As authorities the world over lift economy-crippling movement restrictions aimed at curbing coronavirus infections, the fear on everyone’s minds can be expressed in two words: second wave. The concern is that, once quelled, the pandemic will resurface with renewed strength, causing a repeat of rising infections, swamped health systems and orders for lockdowns. Rather, it’s used to refer to a subsequent, serious increase in cases that occurs after the original surge has been quashed in a given area. Most countries responded to the pandemic by restricting movement, which slows the virus’s spread but leaves many people vulnerable to infection once they begin to venture out again. South Korea, which began easing its social distancing measures in April, suspended plans for further relaxation in June as new infections emerged. Testing, isolating and contact-tracing remain the main tools to stop a cluster from growing into a wave.In the case of the coronavirus that causes Covid-19, countries around the world adopted movement restrictions on an unprecedented scale and social-distancing measures that combined kept people far enough apart that the virus couldn’t easily spread.The WHO has recommended lifting movement restrictions in stages to test the effect of each before moving to greater openness. Health authorities need to find infected people, isolate them and identify their recent contacts, so they can be tested as well and isolated if necessary. Eventually, it’s possible that enough people will become exposed to the coronavirus that herd immunity will develop and it will stop spreading, or that a vaccine against it will be licensed.