Bivalent COVID-19 vaccines boost the capacity of pre-existing SARS-CoV-2-specific memory B cells to cross-recognize Omicron subvariants (preprint)

Bivalent COVID-19 vaccines comprising ancestral Wuhan-Hu-1 (WH1) and the Omicron BA.1 or BA.5 subvariant elicit enhanced serum antibody responses to emerging Omicron subvariants. We characterized the memory B-cell (Bmem) response following a fourth dose with a BA.1 or BA.5 bivalent vaccine, and compared the immunogenicity with a WH1 monovalent fourth dose. Healthcare workers previously immunized with mRNA or adenoviral vector monovalent vaccines were sampled before and one-month after a monovalent, BA.1 or BA.5 bivalent fourth dose COVID-19 vaccine. RBD-specific Bmem were quantified with an in-depth spectral flow cytometry panel including recombinant RBD proteins of the WH1, BA.1, BA.5, BQ.1.1, and XBB.1.5 variants. All recipients had slightly increased WH1 RBD-specific Bmem numbers. Recognition of Omicron subvariants was not enhanced following monovalent vaccination, while both bivalent vaccines significantly increased WH1 RBD-specific Bmem cross-recognition of all Omicron subvariants tested by flow cytometry. Thus, Omicron-based bivalent vaccines can improve recognition of descendent Omicron subvariants by pre-existing, WH1-specific Bmem, beyond that of a conventional, monovalent vaccine. This provides new insights into the capacity of variant-based mRNA booster vaccines to improve immune memory against emerging SARS-CoV-2 variants.

Third dose COVID-19 mRNA vaccine enhances IgG4 isotype switching and recognition of Omicron subvariants by memory B cells after with mRNA but not adenovirus priming (preprint)

Background: Booster vaccinations are recommended to improve protection against severe disease from SARS-CoV-2 infection. With primary vaccinations involving various adenoviral vector and mRNA-based formulations, it remains unclear if these differentially affect the immune response to booster doses. We here examined the effects of homologous (mRNA/mRNA) and heterologous (adenoviral vector/mRNA) vaccination on antibody and memory B cell (Bmem) responses against ancestral and Omicron subvariants. Methods: Healthy adults who received primary BNT162b2 (mRNA) (n=18) or ChAdOx1 (vector) (n=25) vaccination were sampled 1-month and 6-months after their 2nd and 3rd dose (homologous or heterologous) vaccination. Recombinant spike receptor-binding domain (RBD) proteins from ancestral, Omicron BA.2 and BA.5 variants were produced for ELISA-based serology, and tetramerized for immunophenotyping of RBD-specific Bmem. Results: Dose 3 boosters significantly increased ancestral RBD-specific plasma IgG and Bmem in both cohorts. Up to 80% of ancestral RBD-specific Bmem expressed IgG1+. IgG4+ Bmem were detectable after primary mRNA vaccination, and expanded significantly to 5-20% after dose 3, whereas heterologous boosting did not elicit IgG4+ Bmem. Recognition of Omicron BA.2 and BA.5 by ancestral RBD-specific plasma IgG increased from 20% to 60% after the 3rd dose in both cohorts. Reactivity of ancestral RBD-specific Bmem to Omicron BA.2 and BA.5 increased following a homologous booster from 40% to 60%, but not after a heterologous booster. Conclusion: A 3rd mRNA dose generates similarly robust serological and Bmem responses in homologous and heterologous vaccination groups. The expansion of IgG4+ Bmem after mRNA priming might result from the unique vaccine formulation or dosing schedule affecting the Bmem response duration and antibody maturation.

Lasting first impression: Pre-existing immunity restricts mucosal antibody responses during Omicron breakthrough (preprint)

Understanding mucosal antibody responses from SARS-CoV-2 infection and/or vaccination is crucial to develop strategies for longer term immunity, especially against emerging viral variants. We profiled serial paired mucosal and plasma antibodies from: COVID-19 vaccinated only vaccinees (vaccinated, uninfected), COVID-19 recovered vaccinees (convalescent, vaccinated) and individuals with breakthrough Delta or Omicron BA.2 infections (vaccinated, infected). Saliva from COVID-19 recovered vaccinees displayed improved antibody neutralizing activity, Fc{gamma}R engagement and IgA compared to COVID-19 uninfected vaccinees. Furthermore, repeated mRNA vaccination boosted SARS-CoV-2-specific IgG2 and IgG4 responses in both mucosa biofluids (saliva and tears) and plasma. IgG, but not IgA, responses to breakthrough COVID-19 variants were dampened and narrowed by increased pre-existing vaccine-induced immunity to the ancestral strain. Salivary antibodies delayed initiation of boosting following breakthrough COVID-19 infection, especially Omicron BA.2, however, rose rapidly thereafter. Our data highlight how pre-existing immunity shapes mucosal SARS-CoV-2-specific antibody responses and has implications for long-term protection from COVID-19.

