1 Natural Antibodies and Severe Acute Respiratory Syndrome Coronavirus 2–Specific Antibodies in Healthy Asymptomatic Individuals TO THE EDITOR—Reports in this journal [1, 2] and others [3] have concluded with varying degrees of certainty that de- tection of specific immunoglobulin G (IgG) and immunoglobulin M (IgM) an- tibodies to severe acute respiratory syn- drome coronavirus 2 (SARS-CoV-2) spike and coat proteins in plasma of asymptomatic healthy individuals might be indicative of prior infection. They sug- gest that these data are consistent with there being individuals infected in late 2019 (September–December) and early 2020, prior to the main 2020 epidemic outbreaks. Data in Apolone et al [3], refer- ring to detection of IgG and IgM antibod- ies in (a small number of) asymptomatic individuals in Italy as early as September 2019, would date the appearance of coro- navirus disease 2019 (COVID-19) to a time that contradicts other molecular analyses, dating the earliest rapid evolu- tion of the virus to “the period between mid-October and mid-November 2019 as the plausible interval when the first case of SARS-CoV-2 emerged in Hubei province, China” [4]. How valid are these conclusions based on serology? In our opinion, all such claims are equivocal. In our collective experience [5–7], naturally arising antibodies at high titers (neutralizing antibodies [NAbs]; reviewed in Holodick et al [8]) are often detected in random patterns like this [1–3] in two main ways: 1. Through continuous endogenous self- antigen stimulation and clonal selec- tion by antigenic epitopes exposed to the normal immune system on “bur- ied” or “masked” self-components as they age and become effete—most are B cells secreting IgM expressing germline encoded variable (V) regions [8]. How such V element specificities become germline encoded in NAbs has been considered again recently by us [9]. Some, like the well-known anti- phosphorylcholine specificity, play de- monstrable roles in both protective im- munity to exogenous pathogens such as Streptococcus pneumoniae and also in internal disposal of effete apoptotic cells and oxidized lipids in cardiovas- cular repair [8]; and 2. Through the chance cross-stimulation and clonal selection by environmental and cross-reactive antigens on other pathogens—many of these NAbs could be both long-lived IgG and shorter half-life high-turnover IgM. In this regard the more conservative report based on IgG antibodies in a very small number of positive detec- tions (7 of 24 079 study participants in the United States) by Althoff et al [2] is more representative of the “norm” than Apolone et al [3], who reported 111 positives out of 959 study participants in Italy. We think it most likely that the data from Apolone et al reflect “background” levels of natural IgM antibodies and some lower-frequency, cross-reactive, exogenous induced long-lived IgG. The Italian study shows a random incidence pattern across the country supporting the hypothesis con- clusion that they are NAbs arising sponta- neously, without prior SARS CoV-2 infection, in asymptomatic humans. While this alternative conclusion is not mentioned by these authors, we con- cede they may have tacitly understood this to be the case. We predict the exis- tence of a predominantly IgM antibody reactivity pattern with lower-level IgG, as reported in Apolone et al [3] and in- cluding Basavaraju et al [1], for almost any randomly chosen antigen in plasma or serum of normal, healthy “asymptom- atic” individuals. Note Potential conflicts of interest. R. A. L. reports participation on a data safety monitoring board or advisory board and a significant number of shares in 3 entities, unrelated to this correspon- dence. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed. Edward J. Steele, 1 Reginald M. Gorczynski, 2 Robyn A. Lindley, 3,4 and N. Chandra Wickramasinghe 5 1 Melville Analytics Pty Ltd and Immunomics, Melbourne, Australia; 2 Institute of Medical Science, Department of Immunology and Surgery, University of Toronto, Canada; 3 Department of Clinical Pathology, Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne, Australia; 4 GMDxgen, Melbourne, Australia; and 5 National Institute of Fundamental Studies, Kandy, Sri Lanka References 1. Basavaraju SV, Patton ME, Grimm K, et al. Serologic testing of U.S. blood donations to identify SARS-CoV-2-reactive antibodies: December 2019– January 2020. Clin Infect Dis 2020; 72:e1004–9. 2. Althoff KN, Schlueter DJ, Anton-Culver H, et al. Antibodies to severe acute respiratory syndrome co- ronavirus 2 (SARS-CoV-2) in All of Us Research Program participants, 2 January to 18 March 2020. Clin Infect Dis 2022; 74:584–90. 3. Apolone G, Montomoli E, Manenti A, et al. Unexpected detection of SARS-CoV-2 antibodies in the prepan- demic period in Italy. Tumori 2021; 107:446–45. 4. Pekar J, Worobey M, Moshiri N, Scheffler K, Wertheim JO. Timing the SARS-CoV-2 index case in Hubei province. Science 2021; 372:412–7. 5. Steele EJ, Cunningham AJ. High proportion of Ig-producing cells making autoantibody in normal mice. Nature 1978; 274:483–4. 6. Cunningham AJ, Steele EJ. Ontogeny of the auto- immune reaction in normal mice to antigens in erythrocytes and gut. Clin Exp Immunol 1981; 44: 38–48. 7. Jonusys AM, Cox KO, Steele EJ. IgM natural autoan- tibodies against bromelain mouse red blood cells rec- ognize carbonic anhydrase. Autoimmunity 1991; 9: 207–16. 8. Holodick NE, Rodrıguez-Zhurbenko N, Hernandez AM. Defining natural antibodies. Front Immunol 2017; 8:872. 9. Steele EJ, Lindley RA. Germline V repertoires: origin, maintenance, diversification. Scand J Immunol 2018; 87:e12670. Correspondence: Edward J. Steele, PhD, Melville Analytics Pty Ltd and Immunomics, Melbourne, VIC, Australia, Unit 14/ 35A Grandview Grove, Prahran 3181 Victoria, Australia (e.j.steele@ bigpond.com). Clinical Infectious Diseases ® © The Author(s) 2023. Published by Oxford University Press on behalf of Infectious Diseases Society of America. All rights re- served. For permissions, please e-mail: journals.permission- s@oup.com https://doi.org/10.1093/cid/ciad004 CORRESPONDENCE • CID • 1 Clinical Infectious Diseases CORRESPONDENCE