Objective To assess the incidence of COVID-19 infection in the absence of a confirmatory test in persons suspecting they contracted COVID-19 and elucidate reasons for their belief. Methods We recruited persons with a confirmed COVID-19 diagnosis and persons who believed they may have contracted COVID-19 between December, 2019 and April, 2021 into a study of immunity against SARS-CoV-2. An intake questionnaire captured their perceived risk factors for exposure and symptoms experienced, including symptom duration and severity. ELISA testing against multiple SARS-CoV-2 antigens was done to detect antibodies against SARS-CoV-2. No participant had received COVID-19 vaccination prior to the time of testing. Results The vast majority of study subjects without Public Health confirmation of infection had no detectable antibodies against SARS-CoV-2. Suspected infection with SARS-CoV-2 generally involved experiencing symptoms common to many other respiratory infections. Unusually severe or persistent symptoms often supported suspicion of infection with SARS-CoV-2 as did travel or contact with travelers from outside Newfoundland and Labrador. Rare cases in which antibodies against SARS-CoV-2 were detected despite negative results of Public Health testing for SARS-CoV-2 RNA involved persons in close contact with confirmed cases. Conclusions Broad public awareness and declaration of pandemic status in March, 2020 contributed to the perceived risk of contracting COVID-19 in Newfoundland and Labrador from late 2019 to April 2021 and raised expectation of its severity. Serological testing is useful to diagnose past infection with SARS-CoV-2 to accurately estimate population exposure rates.

Background: Suicide rates are higher in rural compared to urban areas. Although this pattern appears to be driven by higher rates among men, there is limited evidence about the characteristics of rural people who die by suicide in Canada. The objective of this study was to examine the demographics, manner of death, and social and clinical antecedents of people who died by suicide in rural areas compared to urban areas. Methods: We conducted an observational study of all suicide deaths that occurred among Newfoundland and Labrador residents between 1997 and 2016 using a linked data set derived from a comprehensive review of provincial medical examiner records. We used t tests and χ2 to assess associations between rural/urban status and variables related to demographics, circumstances, and manner of death, as well as social and medical history. Logistic regression was utilized to assess the independent contribution of any variable found to be significant in univariate analysis. Results: Rural people who died by suicide accounted for 54.8% of all deaths over a 20-year period. Overall, 81.6% of people who died were male. Compared to urban, rural people who died by suicide were younger, more likely to use firearms or hanging, and had a higher mean blood alcohol content at the time of death (27.69 vs. 22.95 mmol/L). Rural people were also less likely to have had a known history of a prior suicide attempt, psychiatric disorder, alcohol or substance abuse, or chronic pain. Discussion: The demographic and clinical differences between rural and urban people who died by suicide underscore the need for suicide prevention approaches that account for place-based differences. A key challenge for suicide prevention in rural communities is to ensure that interventions are developed and implemented in a manner that fits local contexts.

Antibodies capable of neutralizing SARS-CoV-2 are well studied, but Fc receptor–dependent antibody activities that can also significantly impact the course of infection have not been studied in such depth. Since most SARS-CoV-2 vaccines induce only anti-spike antibodies, here we investigated spike-specific antibody-dependent cellular cytotoxicity (ADCC). Vaccination produced antibodies that weakly induced ADCC; however, antibodies from individuals who were infected prior to vaccination (hybrid immunity) elicited strong anti-spike ADCC. Quantitative and qualitative aspects of humoral immunity contributed to this capability, with infection skewing IgG antibody production toward S2, vaccination skewing toward S1, and hybrid immunity evoking strong responses against both domains. A combination of antibodies targeting both spike domains support strong antibody-dependent NK cell activation, with 3 regions of antibody reactivity outside the receptor-binding domain (RBD) corresponding with potent anti-spike ADCC. Consequently, ADCC induced by hybrid immunity with ancestral antigen was conserved against variants containing neutralization escape mutations in the RBD. Induction of antibodies recognizing a broad range of spike epitopes and eliciting strong and durable ADCC may partially explain why hybrid immunity provides superior protection against infection and disease compared with vaccination alone, and it demonstrates that spike-only subunit vaccines would benefit from strategies that induce combined anti-S1 and anti-S2 antibody responses.

Antibodies capable of neutralising SARS-CoV-2 have been well studied, but the Fc receptor-dependent antibody activities that also significantly impact the course of infection have not been studied in such depth. SARS-CoV-2 infection induces antibody-dependent NK cell responses targeting multiple antigens, however, as most vaccines induce only anti-spike antibodies, we investigated spike-specific antibody-dependent cellular cytotoxicity (ADCC). Vaccination produced antibodies that only weakly induced ADCC, however, antibodies from individuals who were infected prior to vaccination (hybrid immunity) elicited much stronger anti-spike ADCC. Quantitative and qualitative aspects of humoral immunity contributed to this capability, with infection skewing IgG antibody production towards S2, vaccination skewing towards S1 and hybrid immunity evoking strong responses against both domains. The capacity for hybrid immunity to provide superior spike-directed ADCC was associated with selectively increased antibody responses against epitopes within both S1 and S2. Antibodies targeting both spike domains were important for strong antibody-dependent NK cell activation, with three regions of antibody reactivity outside the receptor-binding domain (RBD) corresponding with potent anti-spike ADCC. Consequently, ADCC induced by hybrid immunity with ancestral antigen was conserved against variants containing neutralisation escape mutations in the RBD [Delta and Omicron (BA.1)]. Induction of antibodies recognizing a broad range of spike epitopes and eliciting strong and durable ADCC may partially explain why hybrid immunity provides superior protection against infection and disease than vaccination alone, and demonstrates that spike-only subunit vaccines would benefit from strategies to induce a combination of S1- and S2-specific antibody responses.

Since COVID-19 emerged in 2019, neutralizing antibody (Ab) responses have dominated as surrogate measures of protection against infection and severe illness. The mostly self-limiting course of SARS-CoV-2 infection suggests the innate immune system, including natural killer (NK) cells, plays a role in its control. Therefore, we investigated SARS-CoV-2-specific antibody-dependent cell-mediated cytotoxicity (ADCC) and if Abs against Wuhan and early SARS-CoV-2 variants activate ADCC against emergent highly transmissible variants. ADCC was evaluated by measuring NK cell cytotoxicity (51Cr) against plasma (1:1000)-sensitized MRC-5 cells stably transduced to express Wuhan or Delta spike. ADCC mediated by plasma from double-vaccinated and convalescent adults was generally low (range 1.2 – 24.9%; median 7.2% (IQR 5.2 – 10.6%), and plasma from those with previous natural infection mediated significant ADCC (> 10% lysis) to the same extent as plasma from fully vaccinated individuals (13/34 vs 11/44). Hybrid immunity imparted robust ADCC (27.9 ± 10.8% target lysis) in 32/34 individuals tested and levels of ADCC significantly correlated with IgG3 responses (P = 0.0093) against spike stalk (S2) domain. A peptide scan identified four distinct ‘hotspot’ regions associated with robust ADCC in individuals with reactive Abs. Overall, a minor decline in ADCC against Delta variant spike-expressing relative to Wuhan targets occurred in all groups; however, individuals with Abs against the region containing D614G mediated greater ADCC against Delta variant than Wuhan targets. Determining which Abs provide robust and broad ADCC can inform future COVID-19 prevention and treatment strategies and predict ADCC responses for emergent variants. Supported by grants from CIHR through the COVID Immunity Task Force (VR1-173202)