The study, published this week by the U.S. National Institutes for Health (NIH), showed that at least seven people in five U.S. 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 species. But Beijing has promoted the theory that COVID-19 entered China from overseas via contaminated frozen food, while a number of foreign politicians are also calling for more investigations into the possibility it leaked from a laboratory.