COVID-19 adenoviral vector vaccination elicits a robust memory B cell response with the capacity to recognize Omicron BA.2 and BA.5 variants (preprint)

Following the COVID-19 pandemic caused by SARS-CoV-2, novel vaccines have successfully reduced severe disease and death. Despite eliciting lower antibody responses, adenoviral vector vaccines are nearly as effective as mRNA vaccines. Therefore, protection against severe disease may be mediated by immune memory cells. We here evaluated plasma antibody and memory B cells (Bmem) targeting the Spike receptor binding domain (RBD) elicited by the adenoviral vector vaccine ChAdOx1 (AstraZeneca), their capacity to bind Omicron subvariants, and compared this to the response elicited by the mRNA vaccine BNT162b2 (Pfizer-BioNTech). Whole blood was sampled from 31 healthy adults pre-vaccination, and four weeks after dose one and dose two of ChAdOx1. Neutralizing antibodies (NAb) against SARS-CoV-2 were quantified at each timepoint. Recombinant RBDs of the Wuhan-Hu-1 (WH1), Delta, BA.2, and BA.5 variants were produced for ELISA-based quantification of plasma IgG and incorporated separately into fluorescent tetramers for flow cytometric identification of RBD-specific Bmem. NAb and RBD-specific IgG levels were over eight times lower following ChAdOx1 vaccination than BNT162b2. In ChAdOx1-vaccinated individuals, median plasma IgG recognition of BA.2 and BA.5 as a proportion of WH1-specific IgG was 26% and 17%, respectively. All donors generated resting RBD-specific Bmem, which were boosted after the second dose of ChAdOx1, and were similar in number to those produced by BNT162b2. The second dose of ChAdOx1 boosted Bmem that recognized VoC, and 37% and 39% of WH1-specific Bmem recognized BA.2 and BA.5, respectively. These data uncover mechanisms by which ChAdOx1 elicits immune memory to confer effective protection against severe COVID-19.

Double-dose mRNA vaccination to SARS-CoV-2 progressively increases recognition of variants-of-concern by Spike RBD-specific memory B cells (preprint)

Background: SARS-CoV-2 vaccination with BNT162b2 (Pfizer BioNTech) has been shown to be 95% effective. Double-dose vaccination generates high levels of spike-specific antibodies, memory B cells (Bmem) and T cells. However, variants of concern (VoC) with mutations in the spike Receptor Binding Domain (RBD) can evade antibody responses. Booster vaccinations improve antibody recognition of VoC, but it is unclear if this is due to higher total antibodies or their capacity to bind VoC. We here addressed the capacity of surface Ig on single Wuhan-specific Bmem after first and second dose BNT162b2 vaccination to recognize variant RBD. Methods: Samples were collected from 30 healthy COVID-19 naive individuals pre-BNT162b2 vaccination, 3 weeks post-dose 1 and 4-weeks post-dose 2. Plasma antibodies and Bmem were evaluated using recombinant RBD proteins of the Wuhan, Gamma and Delta strains. Results: All individuals generated a robust antibody response to BNT162b2 vaccination with all participants producing neutralizing antibodies following dose 2. IgM+ and IgG+ RBD-specific Bmem were generated after one vaccine dose, and those expressing IgG1 increased in absolute number after dose 2. The majority of RBD-specific Bmem bound the Gamma and/or Delta variants, and this proportion significantly increased after the second dose. Conclusion: The second dose of BNT162b2 increases the number of circulating Ig-class switched RBD-specific Bmem. Importantly, the second dose of vaccination is required for a high frequency of RBD-specific Bmem to recognize Gamma and Delta variants. This suggests that dose 2 not only increases the number of RBD-specific Bmem but also the affinity of the Bmem to overcome the point mutations in VoC.

