An estimated 168,000 people were infected by the coronavirus during the epidemic in the central Chinese city of Wuhan, with at least two-thirds of the infections asymptomatic, according to a national study of blood antibodies.
“A large amount of asymptomatic carriers of Sars-CoV-2 [the virus that causes Covid-19] existed after elimination of clinical cases of Covid-19 in Wuhan. Therefore, [the virus] may exist in a population without clinical cases for a long period,” the researchers concluded.
The study was conducted by scientists at the State Key Laboratory of Virology at Wuhan University and an independent medical laboratory Wuhan CMLabs. The results were published in the peer-reviewed journal PLOS Neglected Tropical Diseases on Thursday.
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The researchers collected blood from more than 63,100 healthy people from March to early May last year from 30 provinces to test for antibody rates, a standard measure for previous infection.
They examined the blood for IgG antibodies, which suggest a person was previously infected, and IgM antibodies, which point to a current or recent infection.
They found that the total IgG and IgM antibody positive rate was 1.68 per cent in Wuhan, much higher than the rate of 0.38 per cent in the rest of the country.
That suggested that roughly 168,000 of the city’s more than 10 million people were infected at some stage.
The researchers also found that the IgM positive rate of 0.46 per cent, indicating that thousands of people in Wuhan were infected asymptomatically between March and early May last year, after the outbreak was believed to be under control.
Detection of IgM antibodies gives a better understanding of the number of asymptomatic infections in the population in real time, as silent carriers can unwittingly transmit the virus to others.
The researchers said one explanation as to why very few people got sick even though they were IgM positive to the virus was that the asymptomatic infections were caused by attenuated – or weaker – viral strains.
The estimates are well above the 50,300 reported hospitalised cases as of May 19 last year, suggesting that only one-third of the patients needed hospital treatment and at least two-thirds of the infected people were asymptomatic.
But the projections are also below the roughly half a million people in the city who might have contracted the virus during the initial outbreak as indicated by a study by the Chinese Centre for Disease Control and Prevention.
In late December, the CDC said a large-scale survey found coronavirus antibodies in more than 4 per cent of blood samples taken from residents of Wuhan.
Wuhan University dean of health sciences Yu Xuejie, who led the research team in the most recent study, said it was very difficult to detect all asymptomatic cases in the country but those cases might also be beneficial.
“There is no need to mass test the whole population to find asymptomatic carriers because there would always be overlooked cases. Now [health authorities] contain the virus wherever patients are detected,” Yu said.
“People should stay vigilant as the coronavirus may still exist. It is not a bad thing if the virus continues to stay in a form that will not cause the disease. Living with non-pathogenic viruses can instead boost our immune system.”
China is experiencing sporadic coronavirus outbreaks, with Hebei province recording 120 new locally transmitted cases on Wednesday, over half of which were asymptomatic.
With 92 confirmed infections and 149 asymptomatic cases as of Wednesday, the Hebei outbreak marks China’s biggest rise in Covid-19 infections in recent months.
China has actively searched for coronavirus patients and hospitalised symptomatic patients, restricting the ability of the viral strains that caused severe disease from spreading to new hosts, according to the team.
But it was difficult to identify less virulent strains, which may still cause symptoms in extremely susceptible individuals and revert to a highly virulent strain to reignite the epidemic, the scientists said.
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