COVID-19 may have continued to spread quietly in Wuhan, China, during the spring of 2020, even after official government proposals suggested that the coronavirus be eliminated, a new study indicates.
SARS-CoV-2, the virus that causes COVID-19, was first discovered in Wuhan in December 2019, and the city soon became the center of what would become the COVID-19 pandemic. Business peaked in Wuhan in February 2020, but declined rapidly, with only a few cases reported at the end of March. By the beginning of April, the city closure ended, and later that month Wuhan was declared free of coronavirus.
But the new study, which was published in the journal on Thursday (January 7) PLOS neglected tropical diseases, tells a different story. The researchers, from Wuhan University, analyzed more than 63,000 blood samples collected in China between March 6 and May 3, 2020 – mainly in Wuhan. All of these participants were healthy and underwent an examination before returning to work, the researchers said.
The blood samples were tested antibodies against SARS-CoV-2. The researchers specifically looked at both IgG antibodies, a type of long-term antibody that indicates a previous infection with SARS-CoV-2, and IgM antibodies, a relatively short-lived antibody that indicates a current or recent infection with the virus.
In Wuhan, the percentage of participants with one of these antibodies was 1.7%. This is much higher than the percentage in areas outside Hubei Province (which includes Wuhan), which was about 0.4%.
What’s more, the researchers found that the IgM positivity rate – indicating an active or recent infection – was almost 0.5% in Wuhan, compared to 0.07% in other parts of China.
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Based on the level of IgM antibodies seen in Wuhan in the spring of 2020, the researchers estimated that thousands of people were asymptomatically infected during this period.
“We conclude that … a large number of asymptomatic carriers of SARS-CoV-2 existed after the elimination of COVID-19 clinical cases in Wuhan City,” the researchers wrote.
Based on the antibody numbers from the study, the researchers estimate that in Wuhan, a city of about 10 million people, about 168,000 people in Wuhan were infected at that time – higher than the approximately 50,000 cases reported.
The authors noted that officials in Wuhan conducted massive COVID-19 testing of 9.9 million people from May 14 to June 1 and found a rate of asymptomatic infection of only 0.3 per 10,000 people, based on PCR testing for the genetic material of SARS-CoV-2.
But the rate found in the current study, based on the IgM test, was hundreds of times higher, the researchers said. This discrepancy may be due to several factors, including a greater sensitivity of blood tests compared to PCR tests and the earlier dates of collection in the current study compared to the observation test by city officials.
Originally published on Live Science.