Altered affinity to ACE2 and reduced Fc functional antibodies to SARS-CoV-2 RBD variants (preprint)

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants remains a formidable challenge to worldwide public health. The receptor binding domain (RBD) of the SARS-CoV-2 spike protein is a hotspot for mutations, reflecting its critical role at the ACE2 interface during viral entry. We comprehensively investigated the impact of RBD mutations, including 6 variants of concern (VOC) or interest (Alpha, Beta, Gamma, Delta, Kappa and Omicron) and 33 common point mutations, on IgG recognition, Fc{gamma}R-engagement, and ACE2-binding inhibition in plasma from BNT162b2-vaccine recipients (two-weeks following second dose) and mild-to-moderate COVID-19 convalescent subjects using our custom bead-based 39-plex array. We observed that IgG-recognition and Fc{gamma}R-binding antibodies were most profoundly decreased against Beta and Omicron RBDs, as well as point mutations G446S, found in Omicron, and N501T, a key mutation found in animal adapted SARS-CoV-2 viruses. Measurement of RBD-ACE2 binding affinity via Biolayer Interferometry showed all VOC RBDs have enhanced affinity to human ACE2. Furthermore we demonstrate that human ACE2 polymorphisms, E35K (rs1348114695), K26R (rs4646116) and S19P (rs73635825), have altered binding kinetics to the RBD of VOCs potentially affecting virus-host interaction and thereby host susceptibility.

A point-of-care lateral flow assay for neutralising antibodies against SARS-CoV-2 (preprint)

As vaccines against SARS-CoV-2 are now being rolled out, a better understanding of immunity to the virus; whether through infection, or passive or active immunisation, and the durability of this protection is required. This will benefit from the ability to measure SARS-CoV-2 immunity, ideally with rapid turnaround and without the need for laboratory-based testing. Current rapid point-of-care (POC) tests measure antibodies (Ab) against the SARS-CoV-2 virus, however, these tests provide no information on whether the antibodies can neutralise virus infectivity and are potentially protective, especially against newly emerging variants of the virus. Neutralising Antibodies (NAb) are emerging as a strong correlate of protection, but most current NAb assays require many hours or days, samples of venous blood, and access to laboratory facilities, which is especially problematic in resource-limited settings. We have developed a lateral flow POC test that can measure levels of RBD-ACE2 neutralising antibodies from whole blood, with a result that can be determined by eye (semi-quantitative) or on a small instrument (quantitative), and results show high correlation with microneutralisation assays. This assay also provides a measure of total anti-RBD antibody, thereby providing evidence of exposure to SARS-CoV-2 or immunisation, regardless of whether NAb are present in the sample. By testing samples from immunised macaques, we demonstrate that this test is equally applicable for use with animal samples, and we show that this assay is readily adaptable to test for immunity to newly emerging SARS-CoV-2 variants. Lastly, using a cohort of vaccinated humans, we demonstrate that our whole-blood test correlates closely with microneutralisation assay data (R 2 =0.75, p<0.0001), and that fingerprick whole blood samples are sufficient for this test. Accordingly, the COVID-19 NAb-test™ device described here can provide a rapid readout of immunity to SARS-CoV-2 at the point of care.

Systems serology detects functionally distinct coronavirus antibody features in children and elderly.

The hallmarks of COVID-19 are higher pathogenicity and mortality in the elderly compared to children. Examining baseline SARS-CoV-2 cross-reactive immunological responses, induced by circulating human coronaviruses (hCoVs), is needed to understand such divergent clinical outcomes. Here we show analysis of coronavirus antibody responses of pre-pandemic healthy children (n = 89), adults (n = 98), elderly (n = 57), and COVID-19 patients (n = 50) by systems serology. Moderate levels of cross-reactive, but non-neutralizing, SARS-CoV-2 antibodies are detected in pre-pandemic healthy individuals. SARS-CoV-2 antigen-specific Fcγ receptor binding accurately distinguishes COVID-19 patients from healthy individuals, suggesting that SARS-CoV-2 infection induces qualitative changes to antibody Fc, enhancing Fcγ receptor engagement. Higher cross-reactive SARS-CoV-2 IgA and IgG are observed in healthy elderly, while healthy children display elevated SARS-CoV-2 IgM, suggesting that children have fewer hCoV exposures, resulting in less-experienced but more polyreactive humoral immunity. Age-dependent analysis of COVID-19 patients, confirms elevated class-switched antibodies in elderly, while children have stronger Fc responses which we demonstrate are functionally different. These insights will inform COVID-19 vaccination strategies, improved serological diagnostics and therapeutics.

