key: cord-0810987-gbi4e0n2
authors: Post, N.; Eddy, D.; Huntley, C.; van Schalkwyk, M. C. I.; Shrotri, M.; Leeman, D.; Rigby, S.; Williams, S. V.; Bermingham, W. H.; Kellam, P.; Maher, J.; Shields, A. M.; Amirthalingam, G.; Peacock, S. J.; Ismail, S. A.
title: Antibody response to SARS-CoV-2 infection in humans: a systematic review
date: 2020-08-30
journal: nan
DOI: 10.1101/2020.08.25.20178806
sha: 303be24ff41741649696c44ab7f5a703b8a9c155
doc_id: 810987
cord_uid: gbi4e0n2

Introduction Progress in characterising the humoral immune response to Severe Acute Respiratory Syndrome 2 (SARS-CoV-2) has been rapid but areas of uncertainty persist. This review comprehensively evaluated evidence describing the antibody response to SARS-CoV-2 published from 01/01/2020-26/06/2020. Methods Systematic review. Keyword-structured searches were carried out in MEDLINE, Embase and COVID-19 Primer. Articles were independently screened on title, abstract and full text by two researchers, with arbitration of disagreements. Data were double-extracted into a pre-designed template, and studies critically appraised using a modified version of the MetaQAT tool, with resolution of disagreements by consensus. Findings were narratively synthesised. Results 150 papers were included. Most studies (75%) were observational in design, and included papers were generally of moderate quality based on hospitalised patients. Few considered mild or asymptomatic infection. Antibody dynamics were well described in the acute phase, and up to around 3 months from disease onset, although inconsistencies remain concerning clinical correlates. Development of neutralising antibodies following SARS-CoV-2 infection is typical, although titres may be low. Specific and potent neutralising antibodies have been isolated from convalescent plasma. Cross reactivity but limited cross neutralisation occurs with other HCoVs. Evidence for protective immunity in vivo is limited to small, short-term animal studies, which show promising initial results in the immediate recovery phase. Interpretation Published literature on immune responses to SARS-CoV-2 is of variable quality with considerable heterogeneity with regard to methods, study participants, outcomes measured and assays used. Antibody dynamics have been evaluated thoroughly in the acute phase but longer follow up and a comprehensive assessment of the role of demographic characteristics and disease severity is needed. The role of protective neutralising antibodies is emerging, with implications for therapeutics and vaccines. Large, cross-national cohort studies using appropriate statistical analysis and standardised serological assays and clinical classifications should be prioritised.

Immunity, antibody, IgG, IgM, neutralising antibody, cross-reactivity, SARS-CoV-2, COVID-19 Experience with other human coronavirus species (HCoV) suggests that partial immunity arises following infection with a variable but generally short (1 to 2 year) duration. 7 Limited data available for the closely related SARS-CoV-1 indicate that antibodies able to block viral infection (neutralising antibodies) may persist for up to 17 years following infection. 8 Early clinical studies suggest that the dynamics of antibody response following acute infection with SARS-CoV-2 is similar to other HCoVs. Antibody responses are generally detected against the nucleocapsid (N) or spike (S) proteins, the S1 subunit of which contains the receptor-binding domain (RBD): antibodies against different antigens may have differential dynamics and neutralising effect. The presence of neutralising antibodies has been demonstrated in studies of vaccine research and therapeutic use of convalescent plasma. 7, 9 Previous lessons from Severe Acute Respiratory Syndrome (SARS-CoV-1), Middle Eastern Respiratory Syndrome (MERS-All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08. 25.20178806 doi: medRxiv preprint CoV) epidemics and other seasonal human coronaviruses suggest that there is the potential for a decline in population level protection from reinfection over a short period of time, but this is somewhat dependent on initial disease severity. 7, 9 Neutralising antibodies (nAbs) are likely to be a key metric for protection against infection by viruses such as SARS-CoV-2. However, their dynamics and role in long-term population immunity are not well understood. 7 Furthermore, understanding of the mechanistic correlates of protective immunity in humans remains limited, including the antibody titre and specificity required to confer protection. 10 This is the first of two linked papers reporting results from a systematic review of peer-reviewed and pre-print literature on the immune response to SARS-CoV-2 infection. 11 This paper has three aims. Firstly, to characterise the antibody response to SARS-CoV-2 infection over time and explore the effects of potential correlates of immune activity (including age, time since symptom onset, clinical severity and ethnicity) on the nature of this response. Secondly, to consider relationships between these variables and indirect or relative quantification of antibodies to SARS-CoV-2. Thirdly, to consider the duration of post-infection immunity conferred by the antibody response.

