Background A rapidly increasing number of serological surveys for antibodies to SARS-CoV-2 have been reported worldwide. We aimed to synthesise, combine, and assess this large corpus of data.Methods In this systematic review and meta-analysis, we searched PubMed, Embase, Web of Science, and five preprint servers for articles published in English between Dec 1, 2019, and Dec 22, 2020. Studies evaluating SARS-CoV-2 seroprevalence in humans after the first identified case in the area were included. Studies that only reported serological responses among patients with COVID-19, those using known infection status samples, or any animal experiments were all excluded. All data used for analysis were extracted from included papers. Study quality was assessed using a standardised scale. We estimated age-specific, sex-specific, and race-specific seroprevalence by WHO regions and subpopulations with different levels of exposures, and the ratio of serology-identified infections to virologically confirmed cases. This study is registered with PROSPERO, CRD42020198253.Findings 16 506 studies were identified in the initial search, 2523 were assessed for eligibility after removal of duplicates and inappropriate titles and abstracts, and 404 serological studies (representing tests in 5 168 360 individuals) were included in the meta-analysis. In the 82 studies of higher quality, close contacts (18•0%, 95% CI 15•7-20•3) and highrisk health-care workers (17•1%, 9•9-24•4) had higher seroprevalence than did low-risk health-care workers (4•2%, 1•5-6•9) and the general population (8•0%, 6•8-9•2). The heterogeneity between included studies was high, with an overall I² of 99•9% (p<0•0001). Seroprevalence varied greatly across WHO regions, with the lowest seroprevalence of general populations in the Western Pacific region (1•7%, 95% CI 0•0-5•0). The pooled infection-to-case ratio was similar between the region of the Americas (6•9, 95% CI 2•7-17•3) and the European region (8•4, 6•5-10•7), but higher in India (56•5, 28•5-112•0), the only country in the South-East Asia region with data.Interpretation Antibody-mediated herd immunity is far from being reached in most settings. Estimates of the ratio of serologically detected infections per virologically confirmed cases across WHO regions can help provide insights into the true proportion of the population infected from routine confirmation data.

Severe Fever with Thrombocytopenia Syndrome (SFTS), an emerging infectious disease caused by a novel phlebovirus, is associated with high fatality. Therapeutic interventions are lacking and disease pathogenesis is yet to be fully elucidated. The anti-viral immune response has been reported, but humoral involvement in viral pathogenesis is poorly understood. Here we show defective serological responses to SFTSV is associated with disease fatality and a combination of B-cell and T-cell impairment contribute to disruption of anti-viral immunity. The serological profile in deceased patients is characterized by absence of specific IgG to viral nucleocapsid and glycoprotein due to failure of B-cell class switching. Expansion and impairment of antibody secretion is a signature of fatal SFTSV infection. Apoptosis of monocytes in the early stage of infection diminishes antigen-presentation by dendritic cells, impedes differentiation and function of T follicular helper cells, and contributes to failure of the virus-specific humoral response.

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Recently emerged SARS-CoV-2 variants may pose a threat to immunity. A systematic landscape of neutralizing antibodies against emerging variants is needed. We systematically searched for studies that evaluated neutralizing antibodies titers induced by previous infection or vaccination against SARS-CoV-2 variants and collected individual data. We identified 106 studies meeting the eligibility criteria. Lineage B.1.351 (Beta), P.1 (Gamma) and B.1.617.2 (Delta) significantly escaped natural-infection-mediated neutralization, with an average of 4.1-fold (95% CI: 3.6-4.7), 1.8-fold (1.4-2.4), and 3.2-fold (2.4-4.1) reduction in live virus neutralization assay, while neutralizing titers against B.1.1.7 decreased slightly (1.4-fold, 95%CI: 1.2-1.6). Serum from vaccinees also led to significant reductions in neutralization of B.1.351 across different platforms, with an average of 7.1-fold (5.5-9.0) for non-replicating vector platform, 4.1-fold (3.7-4.4) for mRNA platform, and 2.5-fold (1.7-2.9) for protein subunit platform. Neutralizing antibodies levels induced by mRNA vaccines against SARS-CoV-2 variants were similar, or higher, than that derived from naturally-infected individuals.