herald wave, to use terminology borrowed from those who study tsunamis, and is the big wave still to come?" he wrote. To understand the complexity of how epidemics evolve it is necessary to go back to the post World War I period, when in three waves the Spanish Flu killed nearly 50 million people -- more than the Great War itself. Then it disappeared.

The question of why has tormented mathematicians. In the late 1920's Scotland's William Ogilvy Kermack and Anderson Gray McKendrick developed models in a bid to understand the dynamics of epidemics.

Immunity Threshold

Kermack and McKendrick discovered that an epidemic does not end because it runs out of vulnerable people, but because as the number of infections increases a so-called "herd immunity" threshold is reached.

"Herd immunity is the proportion of people immunised against the virus (either through infection or vaccination when it exists) that needs to be achieved to stop any risk of resurgence", said Flahault, head of Geneva University's Institute of Global Health.

That proportion depends on the ease with which the virus is transmitted from an infected to a healthy person. The more contagious the disease, the higher the number of immunised people has to be to stop it in its tracks.

For COVID-19 "there would need to be between 50 and 66 percent of people infected and then rendered immune to wipe out the pandemic", he said. The level of contagion itself is open to variation, according to the kind of preventative measures taken, such as quarantine, confinement, and also potentially weather conditions.

If an infected person infects on average less than one other person "then the epidemic comes to an end", he said.

'Resurgences'

But that will not necessarily be an end of the epidemic, which might just be taking a break, as he argues it is "currently doing in China and South Korea".

Because health measures during an epidemic are only temporary "and when you relax them the epidemic starts again until it reaches an ad-hoc herd immunity, sometimes over several months or years," he said.

The head of the infectious diseases service at the Pitie Salpetriere hospital in Paris, Professor Francois Bricaire, also warned of possible "resurgences". "The reappearance of COVID-19 is a possibility, with eventually a seasonal resurgence" he told AFP.

Sharon Lewin, an Australian infectious diseases expert, also wonders about the possibility of a return: "Will it come back? We don’t know." However, she said SARS (Severe Acute Respiratory Syndrome), also a coronavirus, disappeared completely following strict social distancing measures, after killing 774 people in 2002 and 2003.

The development of a vaccine and its global distribution, which the pharmaceutical industry has promised to deliver within 12-18 months, would radically change the outlook.