Is the coronavirus pandemic entering a second wave?

In the United States, where new cases had levelled off at roughly 20,000 a day for a period of weeks, infections have again spiked.

The US on Friday reported one of its largest single-day increases since the start of the pandemic, with more than 40,000 new cases on the previous day, according to data from the Centres for Disease Control and Prevention.

On Thursday, the World Health Organisation’s regional director for Europe Hans Kluge said 30 countries and territories in the region had seen increases in new cumulative cases in the past two weeks as they eased social distancing measures, with 11 of those experiencing a “significant resurgence”.

But whether this means such areas are seeing a second wave remains unclear, largely due to the ambiguity of the term, experts say.

Many caution against declaring a new rise in case numbers in areas or countries where cases had appeared to decline as a “second wave”, since an uptick of cases as social distancing restrictions are relaxed did not necessarily mean the start of a new cycle – or the end of old one – especially if there was still a significant amount of transmission.

John Mathews, an honorary professor at the University of Melbourne’s School of Population and Global Health, said a second wave would typically be characterised by a dramatic decline followed by a sudden comeback in the numbers of cases.

“But no one has really defined the scale that is required to call a second wave, either in terms of the time, or space, or the scale of the [case] numbers involved.” Mathews, a former deputy medical officer to the Australian government, said “second wave” was an ambiguous term, and not one “to use loosely”.

The second wave phenomenon is most widely associated with past influenza pandemics. The 1918 flu pandemic, which infected 500 million people and killed 50 million worldwide, is infamous for its far deadlier second wave in the autumn, months after the first wave. A third wave occurred in a number of countries in 1919.

Mathews said influenza-like second waves could be driven by a change in the virus or shifts in people’s behaviour, with changes in the virus thought to play a role in the second wave in 1918. Immunity had developed among a sufficient proportion of the population which drove the flu virus to evolve to “dodge immune response” and continue to infect people, he said.

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