This systematic review was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The protocol was pre-registered with PROSPERO (CRD42020192528). correspondence pieces or letter responses, consensus statements or guidelines and study protocols.

We focused on studies reporting measured titres (total antibody, IgA, IgG and/or IgM) with followup duration of greater than 28 days (which we defined as the limit of the acute phase of illness).

Shorter follow-up studies were included if they reported on protective immunity, or immune response correlates. We defined "correlates" as encompassing, among other factors: primary illness severity -proxied by the WHO's distinction between "mild", "moderate", "severe" and "critical" illness; 13 subject age; gender; the presence of intercurrent or co-morbid disease e.g. diabetes, cardiovascular and/or chronic respiratory disease; and ethnicity.

Studies were independently screened for inclusion on title, abstract and full text by two members of the research team (working across 4 pairs), with arbitration of disagreements by one review lead.

Data extraction, assessment of study quality, and data synthesis Data were extracted in duplicate from each included study. Extraction was performed directly into a dedicated Excel template (Supplementary Appendix B) . Pre-prints of subsequently published peer reviewed papers were included and results extracted where substantial differences in reported data were identified; if little difference was observed only the peer-reviewed version was retained.

Critical appraisal for each included study was performed in duplicate using a version of the MetaQAT 1.0 tool, adapted for improved applicability to basic science and laboratory-based studies. MetaQAT was selected for its simplicity and versatility in application to studies of all design types. 14 Principal adaptations to the MetaQAT tool are described in Supplementary Appendix C.

The adapted MetaQAT tool was used to gather both qualitative feedback on study quality, and scaled responses (yes/no/unclear) for answers to key questions around study reliability, internal and external validity, and applicability, among other fields. Scaled responses were converted into weighted scores for each paper. Accordingly, studies were assigned a "high", "medium" or "low" quality grading. All rights reserved. No reuse allowed without permission.

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The PRISMA flowchart for the review is given in figure 1.

150 studies were included, of which 108 (72%) contained data pertaining to antibody response, and 70 (47%) to protective immunity (descriptive statistics for included studies are given in table   1 ). The vast majority focused on hospitalised patients (i.e. higher severity disease). Eleven studies considered antibody responses in asymptomatic individuals in the community and only five investigated protective immunity in this group. Most studies were of moderate quality. Assays used to detect and quantify antibody response were diverse, with target antigens including spike (S), S1 and S2 subunits, receptor binding domain (RBD) and nucleocapsid (N). Details of assays used, and an overview of strengths and limitations of these is provided in Supplementary Appendix D.

The majority of individuals in the included studies mounted a SARS-CoV-2-specific antibody response during the acute phase of illness, with many studies reporting 100% seroconversion.

Overall seroconversion rates depended on the timepoint at which testing was conducted in the disease course, the populations under study, the serology assay platforms used and their specific target proteins. Studies considered time to seropositivity for total antibody and/or individual antibody classes (IgA/IgG/IgM) (figure 2), although this was often not clearly defined with respect to symptom onset or first positive PCR test. In addition, whilst some studies described specific target proteins of assays used, others were either non-specific or not described. This limited assessment of dynamics of antibodies against specific viral targets, in particular anti-N versus anti-S, the latter of which may be more closely related to protection. All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint A number of studies reported seroconversion for total antibody (combined IgG, IgM and/or IgA), [15] [16] [17] [18] [19] [20] [21] however the focus of findings presented is for specific antibody isotypes. For IgG, mean or median time to seroconversion ranged from 12 -15 days post symptom onset, 7, 9, 15, [22] [23] [24] [25] [26] with wide variation in first to last detection of IgG from 4 -73 days post symptom onset although reporting methods varied by study. 15, [27] [28] [29] [30] [31] [32] [33] For IgM, mean or median time to seroconversion ranged from 4-14 days post symptom onset, 7, 9, 15, [22] [23] [24] 26, 31, 34 again with variations in reporting methods, study quality, and sample size giving rise to uncertainty around findings. Time to seroconversion for IgA was measured in fewer studies, ranging from 4 -24 days post symptom onset, although most were within 4-11 days, 23, 35, 36 with some outliers, including two reports of 24 days to first detection. 37,38

