As the highly infectious Delta variant challenges pandemic control efforts around the world, a great unknown looms large: what might the virus behind Covid-19 do next?Scientists widely agree that new mutations of Sars-CoV-2 are likely to emerge as time goes on, bolstering a roster of Greek letters that already includes Alpha, Beta, Gamma and Lambda.The question is whether the virus behind the worst pandemic in a century will evolve to become more or less threatening in terms of its harmfulness…