Seroprevalence studies of coronavirus disease 2019 (COVID-19) effectively offer insight into how well the virus is being contained. Several of these studies have been conducted across the globe. Here is a summary of this research and the potential clinical and global impact of the findings.
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Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was first confirmed on January 20, 2020. Since this first case of COVID-19, several seroepidemiologic investigations have been conducted to understand the spread of asymptomatic and subclinical infections in the general population.
Additionally, seroprevalence studies offer greater insight into how effective containment strategies, such as social distancing and quarantines, are working to control transmission rates.
Global COVID-19 Seroprevalence: What does the research say?
Many seroprevalence studies conducted across the world have shown that the number of undiagnosed, missing cases is probably greater than confirmed cases.
In a study published in the JAMA of Internal Medicine, investigators collected serum samples from 16,025 geographically diverse individuals in the United States between March 23 through May 2020. There was no evidence of antibodies to SARS-CoV-2 in most specimens. The adjusted estimates of the percentage of people who were seroreactive to the spike protein antibodies of SARS-CoV-2 were between 1.0% in people from San Francisco to 6.9% in people from New York City.
The study also found that the estimated number of COVID-19 infections were between 6 to 24 times higher than the number of reported cases. According to the researchers, their results may be explained by individuals who had mild disease or no symptoms who did not seek medical attention or undergo testing. These people, however, likely contribute to a viral transmission.
In another cross-sectional study from Korea, researchers tested the seroprevalence of anti-SARS-CoV-2 antibodies in outpatients. The goal of the study was to understand the burden of COVID-19 as well as the level of herd immunity in this population. In total, the study included 1,500 serum samples which were obtained from outpatients who attended two hospitals between May 25 and May 29 in southwestern Seoul, Korea.
Using electrochemiluminescence immunoassay to detect immunoglobulin G (IgG), among other antibodies, against SARS-CoV-2, the investigators found an overall 0.07% anti-SARS-CoV-2 antibody seropositivity. According to the researchers, this low seroprevalence indicated that the pandemic, at least in certain regions in Korea, was under control via its social distancing and contact tracing programs in place.
Another Korean study, conducted in Daegu, Korea, estimated that the seroprevalence in that region was 7.6% between May 25 to June 5. The study also tested serum samples for IgG from 103 patients and 95 guardians without a confirmed COVID-19 diagnosis. No difference was found between patients and guardians in terms of seroprevalence. The investigators concluded that, based on their estimated figure, the number of missing cases in the region were likely 12 times more than the number of PCR-confirmed COVID-19 cases.
A Spanish study of more than 60,000 participants found a nationwide seroprevalence of approximately 5.0% to 6.2%. Urban areas surrounding Madrid had an estimated seroprevalence of 13.6%, which was reflected by PCR-confirmed cases showing much higher COVID-19 prevalence in urban compared with rural areas.
In a rapid communication study published in European Surveillance, researchers from Germany tested seroprevalence of IgG antibodies against SARS-CoV-2 in 3,186 blood donors from three German states between March and June 2020. The seroprevalence rate ranged from 0.66% to 1.22%. The investigators noted since most COVID-19 cases are generally mild or asymptomatic, many cases remain recorded. Despite this suggestion, the researchers did not provide an estimated seroprevalence rate in the studied German population.
Limited data are available on the seroprevalence of COVID-19 in healthcare workers. European research has investigated the seroprevalence of COVID-19 antibodies in cleaning and oncological staff at clinics. Strict hygiene is crucial for specialized health wards, particularly during the pandemic, to prevent the shedding of COVID-19 among vulnerable patients.
In one study, researchers from Germany measured antibody titers against COVID-19, including IgG and immunoglobulin A (IgA), in samples from 45 hospital cleaning staff and 20 members of the hospital’s oncology ward. Samples were obtained between June 15 and June 30. There were significantly increased levels of IgA antibody titers in 1 person each in the cleaning staff and the oncology ward. Only 1 person from the oncology ward had significantly elevated IgG antibody titers.
All swabs taking for virus detection came back negative. Despite the lower seroprevalence in this group, the investigators suggest that monitoring hospital cleaning staff may be a “blind spot” in most surveillance programs. Stricter surveillance in this population, they argue, is needed. A similar study from Croatia found that the SARS-CoV-2 seroprevalence is low among healthcare workers in the country.
Where to go from here?
Based on the findings from these studies, many scientists, clinical researchers, and governing bodies suggest proposed approaches to achieve herd immunity, such as through natural infection, is highly unethical. Additionally, professionals indicate suggest achievement of herd immunity may also be unachievable, based on current data. Effective and safe COVID-19 vaccines and therapies may be the priority approach for the pandemic.
But even when these prevention and treatment options receive approval and become available, researchers note that seroprevalence studies will still be needed to continue offering information about the duration and extent of vaccine-induced herd immunity.
- Havers FP, Reed C, Lim T, et al. Seroprevalence of antibodies to SARS-CoV-2 in 10 Sites in the United States, March 23-May 12, 2020 [published online ahead of print, 2020 Jul 21]. JAMA Intern Med. 2020;10.1001/jamainternmed.2020.4130. doi:10.1001/jamainternmed.2020.4130
- Noh JY, Seo YB, Yoon JG, et al. Seroprevalence of anti-SARS-CoV-2 antibodies among outpatients in Southwestern Seoul, Korea. J Korean Med Sci. 2020;35(33):e311. Published 2020 Aug 24. doi:10.3346/jkms.2020.35.e311
- Song SK, Lee DH, Nam JH, et al. IgG seroprevalence of COVID-19 among individuals without a history of the coronavirus disease infection in Daegu, Korea. J Korean Med Sci. 2020;35(29):e269. Published 2020 Jul 27. doi:10.3346/jkms.2020.35.e269
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- Epstude J, Harsch IA. Seroprevalence of COVID-19 antibodies in the cleaning and oncological staff of a municipal clinic. GMS Hyg Infect Control. 2020;15:Doc18. Published 2020 Jul 23. doi:10.3205/dgkh000353
- Vilibic-Cavlek T, Stevanovic V, Tabain I, et al. Severe acute respiratory syndrome coronavirus 2 seroprevalence among personnel in the healthcare facilities of Croatia, 2020. Rev Soc Bras Med Trop. 2020;53:e20200458. doi:10.1590/0037-8682-0458-2020