A meat-processing plant in Germany. A church in Seoul. A squat and a hospital in Italy. A wholesale market in Beijing. All of them have experienced clusters of infections after national authorities declared the disease suppressed in their country and launched a slow return towards normal life. Months into the pandemic, fear of a second wave of infections now hangs over countries that have managed to stamp out the disease, mostly through economically and socially painful lockdowns. An incubation period of up to two weeks, the fact that some infected people can spread the disease before symptoms show and others remain asymptomatic … all this helps the virus to spread undetected before an outbreak flares. South Korea grabbed international attention last week when its disease control authorities were the first in the world to announce that the country had entered a second wave, focused around the capital and apparently triggered by gatherings over a May holiday. The term “second wave” has no agreed-upon scientific definition. It can mean anything from localised spikes in infection to a full-blown national crisis, and some experts avoid it for this reason. The World Health Organization avoided using it to describe South Korea’s status when asked in a news conference. There is more consensus on both prevention and how to handle new outbreaks using approaches honed rapidly over the past few months to find people who are infected, and prevent them spreading the disease. “In the absence of a vaccine or effective medication, it’s all about the data. We need to know where the cases are,” said Linda Bauld, professor of public health at the University of Edinburgh. “Identifying who has the disease: testing, tracing and isolating.” With these steps, a cluster of local infections does not inevitably have to lead to a full-blown national outbreak if it is identified and contained quickly enough. Preventative measures also help limit the speed with which any new cluster can spread, said Keith Neal, emeritus professor in the epidemiology of infectious diseases, University of Nottingham. He encouraged people to think that there was no single magic bullet, but instead “every little helps”, adding: “If you reduce every risk by 10% or 20%, overall, you have a big reduction. So meet as few people as possible and distance when you do, wear masks, work from home.” However, these approaches to fending off a second wave only work in countries that have been able to contain the virus, and have the resources to pay for testing, tracing and lockdown. In the US, some states started relaxing lockdowns before the virus had been contained, and weeks after infections initially peaked they have surged again, a grim reminder of how easily the virus can regain a foothold. Dr Anthony Fauci, the country’s top infectious disease expert, warned that it was premature to talk of a second wave, however, because the country was not yet out of its first wave. And in countries like Afghanistan, poverty and war made the virus almost impossible to contain; when staying at home means going without food, even harshly enforced orders are likely to be disobeyed. “In parts of the global south, mass testing, tracing and isolating is not going to be possible,” Bauld said. That disparity in controls and the constant vigilance about a second wave mean travel restrictions are likely to stay until an effective vaccine or cure is discovered.