A dispute over interpretation of blood tests has raised questions about an Italian study that suggested SARS-CoV-2 was circulating outside China much earlier than previously believed.

The spat between two universities tasked by the WHO with checking the data highlights the challenges of trying to determine when the virus emerged.

Last year, Italian scientists from the VisMederi laboratory at the University of Siena and Milan Cancer Institute published a study that showed neutralising antibodies against the SARS-CoV-2 virus were found in blood taken from healthy volunteers in the country in October 2019 during a lung cancer screening trial.

COVID-19 was first identified in the central Chinese city of Wuhan in December 2019, while Italy’s first patient was detected on 21 February, 2020, in a small town near Milan.