In line with the expected sequential appearance of antibody isotypes, the majority of studies reported detection of IgM followed by IgG. 15, 23, 39, 40 Nevertheless, this finding was not consistent across all studies. One study measured time to seroconversion for IgA, IgM, and IgG and demonstrated detection of IgA and IgM simultaneously, followed by IgG. 23 . One study detected IgG seroconversion in advance of IgM, 26 and a study involving African green monkeys reported simultaneous IgM and IgG responses. 41 These disparities may reflect the use of differing antibody assays across a range of species and without standardisation.

IgG dynamics appeared to follow a pattern of peak, plateau, and persistence at lower levels (figure 3). After appearance, IgG titres rose to a peak between three and seven weeks post symptom onset, 7, 23, 30, [42] [43] [44] [45] [46] [47] [48] with studies recording the presence of IgG in and beyond weeks four, 40,49 five, 50 six, 23,51 seven, 52,53 and eight 17,45,54-56 post symptom onset. Some studies reported a plateau in virus-specific IgG beyond week three but levels beyond the peak were not well described. 32,57-59 A decrease in antibody levels was reported in the eighth week post symptom onset by two studies, 17,38 while another reported a decline from the second month after symptom onset. 58 Evidence from a cohort of 40 UK patients suggests a decline in titres after eight weeks, 58 although persistence of virus-specific IgG has been described at varying levels up to 12 weeks post symptom onset, 43 the longest follow up period among included studies. Dates of last detection were limited by the length of the study follow-up period, rather than confirmation of disappearance of detectable antibody titres. All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint IgM dynamics follow a 'rise and fall' pattern, with a peak two to five weeks post symptom onset 7, 26, 30, 34, 43, 46 ,47,53,60,61 then decline over time to below the detection limit. 38,43,62 Beyond the peak, IgM is consistently reported to decrease from as early as two to three weeks, 53,61 to as late as eight weeks 55 post symptom onset, with the majority of studies reporting this decline to occur at between three to five weeks. 40,43,62,63 Virus-specific IgM became undetectable in almost all cases by around six weeks after disease onset in two small but high quality cohort studies. 53, 64 Fewer studies describe IgA dynamics compared to IgM or IgG. IgA levels are reported to peak between 16 -22 days post symptom onset, although there is no consensus on trends over time. 23, 61 

Key findings regarding correlates of the antibody response to SARS-CoV-2 infection are summarised in table 2. Included papers addressed clinical factors (disease severity, co-morbid disease status and symptom profile) and demographic factors (age, sex and ethnicity) although results for many of these factors were conflicting or inconclusive. Across all papers, the definitions of comparator groups were highly variable, including disease severity classifications (severe/mild), outcomes (deceased/mild), and treatment categories (ICU/Non-ICU). The lack of consistency in methods, comparison groups and study design means it is not possible to determine whether or how disease severity affects, or is affected by, the antibody response. Most studies showed no association between antibody response and age or sex, and, when taken together, studies that did show associations had inconclusive results and lacked statistical analysis to relate these findings to disease severity. There were virtually no data to describe the immune response according to ethnicity.

Across the included studies, the majority of subjects developed detectable neutralising antibodies in response to SARS-CoV-2 infection in both human 7, 18, 22, 29, and animal 41,89-92 participants.

