Long-term data on the humoral immune response and cross-neutralization against variants of concern in children >1 year after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection are currently missing and urgently needed to predict the likelihood of reinfections and guide vaccination strategies.

We assessed in a prospective observational cohort study the humoral immune response and live-virus neutralization of B.1.617.2 (delta variant) at 2 months (T1) and 14 months (T2) after mild or asymptomatic, wild-type SARS-CoV-2 infection. Participants were recruited by local health authorities. The study was conducted according to the Declaration of Helsinki at the University Children’s Hospital in Heidelberg from May 11 to June 20, 2020 (T1) and May 20 to 28, 2021 (T2) with approval by the review board of the Medical Faculty Heidelberg (S-294/2020). Details on inclusion/exclusion criteria (Supplemental Table 5), questionnaire items (Supplemental Table 6), detection of SARS-CoV-2 reactive antibodies, live-virus neutralization, and statistical analysis are given in the Supplemental Information. Results on SARS-CoV-2 transmission were published previously as part of a multicenter study in the Federal State of Baden-Württemberg, Germany.1 

Fifty children and 172 adults were classified as seropositive at 2 months (T1) after the acute SARS-CoV-2 infection, and 35 children and 45 adults were followed up at 14 months (T2) (Supplemental Fig 2). Age, sex, and symptom distribution within the groups of adults and children were in balance at T1 and T2 (Table 1).

TABLE 1

Demographic Characteristics of Seropositive Study Participants in May/June 2020 (T1) and May 2021 (T2)

2 Mo Postinfection (T1)14 Mo Postinfection (T2)
AllAdultsChildrenAllAdultsChildren
Total No. of participants 222 172 50 80 45 35 
Age, y mean ± SD (range) 35.9 ± 16.5 (1–75) 43.2 ± 10.2 (18–75) 10.6 ± 5.15 (1–17) 29.6 ± 18.4 (1–68) 44.1 ± 10.1 (18–68) 11.0 ± 5.10 (1–17) 
Sex, No. (%)       
 Female 108 (48.6) 86 (50) 22 (44) 42 (52.5) 24 (53) 18 (51) 
 Male 114 (51.5) 86 (50) 28 (56) 38 (47.5) 21 (47) 17 (49) 
Symptomatic SARS-CoV-2 infection,a No. (%)       
 No 32 (14.6) 12 (7) 20 (40) 20 (25) 5 (11) 15 (43) 
 Yes 187 (85.4) 157 (93) 30 (60) 60 (75) 40 (89) 20 (57) 
COVID-19 severity,a No. (%)       
 Outpatient 207 (94.5) 158 (93) 49 (98) 79 (99) 44 (98) 35 (100) 
 Hospitalized 12 (5.5) 11 (7) 1 (2) 1 (1) 1 (2) 0 (0) 
2 Mo Postinfection (T1)14 Mo Postinfection (T2)
AllAdultsChildrenAllAdultsChildren
Total No. of participants 222 172 50 80 45 35 
Age, y mean ± SD (range) 35.9 ± 16.5 (1–75) 43.2 ± 10.2 (18–75) 10.6 ± 5.15 (1–17) 29.6 ± 18.4 (1–68) 44.1 ± 10.1 (18–68) 11.0 ± 5.10 (1–17) 
Sex, No. (%)       
 Female 108 (48.6) 86 (50) 22 (44) 42 (52.5) 24 (53) 18 (51) 
 Male 114 (51.5) 86 (50) 28 (56) 38 (47.5) 21 (47) 17 (49) 
Symptomatic SARS-CoV-2 infection,a No. (%)       
 No 32 (14.6) 12 (7) 20 (40) 20 (25) 5 (11) 15 (43) 
 Yes 187 (85.4) 157 (93) 30 (60) 60 (75) 40 (89) 20 (57) 
COVID-19 severity,a No. (%)       
 Outpatient 207 (94.5) 158 (93) 49 (98) 79 (99) 44 (98) 35 (100) 
 Hospitalized 12 (5.5) 11 (7) 1 (2) 1 (1) 1 (2) 0 (0) 
a

Information on symptomatic SARS-CoV-2 infection and hospitalization were missing in 3 adults at T1.

At T1, immunoglobulin G (IgG) antibodies against the S1 domain (S1), receptor-binding domain (RBD) of the S1 glycoprotein, and pan-immunoglobulin against nucleocapsid (N) were highest in the youngest subgroup (<5 years of age) and decreased with increasing age (Supplemental Fig 3), with a significant overall difference among age groups (P ≤ .001) and between sexes (P ≤ .02) (Supplemental Table 2). Most pediatric age groups exerted significantly higher IgG antibody levels against S1, RBD, and N than adults (P ≤ .02), except for anti-RBD IgG between adults and adolescents (P = .66).

The time course of antibody levels from T1 to T2 was analyzed in a mixed effects regression model (Supplemental Table 3). Mean anti-S1, anti-RBD, and anti-N antibody levels fell significantly from T1 to T2 in all age groups (Fig 1A1C). The decline was significantly different between age groups for anti-S1 (P = .02) and anti-N (P < .001), and resulted in comparable levels among age groups at T2 (P = .51–1.00).

FIGURE 1

SARS-CoV-2 antibodies against (A) S1 (children n = 32, unvaccinated adults n = 17), (B) the RBD of the S1 glycoprotein (children n = 34, unvaccinated adults n = 15), and (C) the N (children n = 35, unvaccinated adults n = 17, vaccinated adults n = 28) at 2 and 14 months after SARS-CoV-2 wild-type infection stratified according to age. P values are from the mixed effects regression model (Supplemental Table 3).

