Two separate studies that involved antibody testing for Covid-19 suggest more than 3 per cent of people in Wuhan – a city of 11 million – may have previously had infections, possibly with no or very mild symptoms.
One team of Chinese scientists tested 17,368 people in Wuhan – where the first cases were reported in December – and other cities in China from March 9 to April 10, aiming to assess how prevalent the disease had been.
While nucleic acid tests are used to identify whether someone has contracted Covid-19 at the time they are ill, testing for antibodies – blood proteins produced by the immune system to fight the disease – is an important way to determine, later on, how many people were infected.
It can also indicate whether they had asymptomatic or subclinical infections, meaning the symptoms were so mild that they did not notice them.
Tests for two types of antibodies found that 3.2 per cent to 3.8 per cent of the blood samples from Wuhan were positive, meaning they had previously been infected with Covid-19, according to the peer-reviewed study published in Nature Medicine last week.
The research was conducted by scientists from Guangzhou, Chongqing, Sichuan, Hong Kong and other Chinese cities.
Among those tested were 714 health care workers, 3.8 per cent of whom were found to have been infected with Covid-19.
The proportion was the same, 3.8 per cent, for 346 staff at hotels where health care workers treating Covid-19 patients were staying.
And among the 219 family members of health care workers tested, antibodies were found in 3.2 per cent.
The study found that people aged over 65 had a higher prevalence of antibodies.
But its authors noted that since the study was not based on a random sample, there could be bias and more research was needed to get a better estimate of the total number of infections in Wuhan during the outbreak.
Their findings were, however, in line with a separate peer-reviewed study by scientists from the University of Hong Kong published in The Lancet Microbe on June 3.
Their testing found 4 per cent of 452 Hongkongers evacuated from Hubei province, where Wuhan is located, had Covid-19 antibodies.
The Hong Kong residents left Hubei on four different flights on March 4 and 5, and 364 of them had been in Wuhan.
In both studies, many samples were believed to be either subclinical or asymptomatic cases.
Based on this, the researchers concluded that the results of nucleic acid testing done during the outbreak may reflect just a fraction of the infections in Wuhan.
They estimated that as many as 2.2 million people across Hubei province and 500,000 in Wuhan could have contracted Covid-19.
As of June 10, China had reported 83,057 cases nationwide, while the official figure for Wuhan as of June 9 was 50,340.
But those figures do not include asymptomatic patients – although the government now includes such cases in its daily reports they are not reflected in the confirmed case numbers.
“Our seroprevalence data [on the prevalence of antibodies] showed that RT-PCR [nucleic acid test] confirmed infections grossly underestimated the actual prevalence of Covid-19,” concluded the HKU researchers, led by infectious disease expert Professor Yuen Kwok-yung.
“Thus 97 per cent of infections in Hubei might have gone undiagnosed at that period of the epidemic,” they said in the paper.
Many scientists have called for antibody tests in different areas to find out the exact number of infections, since many people may have been unaware they were infected if they had mild or no symptoms.
Such data could potentially reveal the scale of transmission, the number of asymptomatic or subclinical cases, and whether a population has herd immunity – when enough people have been infected that the virus stops spreading.
Health authorities have carried out nucleic acid tests on all Wuhan residents but only some of them have had antibody tests.
The authorities said the nucleic acid tests found traces of the coronavirus in 300 asymptomatic carriers but not enough to make others sick.
The mass testing – which reportedly cost 900 million yuan (S$180 million) – did not indicate how many asymptomatic or subclinical infections there had been, and authorities have not released any figures for the antibody tests.
The scientists also tested blood samples from people in other Chinese cities for the study in Nature Medicine, finding that the further away they were from Wuhan, the lower the prevalence of antibodies.
In Jingzhou and Honghu, two cities west of Wuhan in Hubei, antibodies were found in 1.3 per cent of 3,091 health care workers.
They also found that 3.6 per cent of 979 patients who regularly went to hospitals for dialysis treatment had Covid-19 antibodies.
In Chongqing, a municipality 750km away from Wuhan, 3.1 per cent of 319 health care workers, 3.8 per cent of 993 hospital outpatients and 0.58 per cent of 9,442 residents were found to have been infected with Covid-19 previously.
The rate was lower in Guangzhou and Foshan, in Guangdong province – about 980km and 1,000km from Wuhan – where 2.8 per cent of 563 patients having dialysis, 1.2 per cent of 260 health care workers, and 1.4 per cent of 442 factory workers had antibodies.
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This article was first published in South China Morning Post.