Tens of thousands of cricket fans and Boxing Day shoppers are being asked to get tested and self-isolate, following Victoria’s first mystery COVID-19 case in more than two months. Health authorities are scrambling to find the source, with a man in his 30s catching the virus after attending the sales at Chadstone Shopping Centre on December 26 and the Boxing Day Test at the MCG the following day. Chadstone and the MCG, billed as the largest shopping centre and sports stadium in the country, have now been listed as possible transmission sites. It presents a major challenge to Victoria’s bolstered testing and contact tracing network, with the state’s COVID response commander Jeroen Weimar describing it as a “needle in a haystack exercise”. Victoria has not had a mystery case since early November, as the state’s deadly second wave wound down to usher in 61 days without community transmission. The streak …