However, neutralising antibody titres were low in a substantial minority of participants. A high All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint quality cohort study found almost all participants (94%, n=19) generated an antibody response capable of neutralising 42-99% of pseudovirus in a carefully validated assay 14 days after exposure. 83 Another high-quality cohort study also found most patients (91%, n=22) developed a neutralising antibody response by 21 days after disease onset. 84 However only three quarters developed titres over 1:80. A larger case-control study including a sample of largely nonhospitalised convalescent patients demonstrated most participants (79%, n=149) had low neutralising antibody titres (<1:1000) after an average of 39 days following disease onset, while only 3% showed titres >1:5000. 87 Notably, RBD-specific antibodies with potent antiviral activity

were found in all individuals tested, suggesting specific neutralising antibodies are produced following infection despite low overall plasma neutralising ability. 87 Neutralising antibodies were generally detectable between 7 -15 days following disease onset, 7, 18, 75, 84, 85, 88, 93, 94 increasing over days 14 -22 before plateauing 22, 68, 69, 88, 93, 94 and declining over a period of six weeks. 69, 85, 88, 95 Evidence from one medium-quality pre-print study suggests neutralising antibody titres reduced significantly among 27 convalescent patients around six weeks following disease onset to a mean neutralisation half maximum inhibitory dilution (ID50) of 596. 51 A second medium quality preprint found neutralising antibodies became undetectable in four of 11 previously detectable cases. 85 Further high-quality evidence is required to fully evaluate the apparent waning of the neutralising antibody response over time. There were no high-quality studies investigating the dynamics of protective immunity over time in a cohort identified in this review. To date no studies have determined neutralising titres in upper respiratory tract samples.

Clinical and demographic correlates of the neutralising antibody response are described in table   3 . Neutralising antibody responses correlated with disease severity in all studies in which this association was tested. 7, 43, 49, 66, 76, 85, 87, [96] [97] [98] Importantly, the few studies that investigated asymptomatic cases found those individuals were considerably less likely to develop detectable serum neutralising antibody responses than cases with symptoms. With regard to age and sex, evidence was mixed and a limitation across all papers was a lack of statistical adjustment for severity.

The level of neutralisation was found to correlate with a wide range of specific antibodies. Most studies, including all those considered high quality, suggested that neutralisation ability broadly correlated with total virus-specific IgG. 29, 49, 67, 74, 87, [99] [100] [101] Specifically, high quality studies found that neutralisation ability correlated positively with anti-S IgG 49,72,87,99 or anti-RBD IgG. 72, 74, 87, 102 There was more limited evidence for correlation with anti-RBD IgM, including one high quality study, 51, 84 and IgA. 93, 99 A number of basic science studies also identified specific neutralising antibodies. The majority of these studies were medium quality, and heterogeneity between assays limits comparability of findings. A high quality study by Rogers et al highlighted the important role of RBD binding antibodies in neutralisation in a pseudovirus assay, with findings supported by an effective animal re-challenge model. 103 This study also reported that SARS-CoV-2 infection elicited a strong response against the S protein. However, few of these antibodies were neutralising, in agreement with other results. 104,105 RBD-specific antibodies were also shown to have potent neutralising activity in a range of other small studies, 70, 104, [106] [107] [108] [109] [110] [111] [112] including one using an IgA isotype. 113 Neutralising ability correlated in particular with competition for the angiotensin converting enzyme -2 (ACE2) receptor. 70, 72, 106 Two studies demonstrated a lack of association with affinity, 73, 106 although a moderate correlation with binding affinity was reported in one study. 107 Potently neutralising N specific antibodies were isolated in other studies, 73, 109 and the potential for antibodies binding to protease cleavage sites as alternatives to RBD isolated from convalescent plasma has also been identified, 114 115 Strong convergence of response emerged across different participants, which was judged to be associated with disease severity, and these findings were consistent with another high quality study. 87 

Several studies investigated the relationship between SARS-CoV-2-specific IgG and viral load 116, 117 or the co-existence of antibodies and viral RNA. 15, 24, 25, 38, 42, 46, 62, 64 In a large cohort study, the presence of SARS-CoV-2 anti-N IgG was significantly correlated with reduced viral load (measured as cycle threshold (Ct) >22, which was also associated with lower mortality). 116 This was consistent with a study by To et al which correlated increasing anti-N IgG titres with All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint decreasing median viral load from 6.7 to 4.9 log10 copies per mL between weeks one to three. 117 Another high-quality cohort study had similar findings but did not quantify viral load. 24 Together these findings suggest the persistence of detectable RNA despite clinical recovery, and although viral loads generally reduced in the convalescent phase, co-existence of SARS-CoV-2 specific IgG and detectable SARS-CoV-2 RNA could be identified in a small number of patients for up to 50 days following seroconversion. 25 Other studies were mixed, with one finding higher levels of specific antibodies correlated with viral clearance within 22 days, 38 and another finding weaker IgG response correlated with viral clearance within seven days after antibodies become detectable, 42 although both of these findings are subject to a number of limitations. Importantly, one included study attempted to associate re-detection of viral RNA with the presence of specific antibodies, finding that IgG titres began to decrease immediately following recovery although this was not associated with whether RNA was re-detected. Across all included studies, high quality evidence for re-infection or lasting immunity was lacking.