First Report of a Phase 1 Randomised Trial of Molecular Clamp-Stabilised Spike Protein-Based and MF59-Adjuvanted Vaccine for SARS-CoV-2 (preprint)

Background: We assessed the safety and immunogenicity of an MF59-adjuvanted subunit vaccine for COVID-19 based on recombinant SARS-CoV-2 spike glycoprotein stabilised in a prefusion conformation by a novel molecular clamp (Sclamp).Methods: Phase 1, double-blind, placebo-controlled trial conducted in Australia (July 2020–ongoing; ClinicalTrials.gov NCT04495933). Healthy adults (18-55 years) received two doses of placebo, 5-μg, 15-μg, or 45-μg SARS-CoV-2 Sclamp, or one 45-μg dose of SARS-CoV-2 Sclamp followed by placebo, 28 days apart (n=120; 24 per group). Safety, humoral immunogenicity (ELISA, microneutralisation, pseudovirus neutralisation), and cellular immunogenicity (antigen-specific CD4+/CD8+ T-cells, antibody-secreting cells) were assessed up to 56 days after the first dose.Findings: The SARS-CoV-2 Sclamp vaccine was very well tolerated with few systemic reactions. All two-dose regimens elicited robust, broadly neutralising humoral responses. Geometric mean titres were higher than in sera from convalescent COVID-19 patients and strongly neutralised spike variants of concern, including N501Y. Moreover, humoral and cellular responses were highly correlated. However, antibodies elicited to a peptide sequence used in the molecular clamp derived from human immunodeficiency virus-1 (HIV-1) gp41 cross-reacted weakly with some HIV diagnostic screening tests.Interpretation: These first-in-human results demonstrate that a subunit vaccine comprising mammalian cell culture-derived, molecular clamp-stabilised recombinant spike protein formulated in a squalene-in-oil adjuvant elicits strong immune responses with an excellent safety profile. However, the gp41 peptide induced diagnostic interference, creates a likely barrier to widespread use and highlights the criticality of potential off-target immunogenicity during vaccine development. Studies are ongoing with alternative molecular clamp trimerisation domains to ameliorate this response.Clinical Trial Registration: ClinicalTrials.gov (NCT04495933).Funding: Coalition for Epidemic Preparedness Innovations; National Health and Medical Research Council, Queensland Government, and philanthropic sources.Declaration of Interests: KJC and DW report grants from the Coalition for Epidemic Preparedness Innovations, the National Health and Medical Research Council of Australia, and the Queensland Government, during the conduct of the study; other from ViceBio Limited, outside the submitted work; and has patents pending (AU 2018241252; BR112019019813.0; CA 3057171; CH 201880022016.9; EP 18775234.0; IN 201917038666; ID P00201909145; IL 269534; JP 2019-553883; MX/a/2019/011599; NZ 757178; KR 0-2019-7031415; SG 11201908280S; US 16/498865). JB reports personal fees from CSL Limited, during the conduct of the study, and other from CSL Limited, outside the submitted work. WZ reports grants from the National Health and Medical Research Council of Australia, the Research Grants Council of the Hong Kong Special Administrative Region, China, and the Jack Ma Foundation, during the conduct of the study. SM-H reports grants from Canarian Foundation Doctor Manuel Morales, during the conduct of the study. KJS reports grants from the the Australian Medical Research Future Fund, during the conduct of the study. AWC reports grants from the Australian Medical Research Future Fund and a National Health and Medical Research Council of Australia Career Development Fellowship, during the conduct of the study. BDW reports grants from the National Health and Medical Research Council of Australia, the Australian Medical Research Future Fund, and the Victorian State Government, during the conduct of the study. PMH reports grants from the Australian Medical Research Future Fund, during the conduct of the study. DP reports grants from the National Health and Medical Research Council of Australia, the A2 Milk Foundation, and the Jack Ma Foundation, during the conduct of the study. CR reports grants from the Coalition for Epidemic Preparedness Innovations, during the conduct of the study. PRY reports grants from the Coalition for Epidemic Preparedness Innovations, the National Health and Medical Research Council of Australia, and the Queensland Government, during the conduct of the study; grants from ViceBio Limited, outside the submitted work; and a patent issued (US 2020/0040042). FLM, Zl, DKW, PE, JAL, STMC, NM, SA, CLH, KH, PG, LH, THON, MHT, PT, JB, PCR, SN, SC, TH, KK, KS, and TPM have nothing to disclose.Ethics Approval Statement: The protocol was approved by the Alfred Health Human Research Ethics Committee (2020001376/334/20).

