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Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Geneva, Switzerland (SEROCoV-POP): a population-based study

Our take —

Overall, 7.9% of individuals in Geneva tested positive for SARS-CoV-2 antibodies in the first 5 weeks of the study. For every confirmed case of COVID-19, there were 11 to 12 estimated additional individuals infected in the community. Importantly, relying on herd immunity is unlikely to be an effective strategy within the population as the vast majority of people did not have detectable antibodies, and so are likely to be susceptible to infection. COVID-19 prevention measures are still needed to reduce the spread of the virus within this population.

Study design

Prospective Cohort

Study population and setting

This study was carried out in Geneva, Switzerland, during the first local wave of the COVID-19 pandemic. The first reported case of COVID-19 in Geneva was on Feb 26, 2020, and by May 9, 2020, there were 5,160 confirmed cases and 266 deaths. The objective of this study was to evaluate the proportion of people with antibodies to SARS-CoV-2 in the Geneva population over a 12-week period. The presence of these antibodies suggest that an individual was infected with the virus that causes COVID-19 at some time in the past. Participants were randomly selected individuals who participated in a previous population-representative study, and their household contacts. Blood samples and data were analyzed for 2766 participants aged 5 years and above from 1339 households who were enrolled over 5 weeks from April 6 to May 9, 2020.

Summary of Main Findings

The proportion of people with antibodies during each week was as follows: 4.8%, 8.5%, 10.9%, 6.6%, and 10.8%, and cases tended to cluster within households. Participants aged 5-9 years and older adults (≥65 years) were less likely to have antibodies compared to persons aged 10-64 years. The authors speculated that the lower prevalence among older adults may indicate that mitigation efforts were working, although it is also possible that older adults have lower antibody response following COVID-19 infection. After taking into account the time it takes for the body to develop antibodies after exposure to the virus, the authors estimated that for every reported confirmed case of COVID-19, there were 11 to 12 infections in the community. However, the vast majority of people in the population did not have antibodies to the virus, and thus could potentially become infected with the virus if exposed.

Study Strengths

Before undertaking the study, the authors performed a validation to evaluate the sensitivity and specificity of the ELISA test used to confirm presence of antibodies in this study. Their ELISA test had high sensitivity (93%) and specificity (100%). In addition, the authors further confirmed positive and indeterminate samples using recombined immunofluorescence assay. The authors took steps to minimize selection bias at entry into the study by supplementing email invitations for study participation with invitations via postal mail. Additional analyses by the authors suggested that their results did not differ substantially when limited to only participants from the population-representative parent study. Furthermore, the prospective design improves understanding of changing antibody prevalence within the population.

Limitations

The study excluded persons in institutionalized residences (e.g., care homes, and prisons), thus findings from this study may only apply to non-institutionalized residences. Study participants were also more socially advantaged than the general population in Geneva (better educated and more technologically proficient) which may underestimate the prevalence estimates of antibodies in the population if health disparities observed in other locations are also experienced in Geneva. Persons in institutionalized residences and persons who are socially disadvantaged may have a greater social and economic susceptibility to COVID-19.

Value added

This study assessed the proportion of individuals who had antibodies on a weekly basis, and so improves understanding of the extent COVID-19 infection in Geneva over time.

This review was posted on: 12 August 2020