Since December 2019, the world has been grappling with the emergence and relentless spread of coronavirus disease 2019 (COVID-19), a highly contagious respiratory illness caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This unprecedented pandemic has presented significant challenges to global health systems and economies (1, 2). In response to the dynamic nature of the virus, the World Health Organization (WHO) has been monitoring and categorizing different variants of the coronavirus. The repertoire of identified variants encompasses a range of strains, notably the Alpha, Beta, Gamma, and Delta variants, and most recently, the Omicron variant. The Omicron variant, scientifically known as B.1.529, was first identified in South Africa in early November 2021 (3). Since then, it has been a subject of intense scrutiny due to its high number of mutations and potential implications for transmissibility, immune escape, and vaccine effectiveness (4).

The Chinese government has applied a series of epidemic prevention and control measures, including SARS-CoV-2 vaccination, routine SARS-CoV-2 nucleic acid testing, wearing masks in public places, and quarantine of confirmed cases. The strategic implementation of vaccination has emerged as a critical weapon in empowering governments across the globe to effectively combat and control the transmission of SARS-CoV-2. The immunogenicity of two inactivated vaccines, namely CoronaVac (Sinovac, Beijing, China) and BBIBP-CorV (Beijing Institute of Biological Products, Beijing, China), were undertaken through a meticulous phase 1/2 trial that specifically targeted individuals aged 3–11 years. The comprehensive assessment of these inactivated vaccines revealed a remarkable outcome, as they unequivocally demonstrated their ability to provoke strong immune responses within the vaccinated cohort. A seroconversion ratio of 100% was observed across all vaccinated individuals, precisely 28 days after the administration of the second vaccine dose (5, 6). This extraordinary efficacy in eliciting humoral responses holds paramount significance in conferring protective immunity, thus establishing a solid foundation to effectively safeguard and fortify the younger age group against the perils of SARS-CoV-2. Subsequently, the vaccines were approved for use in children older than 3 years of age in China from November 2021, with primary immunization consisting of two doses administered over a 28-day period. A recent study conducted in Chile demonstrated that a complete primary immunization for CoronaVac in children aged 6–16 years provided effective protection against hospitalization and severe COVID-19 (7).

The SARS-CoV-2 Omicron variant has been circulating extensively in China since December 2022, with BA.5.2 and BF.7 being the prevalent circulating strains (8). More importantly, children were susceptible to the virus. Real-world evaluations investigating the humoral immune response triggered by Omicron variant infection in pediatric populations, who have received the inactivated SARS-CoV-2 vaccine, are exceptionally scarce. In the quest for comprehensive pandemic management and control, the monitoring of serological antibodies in children emerges as a pivotal aspect. With the aim of shedding light on this critical subject, an investigation was undertaken in this study, concentrating on the analysis of serum SARS-CoV-2 antibody levels in a distinct population of children aged 3-11 years. Within the scope of the study, a comparison was conducted to evaluate the SARS-CoV-2 IgG antibody and Nab levels within 3 months post-SARS-CoV-2 infection. The distinction laid in segregating the participants into two cohorts: those who had received the inactivated SARS-CoV-2 vaccine and those who had not undergone vaccination before falling prey to the virus. The study aimed to furnish public health agencies, guiding vaccination strategies and formulating control policies.

Continuous variables were presented as median and interquartile range (IQR). SARS-CoV-2 IgG and Nab levels were compared using the Mann‐Whitney U test or Wilcoxon test. Categorical variables were expressed as number and percentage. Nabs grades in different vaccination status post-infection were compared using the Chi-squared test. P < 0.05 was considered statistically significant. The statistical analysis and illustration of graphs were performed by GraphPad Prism 8.0 software (GraphPad Software Inc., San Diego, CA, USA).

The SARS-CoV-2 Omicron variant has been circulating extensively in China mainland since December 2022, 70% of cases in Beijing area were caused by the BF.7 variant (8). Children at different ages were mostly susceptible to the Omicron variant (9).

