Viruses mutate all the time, including the coronavirus that’s caused the global Covid-19 pandemic. Although most of the changes are innocuous, some specific mutations have sparked alarm, and four variants that emerged in the U.K., South Africa, Brazil and India have caused particular concern as they spread worldwide. Studies suggest they are more contagious and some evidence points to one of them being more deadly. At least one other is driving reinfections. Developers are working on second-gene