(CN) — Covid-19 was passed around Wuhan and other areas of China by asymptomatic carriers long after officials said the infectious disease was under control, according to a study published Thursday.
Xue-jie Yu of China’s Wuhan University studied the antibodies collected from rapid tests between March 6 and May 3, 2020, from more than 63,100 healthy people in China. The research was published Thursday in the scientific journal PLOS Neglected Tropical Diseases.
“A large amount of asymptomatic carriers of SARS-CoV-2 [the novel coronavirus] existed after elimination of clinical cases of Covid-19 in Wuhan City,” the researchers said in a statement. “Avirulent SARS-CoV-2 strains may still cause symptoms in extremely susceptible individuals and it may also revert to a highly virulent strain to reignite the epidemic of Covid-19 in China.”
Wuhan was ground zero for the worldwide pandemic that began in December 2019 and infected at least 86 million people worldwide since. There, the first community-spread cases of the novel coronavirus were flagged as a viral pneumonia outbreak stemming from a seafood market.
In March 2020, Chinese President Xi Jinping took a trip to Wuhan to indicate his confidence that the outbreak was under control. The Chinese government had quickly built multiple hospitals in a matter of weeks, and restricted travel to and from the region.
After the city’s cases peaked in February, Wuhan declared itself Covid-free when its last dozen patients were discharged from a hospital in late April, though the viral disease returned in small pockets in the months following.
Thursday’s study draws on rapid antibody tests that check patients for two antibodies: the presence of IgG antibodies in a patient’s sample suggests a previous infection, while IgM antibodies indicate recent or current infections.
“On June 3 the Wuhan Municipal Health Commission reported that the asymptomatic infection rate was 0.303/10,000 based on nucleic acid test[s] of 9.9 million people in Wuhan City from May 14 to June 1. The asymptomatic infection rate based on IgM positive rate in this study was hundreds times of that based on the nucleic acid test,” the scientists write. “The discrepancy between our result and the nucleic acid test could be caused by difference[s] in sample collection time and the sensitivity of the methods.”
Yu and his team retrospectively examined the results of 63,107 tests; 11,086 were from Wuhan, 38,171 were from elsewhere in the Hubei Province, and 13,850 were from other provinces of China.
The scientists found that about 1.68% of the people tested in Wuhan had either IgG or IgM antibodies for the novel coronavirus, which is much higher than the 0.38% positivity rate reported by the tests taken outside of Hubei province.
Because IgM antibodies were found in 0.46% of the Wuhan test recipients, Yu and his colleagues conclude that thousands of Wuhan residents suffered asymptomatic infections between March and May last year, when the city’s clinics were not reporting any Covid-19 cases.
“A large amount of asymptomatic carriers of SARS-CoV-2 existed after elimination of clinical cases of Covid-19 in Wuhan City,” the authors wrote in the study. “Therefore, SARS-CoV-2 may exist in a population without clinical cases for a long period.”
The scientists also note that women and older Chinese residents were more likely than males and younger Chinese people to test positively for the coronavirus antibodies.
“The question is why so many people were IgM positive to SARS-CoV-2, but very few persons got sick. Our hypothesis is that strains that cause asymptomatic infection in Wuhan City and other places in China are attenuated viral strains,” the scientists wrote. “It is difficult to identify the avirulent SARS-CoV-2 strains that cause asymptomatic infection in the population.”