SHANGHAI : A senior Chinese epidemiologist said the United States should be the priority in the next phase of investigations into the origin of COVID-19 after a study showed the disease could have been circulating there as early as December 2019, state media said on Thursday.The study, published this week by the US National Institutes for Health (NIH), showed that at least seven people in five US states were infected with SARS-CoV-2, the virus that causes COVID-19, weeks before the United States reported its first official cases.A China-World Health Organization (WHO) joint study published in March said COVID-19 most likely originated in the country's wildlife trade, with the virus passing into humans from bats via an intermediary.