We conducted a nationwide study of the prevalence of severe acute respiratory syndrome coronavirus 2 infection in the Faroe Islands. Of 1,075 randomly selected participants, 6 (0.6%) tested seropositive for antibodies to the virus. Adjustment for test sensitivity and specificity yielded a 0.7% prevalence. Our findings will help us evaluate our public health response.

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Background The durability of SARS‐CoV‐2 antibody response and the resulting immunity to COVID‐19 is unclear. Objectives To investigate long‐term humoral immunity to SARS‐CoV‐2. Methods In this nationwide, longitudinal study, we determined antibody response in 411 patients aged 0–93 years from two waves of infections (March to December 2020) contributing 1063 blood samples. Each individual had blood drawn on 4–5 occasions 1–15 months after disease onset. We measured total anti‐SARS‐CoV‐2 receptor‐binding domain (RBD) antibody using a qualitative RBD sandwich ELISA, IgM, IgG and IgA levels using an quantitative in‐house ELISA‐based assay  and neutralizing antibodies (NAbs) using an in‐house ELISA‐based pseudoneutralizing assay. IgG subclasses were analyzed in a subset of samples by ELISA‐based assay. We used nonlinear models to study the durability of SARS‐CoV‐2 antibody responses and its influence over time. Results After 15 months, 94% still had detectable circulating antibodies, mainly the IgG isotype, and 92% had detectable NAbs. The distribution of IgG antibodies varied significantly over time, characterized by a biphasic pattern with an initial decline followed by a plateau after approximately 7 months. However, the NAbs remained relatively stable throughout the period. The strength of the antibody response was influenced by smoking and hospitalization, with lower IgG levels in smokers and higher levels in hospitalized individuals. Antibody stability over time was mainly associated with male sex and older age with higher initial levels but more marked decrease. Conclusions The humoral immune response to SARS‐CoV‐2 infection varies depending on behavioral factors and disease severity, and antibody stability over 15 months was associated with sex and age.

R everse transcription PCR (RT-PCR) is a standard tool for diagnosing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. However, different testing strategies might cause wide variation in the number of identifi ed subclinical and asymptomatic cases, which could remain undetected (1). In May 2020, the Faroe Islands, an autonomous country that is part of the kingdom of Denmark with a population of 52,554 persons, had a 0.6% seroprevalence of antibodies against SARS-CoV-2 (2), among the lowest reported seroprevalences worldwide (3,4). This low seroprevalence is probably infl uenced by large-scale testing in the Faroes (5,6). However, the study also identifi ed a few previously undetected cases, implying that persons had been infected without knowing and without spreading the contagion (2).Since the identifi cation of the fi rst imported case of coronavirus disease (COVID-19) in the Faroe Islands on March 3, 2020, the territory has complied with World Health Organization recommendations to use an active suppression strategy focusing on testing and isolating patients and their close contacts. Accordingly, all close contacts of COVID-19 patients in the Faroe Islands were advised to quarantine for 2 weeks (5). In the Faroe Islands, the fi rst wave of the COVID-19 pandemic ended on April 22, 2020; no local cases were detected until August 3, 2020, when a second surge began (6). During the fi rst wave, the Faroe Islands' per capita testing rates were among the highest in the world. The seroprevalence study showed that, perhaps because of low levels of community transmission, few cases remained undetected (2). As a result, this context provides a unique opportunity to investigate the transmission dynamics of SARS-CoV-2 using serologic tests. SARS-CoV-2 is highly contagious. Family members and other close contacts of COVID-19 patients are at higher risk for SARS-CoV-2 infection, potentially furthering the transmission of disease (7). Most studies estimating the secondary attack rate of Seroprevalence of SARS-CoV-2-Specifi c Antibodies among Quarantined Close Contacts of COVID-19 Patients,

We conducted a second nationwide severe acute respiratory syndrome coronavirus 2 seroprevalence study in the Faroe Islands during November 2020. We found crude seroprevalence was 0.3% and prevalence was 0.4% after adjusting for test sensitivity and specificity. This low seroprevalence supports the prevention strategies used in the Faroe Islands.

Only a few studies have assessed the long-term duration of the humoral immune response against severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2). In this nationwide longitudinal study from the Faroe Islands with close to full participation of all individuals on the Islands with PCR confirmed COVID-19 during the two waves of infections in the spring and autumn 2020 (n=172 & n=233), samples were drawn at three longitudinal time points (3, 7 and 12 months and 1, 3 and 7 months after disease onset, respectively). Serum was analyzed with a direct quantitative IgG antibody binding ELISA to detect anti–SARS–CoV–2 spike RBD antibodies and a commercially available qualitative sandwich RBD ELISA kit measuring total antibody binding. The seropositive rate in the convalescent individuals was above 95 % at all sampling time points for both assays. There was an overall decline in IgG titers over time in both waves (p < 0.001). Pairwise comparison showed that IgG declined significantly from the first sample until approximately 7 months in both waves (p < 0.001). After that, the antibody level still declined significantly (p < 0.001), but decelerated with an altered slope remaining fairly stable from 7 months to 12 months after infection. Interestingly, the IgG titers followed a U-shaped curve with higher antibody levels among the oldest (67+) and the youngest (0–17) age groups compared to intermediate groups (p < 0.001). Our results indicate that COVID–19 convalescent individuals are likely to be protected from reinfection up to 12 months after symptom onset and maybe even longer. We believe our results can add to the understanding of natural immunity and the expected durability of SARS–CoV–2 vaccine immune responses.