Decay of Fc-dependent antibody functions after mild to moderate COVID-19 (preprint)

The capacity of antibodies to engage with innate and adaptive immune cells via the Fc region is important in preventing and controlling many infectious diseases, and is likely critical in SARS-CoV-2 infection. The evolution of such antibodies during convalescence from COVID-19 is largely unknown. We developed novel assays to measure Fc-dependent antibody functions against SARS-CoV-2 spike (S)-expressing cells in serial samples from a cohort of 53 subjects primarily with mild-moderate COVID-19, out to a maximum of 149 days post-infection. We found that S-specific antibodies capable of engaging dimeric FcγRIIa and FcγRIIIa decayed linearly over time. S-specific antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent phagocytosis (ADP) activity within plasma declined linearly as well, in line with the decay of S-specific IgG. Although there was significant decay in S-specific plasma ADCC and ADP activity, they remained readily detectable by all assays in 94% of our cohort at the last timepoint studied, in contrast with neutralisation activity which was only detectable in 70% of our cohort by the last timepoint. Our results suggest that Fc effector functions such as ADCC and ADP could contribute to the durability of SARS-CoV-2 immunity, particularly late in convalescence when neutralising antibodies have waned. Understanding the protective potential of antibody Fc effector functions is critical for defining the durability of immunity generated by infection or vaccination.

Rapid and lasting generation of B-cell memory to SARS-CoV-2 spike and nucleocapsid proteins in COVID-19 disease and convalescence (preprint)

BackgroundLasting immunity to SARS-CoV-2 following infection is questioned because serum antibodies decline in convalescence. However, functional immunity is mediated by long-lived memory T and B (Bmem) cells. ObjectiveTo determine the longevity and immunophenotype of SARS-CoV-2-specific Bmem cells in COVID-19 patients. MethodsRecombinant spike receptor binding domain (RBD) and nucleocapsid protein (NCP) were produced for ELISA-based serology, and biotinylated for fluorescent tetramer generation to identify SARS-CoV-2-specific Bmem cells by flow cytometry with a panel of 13 mAbs. 36 blood samples were obtained from 25 COVID-19 patients (11 paired) between 4-242 days post-symptom onset for detection of neutralizing antibodies, IgG serology and flow cytometry. ResultsThe recombinant RBD and NCP were specifically recognized by serum IgG in all patients and reactivity declined >20 days post-symptom onset. All patients had detectable RBD- and NCP-specific Bmem cells at 8.23-267.6 cells/ml of blood (0.004-0.13% of B cells) regardless of sampling time. RBD- and NCP-specific Bmem cells predominantly expressed IgM or IgG1, with the latter formed slightly later than the former. RBD-specific IgG+ Bmem were predominantly CD27+, and numbers significantly correlated with circulating follicular helper T cell numbers. ConclusionRBD- and NCP-specific Bmem cells persisted for 8 months, indicating that the decline in serum antibodies after 1 month does not indicate waning of immunity but a contraction of the immune response. Flowcytometric detection of SARS-CoV-2-specific Bmem cells enables detection of long-term functional immunity following infection or vaccination for COVID-19.

Plasma ACE2 activity is persistently elevated following SARS-CoV-2 infection: implications for COVID-19 pathogenesis and consequences (preprint)