Studies exploring re-exposure to SARS-CoV-2 virus were limited to seven animal studies of variable quality. Broadly, two areas were explored; exposure following a primary infection with SARS-CoV-2 [89] [90] [91] 118 and re-exposure following passive transfer of neutralising antibodies. 92, 103 Following primary infection, timing of re-challenge varied between 20-43 days post inoculation.

All studies but one 90 demonstrated some level of protection from reinfection with a high-quality study in nine macaques showing a significant reduction in viral titres (p<0.00001) and reduced clinical symptoms. 89 Similar findings were reported in a hamster model. 92, 118 In a smaller ferret study, clinical findings following reinfection were mixed with the re-challenged group demonstrating increased weight loss compared to naive ferrets. However, the authors acknowledged that the sample size (n=4) was too small to draw wider inference. 90 Two studies examined protection from SARS-CoV-2 infection following the passive transfer of neutralising antibodies in Syrian hamster models. 92, 103 Following transfer of highly potent neutralising antibodies 12 hours prior to infection, hamsters showed lower viral titres and fewer clinical symptoms of COVID-19. However, following transfer of less potent neutralising antibodies, 1-2 days prior to infectious challenge, results were mixed demonstrating their inability to fully neutralise the virus. 103 Data on protection from re-infection in humans was not identified in the included papers, therefore conclusions on protective immunity are limited. All rights reserved. No reuse allowed without permission.

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There is limited evidence on the cross-reactivity of antibodies specific to other coronaviruses. 49,51,94,119-121 Using a variety of assays, several in-vitro studies explored both crossreactive antibody-binding responses and cross-neutralisation between SARS-CoV-2 and seasonal coronaviruses, MERS-CoV and SARS-CoV-1. Cross-reactive antibody-binding responses appear to be highest between SARS-CoV-1 and SARS-CoV-2, however crossneutralisation is rare and where reported is weak. 49,94,119 Whilst seasonal HCoVs are more common in the population, only 10% of sera exposed to HCoVs demonstrated cross-reactivity again with very little neutralisation activity. 120 A study comparing cross reactivity in children and older participants found children had elevated CoV-specific IgM compared to more mature classswitched specific IgA and IgG. 122 All studies were performed in-vitro and recognised the need for in-vivo investigation.

Most people who experience symptomatic SARS-CoV-2 infection undergo seroconversion to produce a detectable, specific antibody response in the acute phase (≤28 days). The kinetics of the antibody response to SARS-CoV-2 follow typical immunological paradigms: virus-specific IgM rises in the acute phase to a peak around two to five weeks following disease onset, then declines over a further three to five weeks before becoming undetectable in many cases; IgG peaks later (three to seven weeks following disease onset), then plateaus, persisting for at least eight weeks with some evidence suggesting a moderate decline over that period. However, understanding of IgG dynamics over time is limited by the understandably short duration of follow up in studies published to date.

Evidence suggests the majority of those infected with SARS-CoV-2 develop nAbsa finding that is consistent with previous findings for SARS-CoV1 and MERS-CoV. 7 The size of this response appears to correlate with disease severity. Neutralising antibodies are initially detectable from around seven to ten days, peaking at around three weeks and then declining. Further evidence is required to evaluate comprehensively the apparent waning of the nAb response over time.