FIGURE 1

SARS-CoV-2 antibodies against (A) S1 (children n = 32, unvaccinated adults n = 17), (B) the RBD of the S1 glycoprotein (children n = 34, unvaccinated adults n = 15), and (C) the N (children n = 35, unvaccinated adults n = 17, vaccinated adults n = 28) at 2 and 14 months after SARS-CoV-2 wild-type infection stratified according to age. P values are from the mixed effects regression model (Supplemental Table 3).

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After T2, the delta variant was predominant in Germany; we therefore measured neutralizing antibodies against this variant. A total of 16 of 17 unvaccinated adults (94%) and 32 of 34 unvaccinated children (94%) exerted neutralizing activity (Supplemental Fig 4). The mean (SD) neutralizing activity in adults (serum dilution that reduces infection by 50% [ID50]) (42.9 [24.4]) was not significantly different from that in children aged <5 years (ID50, 72.0 [17.9]), 5.0 to 11.9 years (ID50, 42.3, [33.2]), and 12.0 to 17.9 years (ID50, 43.8 [23.4]) in a linear regression model with the variables age (P = .32) and sex (P = .58) (Supplemental Table 4) or when applying a Mann-Whitney U test (P ≥ .19).

This study describes the longitudinal humoral immune response and live-virus neutralization against the SARS-CoV-2 variant delta in children compared with adults >1 year after a mild or asymptomatic SARS-CoV-2 wild-type infection. Although we confirm the observation of a higher humoral immune response in children in the early period (at 2 months) postinfection,2,3  we found that children have a greater absolute and relative decline of SARS-CoV-2 IgG antibodies at 14 months postinfection, resulting in comparable antibody levels and live-virus neutralization as adults. Former studies on long-term neutralizing activity against SARS-CoV-2 in children were followed up for <1 year, and results were heterogenous.37 

One limitation is missing data on cell-mediated immunity that has been shown to correlate with peak antibody levels,8  but remains stable for several months after infection and protects from severe disease of immune-escape variants.9  The findings regarding neutralization apply to the delta variant; one can expect a several-fold, lower neutralization capacity against the immune escape variant omicron.10 

In conclusion, our data indicate that humoral immunity and live-virus neutralizing of Delta 14 months after mild or asymptomatic, wild-type infection is comparable in children and adults, and will most likely be insufficient to neutralize the current omicron variant. Hence, an additional vaccination also in children may be indicated. Studies assessing the impact of vaccinations on virus neutralization in convalescent children are urgently needed to guide future vaccination strategies.

We thank all households who participated in this study. We also thank the following staff members for data and sample collection, and excellent organizational support: Jürgen Grulich-Henn, Kristine Chobanyan-Jürgens, Julia Euler, Michal Fischer, Iris Schelletter, and Heike Matzkuhn. We thank Christina Klose for database assistance. We thank Stefanie Wolf, Maria Anders-Össwein, Ira Pistorius-Knop, Sylvia Parthé, and Markus Zorn for support in laboratory analyses.

Drs Stich and Tönshoff conceived and designed the study, drafted the first version of the manuscript, and collected and analyzed data; Drs Benning and Speer conceived and designed the study, provided key resources, and collected and analyzed data; Dr Garbade contributed to drafting sections of the manuscript and did statistical analysis; Ms Bartenschlager and Drs Kim, Waterboer, and Schnitzler performed laboratory analysis; Ms Gleich, Ms Haase, and Dr Jeltsch collected and analyzed data; Drs Janda, Renk, and Elling conceived and designed the study; Drs Hoffmann and Kräusslich conceived and designed the study, and provided key resources; Dr Müller conceived and designed the study, performed laboratory analysis, and contributed to drafting sections of the manuscript; Dr Bartenschlager provided key resources, contributed to drafting sections of the manuscript, and performed laboratory analysis; and all authors revised the manuscript for important intellectual content, approved the final manuscript as submitted, and agree to be accountable for all aspects of the work.

This study is registered at German Registry for Clinical Studies, D 00021521, https://www.drks.de/drks_web/navigate.do?navigationId=trial.HTML&TRIAL_ID=DRKS00021521. Deidentified data will be made available upon publication to researchers who provide a methodologically sound proposal for use in achieving the goals of the approved proposal. Proposals should be submitted to maximilian.stich@med.uni-heidelberg.de.

Neutralization of B.1.617.2 (delta) is comparable in children and adults 14 months after wild-type infection.

FUNDING: Funded by the Ministry of Science, Research and the Arts Baden-Württemberg, Germany, within the framework of the special funding line for coronavirus disease 2019 research, part of the measures to combat the severe acute respiratory syndrome coronavirus 2 pandemic in the field of medical research, as well as the Dietmar Hopp Foundation, St. Leon-Rot, Germany. Dr Waterboer was supported by the Dieter Morszeck Foundation. The funder had no role in the design or conduct of this study.

CONFLICT OF INTEREST DISCLAIMER: The authors have indicated they have no conflicts of interest relevant to this article to disclose.

ELISA

enzyme-linked immunosorbent assay

ID50

serum dilution that reduces infection by 50%

IgG

immunoglobulin G

N

nucleocapsid

RBD

receptor-binding domain

S1

S1 domain

SARS-CoV-2

severe acute respiratory syndrome coronavirus 2

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Supplementary data