In our previous study, we conducted an investigation into the trends of SARS-CoV-2 IgG antibodies in children aged 3-11 years who had undergone primary immunization and had no previous history of SARS-CoV-2 infection. The study revealed that after receiving the second dose of the vaccine (10), SARS-CoV-2 IgG antibodies peaked at 1 month and remained at elevated levels for 3 months. Subsequently, the antibody levels gradually declined and closed to baseline after 6-7 months. Other research has also shown that vaccination with two doses of mRNA vaccine more than 6 months earlier, with no prior infection, offered limited protection against symptomatic Omicron infection, while demonstrated strong effectiveness in preventing severe conditions (11). In the current study, all children who received two doses of the inactivated SARS-CoV-2 vaccine had an interval of more than 6 months between the second dose and the infection. Consequently, the vaccine-induced antibodies against SARS-CoV-2 had nearly disappeared by the time of infection. Surprisingly, within 3 months post Omicron infection, the Nabs positive rate reached 100% in vaccinated individuals, and an impressive 99.4% of children who had received two doses prior to infection displayed elevated levels of Nabs, ranging from 100.00 AU/ml to 120.00 AU/ml. These findings highlight that children who completed two doses of the SARS-CoV-2 vaccine were capable of activating a robust humoral immune response, generating high levels of Nab, thereby offering effective protection against SARS-CoV-2 infection. Moreover, these high levels of Nab persisted in two doses vaccinated children for at least three months following infection.

We compared the post-infection humoral immune response to previous primary administration of either Corona Vac or BBIBP-CorV. The results showed that children who had previously received two doses of inactivated vaccine from different manufacturers were able to produce elevated levels of SARS-CoV-2 antibodies for at least three months after infection. In addition, we observed that children of different genders and ages who had received the primary SARS-CoV-2 vaccine prior to infection were capable of producing high levels of SARS-CoV-2 Nabs. Given that the majority of Chinese children over the age of 3 have received two doses of the inactivated SARS-CoV-2 vaccine prior to infection, the significant post-infection elevation in Nabs within the pre-vaccinated pediatric population played a crucial role in controlling the spread of the virus. Recent research has demonstrated that Omicron BA.1 infection after receiving three doses of the CoronaVac predominantly stimulates humoral immune memory, resulting in the production of antibodies that neutralized both the ancestral virus and BA.1 variant (12). Another study demonstrated that SARS-CoV-2 IgG increased two-fold in individuals who had received triple doses of mRNA vaccine and had no previous infection, at 2-5 weeks post Omicron infection (13). Additionally, the latest research has shown that BNT162b2 full vaccination reduced the risk of omicron-associated hospitalization by two-thirds among children who aged 5-11 years (14). In the study, among the unvaccinated children who were infected with SARS-CoV-2, the positive rate of SARS-CoV-2 Nabs was only 50.2%, and the majority of children had low expression levels. The results indicated that children had a weak humoral immune response against SARS-CoV-2 following natural infection.

In the realm, recent studies have compared the relationship between inactivated SARS-CoV-2 vaccines administered prior to or post infection and Nab levels in COVID-19 convalescent individuals. A study based on population over the age of 12 showed that the fully vaccinated recipients had higher levels of SARS-CoV-2 Nabs than the partially vaccinated and unvaccinated individuals, as well as a similar tendency was observed in positive rates (15).Another adult study revealed recipients of the CoronaVac vaccine exhibited significantly higher geometric mean Nabs titers and seropositivity rates in response to the Omicron variant compared to the unvaccinated in COVID-19 recovered patients (16). Recent research compared the effects of two doses of inactivated SARS-CoV-2 vaccine (CoronaVac) on previously naturally infected individuals and uninfected recipients. The results showed that anti-RBD antibody titers were significantly higher in those with a previous natural infection than in uninfected populations with two doses vaccine (17). The above results indicated that primary immunization prior to infection or complementary vaccination with two doses of inactivated SARS-CoV-2 vaccine during recovery from COVID-19 could effectively evoke immune memory effects and activate the humoral immune response to produce elevated antibodies.

In the landscape of SARS-CoV-2 Nabs detection, the traditional virus neutralization test and pseudovirus-based platforms, though classical methods of the field, present inherent challenges. Both methods require use of either live virus that requires a biocontainment level 3 laboratory, or extensive tissue culture capabilities, necessitating skilled technicians and protracted timelines, spanning several days to obtain conclusive results (18, 19). In the quest for more efficient and reliable alternatives, our study embraced a commercial Nabs assay, designed to gauge SARS-CoV-2 Nabs in the pediatric population following infection.

