Antibody Tests Suggest That Coronavirus Infections Vastly Exceed Official Counts
9 months ago
Study estimates a more than 50-fold increase in coronavirus infections compared to official cases, but experts have raised concerns about the reliability of antibody kits.
Widespread antibody testing in a Californian county has revealed a much higher prevalence of coronavirus infection than official figures suggested. The findings also indicate that the virus is less deadly than current estimates of global case and death counts suggest. But some scientists have raised concerns about the accuracy of kits used in such studies because most have not been rigorously assessed to confirm they are reliable.
An analysis of the blood of some 3,300 people living in Santa Clara county in early April found that one in every 66 people had been infected with SARS-CoV-2. On the basis of that finding, the researchers estimate that between 48,000 and 82,000 of the county’s roughly 2 million inhabitants were infected with the virus at that time — numbers that contrast sharply with the official case count of some 1,000 people reported in early April, according to the analysis posted today on medRxiv. The work has not yet been peer reviewed.
The results are some of the first of more than a dozen ‘sero-prevalence surveys’ being carried out in cities worldwide to try to estimate populations’ true infection rates, in the absence of widespread diagnostic testing. The World Health Organization is also running a global sero-prevalence study, known as Solidarity II.
Many surveys are using commercial antibody kits to detect antibodies against the virus in blood samples. The presence of SARS-CoV-2-specific antibodies reveals that a person had been infected for at least a week earlier, even if they have had no symptoms.
“A sero-survey gives you a snapshot in time of who is infected in your given population,” says Kanta Subbarao, a virologist at the Peter Doherty Institute for Infection and Immunity in Melbourne. This is especially important for an infection such as SARS-CoV-2, for which some people show no symptoms, or only mild ones, she says.
When combined with information about age, gender, symptoms, co-morbidities and socioeconomic status, these surveys can also help to answer questions about factors such as the role of children in spreading the infection, and the portion of cases that are asymptomatic.
“This is a really inexpensive way to get an incredible amount of information,” says Jayanta Bhattacharya, a health economist at Stanford University in California and a co-author of the study.
News of the Santa Clara analysis follows preliminary results from a similar study in Germany, released on 9 April, that tested some 500 people in a village of more than 12,000 and found that one in seven had been infected with SARS-CoV-2. The German team also looked for active infections, using diagnostic tests based on the polymerase chain reaction, and when those figures were combined with those who had antibodies, estimate that the town’s overall infection rate was 15%.
But this result might not be indicative of what’s happening across Germany, says virologist Christian Drosten, who heads the Institute of Virology at the Charité university hospital in Berlin, because many people in the town celebrated at a carnival in February. “There was a big point source outbreak in that village,” he says.
The fact that both studies detected much higher rates of infection than official figures suggest is not surprising, says Peter Collignon, a physician and microbiologist at the Australian National University in Canberra. The virus had been spreading in the United States and parts of Europe for at least a month before it was detected as spreading in the community.
But Collignon notes that the commercial antibody tests used in both studies were evaluated onusing only a small number of people, which could also affect the accuracy of the survey results.
Antibody kits aren’t just being used for population studies. Kits are also being marketed for testing whether individuals have had the disease. But experts warn that most tests haven’t been rigorously evaluated to ensure they are reliable.