Although nAb may be detectable, high quality studies suggest that titres are generally low, and All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint the response is short lived. This is supported by recently published data (beyond the date cut off for inclusion in this study) from a UK cohort of healthcare professionals. 123 A number of potent, specific nAbs have been identifiedin line with findings for other HCoVs. This is particularly the case for neutralising anti-RBD antibodies, 124 and is consistent with data emerging from vaccine development studies showing that protective antibodies can be induced. [125] [126] [127] Ongoing vaccine research has, however, highlighted a need for evidence of longerterm protection due to nAbs, and the titres at which these effects are achievedneither of which were fully addressed by studies included in this review. This is a significant gap in the evidence base on the immune response to SARS-CoV-2.

Data on correlates of the antibody response is incomplete, inconsistent or contradictory. It is not possible to draw robust conclusions on the associations of antibody response with age, sex, ethnicity or comorbidities, and although disease severity positively correlated with higher IgG antibody titres in a number of studies, distinguishing causation from correlation is not possible.

The size of the detectable nAb response appears to be associated with male sex (although the effect of disease severity was not controlled for); this is a surprising finding given the now wellrecognised association between male sex and poor COVID-19 outcomes. 128 Available data on protection following primary infection are limited to small scale animal models which consider re-exposure rather than reinfection. Primary infection appears to provide a degree of protection to reinfection up to day 43 post primary inoculation but no further data are available at later time points. The success of passive transfer of nAbs for protection against SARS-CoV-2 infection appears to be dose dependent, although no data exist around the importance of affinity, isotype or immunoglobulin subclass. Given the probable reduction in nAb titre over time, the protection they provide is likely to be limited. Limited cross reactivity is evident between SARS-CoV-2 and other HCoVs, but cross-neutralisation is rare and when it does occur, fails to fully neutralise the SARS-CoV-2 virus.

Our review is the first to provide an overview and critical appraisal of literature published since the beginning of 2020 on the immune response in the round. Our findings are nevertheless limited both by aspects of the review methodology and by shortcomings in the included literature. The comprehensiveness of systematic reviews is always dependent on search strategy, and some All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint results relevant to the research question may have been missed. As with all systematic reviews, our findings cannot account for unpublished negative results.

Limitations of the underlying evidence base were considerable. A majority of included studies were of moderate quality. Study populations were highly variable, as were the assays used, along with the rigour with which they were described, verified and validated against their target populations. There are efforts in the UK to standardise laboratory SARS-CoV-2 assays use through the National External Quality Assessment Service (NEQAS), but these are early stage and no comparable international initiatives yet exist to support comparability of research findings.

Longitudinal follow-up for durations greater than 50-60 days was rare. Many studies did not perform statistical analysis of findings; in particular, studies of putative correlates of immune response usually failed to control for the effects of potential confounders. Small sample sizes were common, as were study populations selected by convenience which, although common for clinical cohort studies, are prone to bias. Additionally, a large body of the evidence drew from preprint publications which have not been subject to peer-review. While efforts were made to account for this during synthesis and reporting, reporting standards in these publications were highly variable and there is no validated system at this time for weighting evidence from pre-print publications relative to peer-reviewed papers. Finally, reporting of ethical approval was limited or absent in many studies.

We identify two main policy implications arising from this work. At individual level, continuing uncertainty concerning the nature of the humoral response to SARS-CoV-2 makes it difficult to determine what the practical meaning of serologically-detected antibody response is with respect to sterilising immunity. Short follow-up periods, as well as the use of binary (positive/negative) serological tests in many studies continue to limit what can be said about the granularity of the immune response over timeand by implication, how best to interpret the results of serological testing with respect to individual susceptibility to infection. 129 We did not identify any studies considering risk of re-infection with SARS-CoV-2, which might provide an alternative perspective on susceptibility to infection.

At population-level, important policy implications arising from these data on antibody response relate to both surveillance and control. Reliance in the published literature on serological tests that have been evaluated predominantly in acutely unwell, hospitalised patients (without All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint appropriate validation against mild disease or in people with asymptomatic infection) means that seroprevalence estimates from this work should be treated with caution. A recent Cochrane review emphasises the risk of false-positive and false-negative results under different population prevalence scenarios. 130 However, in the UK, nationally validated assays have been evaluated with convalescent samples from community participants and a number of large-scale serosurveys now use these. [131] [132] [133] Clear understanding of the kinetics of the response, particularly for the specific N and S antigens, is important for the interpretation of seroprevalence studies.

