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Ten months into the COVID-19 pandemic we still need a better understanding of many features of this disease, including the overall case-fatality rate and the durability of immunity to SARS-CoV-2.
Now, investigators in Iceland have been able to perform a population-based study of the serologic response to SARS-CoV-2, thanks to the country's geographic isolation and centralized healthcare system. The researchers used six different SARS-CoV-2–specific antibody assays to survey 1237 individuals who had COVID-19 based on a positive quantitative polymerase chain reaction (qPCR) test, 4222 individuals quarantined due to COVID-19 exposure, and 23,452 individuals without known exposure. SARS-CoV-2 seropositivity was defined as a positive result on two separate pan-immunoglobulin (pan-Ig) assays.
Screening of 472 samples from 2017 and early 2020 found no positive results, confirming the specificity of the assays and the lack of viral spread in Iceland before February 2020. Among individuals who recovered from COVID-19, more than 90% were seropositive on both pan-Ig assays by day 25 after initial qPCR positivity; hospitalized patients seroconverted more frequently and more quickly than nonhospitalized individuals. In individuals with multiple samples, antibody titers increased during the first 2 months after diagnosis and remained stable for the next 2 months. Among 422 quarantined qPCR-negative individuals, 2.3% became seropositive, and those with household contacts were 5.2 times more likely to become positive than other groups of exposed quarantined individuals; overall, 26.6% of quarantined household contacts and 5.0% of other quarantined individuals became infected.
Based on the seroprevalence in these groups as well as in the population screened without known exposure, approximately 0.9% of the population of Iceland had been infected by SARS-CoV-2 by early July 2020, with 56% diagnosed by qPCR. The infection fatality risk was 0.3% overall and 4.4% in those over 70 years of age.
Gudbjartsson DF et al. Humoral immune response to SARS-CoV-2 in Iceland. N Engl J Med 2020 Sep 1; [e-pub]. (https://doi.org/10.1056/NEJMoa2026116)
Alter G and Seder R.The power of antibody-based surveillance. N Engl J Med 2020 Sep 1; [e-pub]. (https://doi.org/10.1056/NEJMe2028079)
Comment
These findings must be viewed with the understanding that the Icelandic population is quite homogeneous and represents essentially a single ethnic community. Nevertheless, the study provides valuable data on the overall case-fatality rate as well as reassurance regarding the durability of the immunoglobulin response, which has been questioned in several recent small studies (NEJM JW Infect Dis Sep 2020 and N Engl J Med 2020 Jul 21; [e-pub]). Editorialists note that the apparent discrepancy between the reports may relate to the immune response mediated by short-lived and long-lived plasma cells.