The innovative assay builds upon the RBD-ACE2 interaction, which was consistent with the principles of traditional viral neutralization testing and could detect specific Nabs in the serum of participants. Robustly validated by previous investigations, surrogate NAb levels have demonstrated a consistent correlation with live viral Nab titers, particularly against homologous ancestral viruses, attesting to their accuracy and relevance (20). A study focusing on analysis of serological biomarker showed that Nab of convalescent serum strongly correlated with Spike and RBD- specific antibody levels (21). Furthermore, a study has shown the correlation between the surrogate NAb with the ancestral virus and the Omicron variant was 0.908 and 0.797 respectively (15). Since there have been no previous outbreaks of SARS-CoV-2 in children in Beijing, the level of Nabs detected in the study may reflect the ability of Omicron variant infection to activate the humoral immune response in both vaccinated and unvaccinated children.

In navigating the scientific landscape, we acknowledge several limitations in the study. Firstly, it is vital to recognize that the Nabs detection reagent utilized in this investigation is a commercial kit, designed to target the RBD epitope of the SARS-CoV-2 S protein from the original strain. However, the dynamic nature of the virus has brought forth the emergence of the Omicron variant, featuring distinct genetic characteristics and mutations. Consequently, the commercial kit’s reliance on the original strain’s epitope may limit its ability to fully capture the specific neutralizing antibodies directed against the Omicron variant (22). Secondly, we confront the practicality of the lower limit for SARS-CoV-2 Nabs detection, set at 120.00 AU/ml. The constraints imposed by this threshold may lead to the underestimation of Nab levels in vaccinated children who subsequently encounter SARS-CoV-2 infection. Meanwhile, most children enrolled in the study experienced mild disease processes from SARS-CoV-2 infection, so we didn’t further analyze the correlation between the expression level of COVID-19 Nabs and the severity of the disease.

Within the realm of 3-11 years pediatric populations, children who had received two doses of the Corona Vac or BBIBP-CorV inactivated SARS-CoV-2 vaccine prior to infection exhibited a strikingly robust humoral immune response, characterized by the generation of an abundance of neutralizing antibodies. Even more remarkable was the sustained elevation of Nab levels for a notable period of 3 months post-infection regardless of age and gender. In contrast, children who remained unvaccinated before encountering the SARS-CoV-2 infection manifested a substantially weaker humoral immune response, with most individuals displaying inadequacy in generating Nabs. The SARS-CoV-2 infection acting as a potent enhancer, propelling the humoral immune response to new heights, bears profound significance for vaccine efficacy and immunization strategies in pediatric populations. Consequently, unvaccinated children who exhibit feeble humoral immunity following primary natural infection would largely benefit from timely intervention through vaccination, thereby establishing robust immunization protection against the ever-present risk of reinfection. However, the SARS-CoV-2 Omicron variant, adorned with pivotal mutations in the spike protein, ushers in current challenges, notably heightening the risk of reinfection and vaccine breakthrough events (23, 24). In light of these emerging uncertainties, the urgent call for long-term monitoring of reinfection cases and Nab levels in pediatric cohorts becomes evident. Such monitoring serves as a cornerstone for the formulation of evidence-driven vaccination strategies.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The studies involving humans were approved by Ethics Committee of Capital Institute of Pediatrics. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation in this study was provided by the participants’ legal guardians/next of kin. Written informed consent was obtained from the minors’ legal guardian/next of kin for the publication of any potentially identifiable images or data included in this article.

JL: Conceptualization, Data curation, Methodology, Project administration, Validation, Writing – original draft, Writing – review & editing. JJL: Data curation, Investigation, Writing – original draft. SD: Data curation, Writing – original draft. LD:Investigation, Writing – original draft. LW: Investigation, Writing – original draft. LC: Investigation, Writing – original draft. XC: Investigation, Writing – original draft. YW: Data curation, Writing – original draft. MG: Methodology, Writing – original draft. WL: Data curation, Writing – original draft. QS: Data curation, Writing – original draft. WX: Data curation, Writing – original draft. LM: Conceptualization, Validation, Writing – original draft, Writing – review & editing.