The Covid-19 pandemic is currently unfolding in “one big wave” with no evidence that it follows seasonal variations common to influenza and other coronaviruses, such as the common cold, the World Health Organization has warned. Amid continued debates over what constitutes a second wave, a resurgence or seasonal return of the disease, Margaret Harris, a WHO spokesperson, insisted that these discussions are not a helpful way to understand the spread of the disease. “People are still thinking about seasons. What we all need to get our heads around is this is a new virus and this one is behaving differently,” Harris told a virtual briefing in Geneva, urging vigilance in applying measures to slow transmission that appears to be accelerated by mass gatherings. She also warned against thinking in terms of virus waves, saying: “It’s going to be one big wave. It’s going to go up and down a bit. The best thing is to flatten it and turn it into just something lapping at your feet.” The reality is that the issue of second waves has been a contentious one, much talked about by politicians – including Boris Johnson – and the media, but often very ill-defined. With no agreed-upon scientific definition, the term “second wave” has been used to mean anything from localised spikes in infection to full-blown national crises, leading some experts to avoid it. “‘Second wave’ isn’t a term that we would use [in epidemiology] at the current time, as the virus hasn’t gone away, it’s in our population, it has spread to 188 countries so far, and what we are seeing now is essentially localised spikes or a localised return of a large number of cases,” said Linda Bauld, professor of public health at the University of Edinburgh. Tom Frieden, former director of the US Centers for Disease Control, is among those arguing that the concept is unhelpful for “implying that Covid-19 will act as the flu acts”. Complicating the issue is perspective. Seen from a global viewpoint – such as that of the WHO – the pandemic appears as a single, large and still-accelerating outbreak, with worldwide numbers doubling in the past six weeks. In terms of regional spread and even within individual countries, from a ground-level view, it becomes more complicated. What can appear like a second wave is sometimes different areas of the same country simply being out of phase with each other in experiencing the epidemic, as in the US where a strong but uneven first wave moved initially in fits and starts and then more quickly. Keith Neal, emeritus professor in the epidemiology of infectious diseases at the University of Nottingham, said that it has become a “media term”, as well as a scientific one. “What we are seeing are spikes in many countries, and in Leicester [in the UK] and other places. Some people might call these waves but if they do we are looking at dozens of waves. “Even in Australia [in Victoria] there is clearly an upturn but the disease was only at low levels to start with, so it’s down to a vague terminology.” As Melissa Hawkins, a professor of health at American University, wrote in the Conversation, looking at the US situation, talking about second waves in countries where the disease has simply progressed unevenly is inappropriate. “The US as a whole is not in a second wave because the first wave never really stopped. The virus is simply spreading into new populations or resurging in places that let down their guard too soon,” she wrote, a comment applicable to other countries that have seen resurgences. As the University of Oxford’s Centre for Evidence-Based Medicine, which examined 10 epidemics of respiratory disease from 1889, points out: “Most of our thinking on second-wave theory arises from the 1918-20 Spanish flu that infected 500 million people worldwide and reportedly killed an estimated 20 million to 50 million.” “‘Waves’ imply a lack of viral circulation which is probably an illusion,” wrote Tom Jefferson and Carl Heneghan early in the outbreak in the UK. “Waves are also visible and mostly rhythmical. There does not appear to be any pattern or rhythm to the epidemics summarised in the table and their comings and going are only visible because of the effects on the human body and their impact on society.” The disease has little respect for land borders, even when authorities have tried to seal them; perhaps the only country that appears to have entirely stamped out the disease is New Zealand, an island nation that has curtailed almost all inbound travel. More important than the description of any rise in cases is public health management of the increase, Neal added, and he cautioned that identifying a true second wave may need the perspective of time. “It is defining when we have [a second wave] that is the issue. In the Spanish flu it was quite apparent. But only after the event. “The WHO is looking at world figures and these are still increasing so as a pandemic we are in the first wave.”