- Studies in Europe and the US find many people may have been exposed to the new coronavirus without knowing
- There remain questions about whether they’re protected against further infection and whether it helps herd immunity
Surveys from Germany, the Netherlands, and the United States have found antibodies in people not previously diagnosed with the novel coronavirus, suggesting more people than previously known were infected with the virus responsible for the global pandemic.
But many of the studies have sparked controversy, with experts raising concerns about the accuracy of antibody testing methods, how well the surveyed subjects represented the population and how much antibodies protected against Covid-19, according to a report in Science magazine.
While some specialists said higher numbers of unconfirmed infections " based on the presence of antibodies " reinforced the need for strict physical-distancing measures, others suggested it could allow for some easing of lockdown measures because there was progress towards herd immunity, or resistance among the general population to the disease.
The World Health Organisation's guidance last Friday said that while some governments had suggested the detection of antibodies to the virus, officially called Sars-Cov-2, could allow people with antibodies to travel or return to work, "there is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection".
While most who contracted the virus would develop an antibody response that could offer some level of protection, it was not yet clear what level of protection was provided or how long it would last, the WHO said.
German virologist Hendrik Streeck, from the University of Bonn, carried out a study this month in Heinsberg, which is an epicentre of the virus in Germany under strict lockdown. Streeck's study found that 14 per cent of 500 people they tested had antibodies to the virus.
The researchers said this meant the virus killed 0.37 per cent of those infected, concluding that "15 per cent of the population can no longer be infected with Sars-CoV-2, and the process of reaching herd immunity is already under way".
But Christian Drosten, a virologist from Charite University Hospital in Berlin, said the lab tests had a "high rate of false positive signals" that could be detecting antibodies against seasonal coronaviruses, and not necessarily infection by Sars-Cov-2. Drosten said he could not draw any conclusions from the study, saying: "There is simply so little explained that you don't understand everything."
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Other antibody studies faced similar scrutiny. A preliminary study by Jayanta Bhattacharya and Eran Bendavid from Stanford University found that between 48,000 to 81,000 people in California's Santa Clara county had been infected with the Covid-19 virus by early April, a number that was 50 to 85 times the official number of cases.
It was followed by one from the University of Southern California last week that found between 221,000 and 442,000 people in Los Angeles county may have been infected " compared to the 8,000 reported cases at the time " after the study found antibodies in 4.1 per cent of adults.
But experts were fiercely critical of the Stanford study in particular. Critics said recruiting participants through Facebook would attract people who were likely to have symptoms associated with Covid-19 and simply wanted to be tested. There were also relatively few participants from low-income and minority populations, as well as concerns about false positives and questions about methods used to adjust for error.
Andrew Gelman, director of the Applied Statistics Centre at Columbia University, wrote in an assessment of the Stanford study that the authors "owe us all any apology" for producing data that "were essentially the product of a statistical error".
In the Netherlands, a study by virologist Hans Zaaijer from the Dutch national blood bank Sanquin, found this month that antibodies were present in about 3 per cent of the population. But he said it showed the country was still far from reaching "herd immunity".
Another study from the Boston suburb of Chelsea by Massachusetts General Hospital pathologists John Iafrate and Vivek Naranbhai found that 31.5 per cent of 200 pedestrians sampled randomly on a street had antibodies for Covid-19 in their blood samples. The researchers acknowledged that the sample might not have been representative of Chelsea and that there could be false positives with the test, adding that the antibodies did not necessarily mean immunity to the virus.
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