COVID-19 causes persistent endothelial inflammation, lung and cardiovascular complications. SARS-CoV-2 utilises the catalytic site of full-length membrane-bound angiotensin converting enzyme 2 (ACE2) for cell entry causing downregulation of tissue ACE2. We reported downregulation of cardiac ACE2 is associated with increased plasma ACE2 activity. In this prospective observational study in recovered COVID-19 patients, we hypothesised that SARS-CoV-2 infection would be associated with shedding of ACE2 from cell membranes and increased plasma ACE2 activity. MethodsWe measured plasma ACE2 catalytic activity using a validated, sensitive quenched fluorescent substrate-based assay in a cohort of Australians aged [≥]18 years (n=66) who had recovered from mild, moderate or severe SARS-CoV-2 infection (positive result by PCR testing) and age and gender matched uninfected controls (n=70). Serial samples were available in 23 recovered SARS-CoV-2 patients. ResultsPlasma ACE2 activity at a median of 35 days post-infection [interquartile range 30-38 days] was 97-fold higher in recovered SARS-CoV-2 patients compared to controls (5.8 [2-11.3] vs. 0.06 [0.02-2.2] pmol/min/ml, p<0.0001). There was a significant difference in plasma ACE2 activity according to disease severity (p=0.033), with severe COVID-19 associated with higher ACE2 activity compared to mild disease (p=0.027). Men (n=39) who were SARS-CoV-2 positive had higher median plasma ACE2 levels compared to women (n=27) (p<0.0001). We next analysed whether an elevated plasma ACE2 activity level persisted following SARS-CoV-2 infection in subjects with blood samples at 63 [56-65] and 114 [111-125] days post infection. Plasma ACE2 activity remained persistently elevated in almost all subjects, with no significant differences between timepoints in post-hoc comparisons (p>0.05). DiscussionThis is the first description that plasma ACE2 activity is elevated after COVID-19 infection, and the first with longitudinal data indicating plasma ACE2 activity remains elevated out to a median of 114 days post-infection. Larger studies are now needed to determine if persistent elevated plasma ACE2 activity identifies people at risk of prolonged illness following COVID-19.

Harnessing the immune system via FcγR function in immune therapy: a pathway to next-gen mAbs.

The human fragment crystallizable (Fc)γ receptor (R) interacts with antigen-complexed immunoglobulin (Ig)G ligands to both activate and modulate a powerful network of inflammatory host-protective effector functions that are key to the normal physiology of immune resistance to pathogens. More than 100 therapeutic monoclonal antibodies (mAbs) are approved or in late stage clinical trials, many of which harness the potent FcγR-mediated effector systems to varying degrees. This is most evident for antibodies targeting cancer cells inducing antibody-dependent killing or phagocytosis but is also true to some degree for the mAbs that neutralize or remove small macromolecules such as cytokines or other Igs. The use of mAb therapeutics has also revealed a "scaffolding" role for FcγR which, in different contexts, may either underpin the therapeutic mAb action such as immune agonism or trigger catastrophic adverse effects. The still unmet therapeutic need in many cancers, inflammatory diseases or emerging infections such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) requires increased effort on the development of improved and novel mAbs. A more mature appreciation of the immunobiology of individual FcγR function and the complexity of the relationships between FcγRs and antibodies is fueling efforts to develop more potent "next-gen" therapeutic antibodies. Such development strategies now include focused glycan or protein engineering of the Fc to increase affinity and/or tailor specificity for selective engagement of individual activating FcγRs or the inhibitory FcγRIIb or alternatively, for the ablation of FcγR interaction altogether. This review touches on recent aspects of FcγR and IgG immunobiology and its relationship with the present and future actions of therapeutic mAbs.

Distinct systems serology features in children, elderly and COVID patients (preprint)

SARS-CoV-2, the pandemic coronavirus that causes COVID-19, has infected millions worldwide, causing unparalleled social and economic disruptions. COVID-19 results in higher pathogenicity and mortality in the elderly compared to children. Examining baseline SARS-CoV-2 cross-reactive coronavirus immunological responses, induced by circulating human coronaviruses, is critical to understand such divergent clinical outcomes. The cross-reactivity of coronavirus antibody responses of healthy children (n=89), adults (n=98), elderly (n=57), and COVID-19 patients (n=19) were analysed by systems serology. While moderate levels of cross-reactive SARS-CoV-2 IgG, IgM, and IgA were detected in healthy individuals, we identified serological signatures associated with SARS-CoV-2 antigen-specific Fc{gamma} receptor binding, which accurately distinguished COVID-19 patients from healthy individuals and suggested that SARS-CoV-2 induces qualitative changes to antibody Fc upon infection, enhancing Fc{gamma} receptor engagement. Vastly different serological signatures were observed between healthy children and elderly, with markedly higher cross-reactive SARS-CoV-2 IgA and IgG observed in elderly, whereas children displayed elevated SARS-CoV-2 IgM, including receptor binding domain-specific IgM with higher avidity. These results suggest that less-experienced humoral immunity associated with higher IgM, as observed in children, may have the potential to induce more potent antibodies upon SARS-CoV-2 infection. These key insights will inform COVID-19 vaccination strategies, improved serological diagnostics and therapeutics.