With regard to control, the evidence here for lasting protective immunity, or lack thereof, may suggest it is too early to recommend the use of 'immunity passports'. A range of promising data have been identified to support further investigation of treatment with convalescent plasma or immunoglobulin, and the basic science underlying the antibody/virus/host cell interaction is starting to be described, with promising findings related to vaccine development. For vaccines, beyond development, strategies for implementation will also require a thorough understanding of the likely impact in different population groups.

Investigating the relationship between antibody response and correlates including age, sex, ethnicity and disease severity through high-quality, large-sample studies using well validated assays and incorporating appropriate statistical testing of results should be prioritised. The limited amount of data on antibody dynamics for mild and asymptomatic cases, which are likely to make up a significant proportion of infections, is a particularly important gap in the literature that will need to be addressed to improve understanding and definition of the varied clinical phenotypes associated with SARS-CoV-2 infection.

Evidence on immunity beyond three months following primary infection or vaccination is urgently needed. Evidence of immunity following vaccination is being explored through various vaccine trials (e.g. ChAdOx1 nCoV-19). 125 However, longitudinal studies of those already infected with SARS-CoV-2 is required to examine the degree of protection arising from prior infection.

Studies on the immune response to SARS-CoV-2 is of variable quality, and comparison of findings is difficult. A longer-term view and a more comprehensive assessment of the role of demographic characteristics and disease severity is required. Larger, high-quality, longitudinal studies, with All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint appropriate statistical analysis, consistent use of established and well-validated serological assays matched to clearly defined clinical phenotypes should be prioritised.

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This was a systematic review based on analysis of openly published secondary data. No ethical approval was required.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint All rights reserved. No reuse allowed without permission.

(which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Central points in the forest plot represent the median reported by each study overall; the range across participants in each individual study is represented by whiskers either side. All rights reserved. No reuse allowed without permission.

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The copyright holder for this preprint this version posted August 30, 2020. . https://doi.org/10.1101/2020.08.25.20178806 doi: medRxiv preprint Figure 3 . Schematic showing scale of IgG/IgM/IgA/Neutralising Ab response over time from disease onset (note that the y axis is illustrativeno scale is given).

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(which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Three studies report an association between co-morbidities and seroconversion or antibody positivity, 64, 153 with one finding immunocompromised individuals developed a lower response. 154 

One study reports antibody responses to be independent of co-morbidities, for both IgA and IgG. 148 

One study on this association was limited by low quality. 155 

Three studies report an association between COVID-19 symptoms and seropositivity, [156] [157] [158] and with higher titres of IgG, 29, 159 IgA, 159 and anti-RBD and anti-S antibodies. 160 

Two study reported that asymptomatic healthcare workers did develop antibodies. 161, 162 • Fever appears to have a consistent relationship with seropositivity and antibody titres, 29, 158, 160 although other symptoms such as ageusia have also been associated. 156, 158 Demographic Sex

• Four studies report no association between antibody titres (either IgG or IgM) and sex. 21, 148, 163, 164 • Two studies report a higher IgG titre in women, 165, 166 although one found this association in severe patients only. 166 • Two studies report a higher proportion of women tested positive for antibodies. 156,164 •

One study found higher anti-RBD and anti-S antibodies in male plasma donors. 160 

Older adults • Five studies found no association between antibody response and age, for both antibody positivity 147 or IgM/IgG titre. 21,58,148,167 • One study found that 'older' patients were more likely to seroconvert, 153 and concentration of IgG was related to age. 34 

Three studies reported that older people had higher titres of IgA and IgG, 23 IgM, 163 and anti-S and anti-RBD IgG. 160 

• Two studies found children generally developed a detectable antibody response to SARS-CoV-2 infection 86, 168 • Two studies found children with pneumonia generally mounted lower IgG 169 and IgA responses 170 and one study reported no differences. 167 

Two studies reported most neonates born to COVID-19 positive mothers had raised IgM, 171, 172 and COVID-19 recovered donor breast milk was found to have reactive IgA in one study. 173 

• One study reported non-white ethnicity was associated with higher antibody levels than white ethnicity. 153 

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