1. IntroductionSince the first report of novel coronavirus infection in the Hubei province of China in late 2019, the Coronavirus disease 2019 (COVID-19) has now been known as a global public-health catastrophe, with significant mortality and morbidity [1]. According to the latest data from the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University (JHU), SARS-CoV-2 has infected approximately 625,344,144 people, causing more than 6.5 million deaths (https://coronavirus.jhu.edu/map.html accessed on 18 October 2022).Like other viral respiratory infections, the novel coronavirus can be transmitted through the respiratory tract, manifesting a broad spectrum of clinical manifestations, from asymptomatic to severe respiratory failure, multiorgan dysfunctions, and death [2,3]. COVID-19, also called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is known to affect adults predominantly with greater risks of severe conditions in vulnerable individuals, such as immunocompromised, pre-existing comorbidities, and older people [2,4,5,6,7]. However, data regarding the burden of COVID-19 in children remain limited. Of note, earlier data suggested that children are less likely to contract the virus than adults [8,9,10,11]. Recent studies have shown that children had a similar prevalence of SARS-CoV-2 infections compared to adults but majorly presented in the asymptomatic form; therefore, many of those were not tested at all [12]. This potentially leads to an underestimated burden of infections, in which the actual number of infected people is substantially higher than the reported cases [13,14]. Remarkably, asymptomatic cases might have a critical role in transmission and infected children remain at risk of severe complications, including hospitalization, multisystem inflammatory syndrome, post-COVID-19 conditions, and death [15,16]. Hence, the ongoing surveillance of the real burden of infections in pediatrics is vital for guiding political decisions to prevent pandemic expansion, counting school closures, and vaccination strategies.The reverse-transcription polymerase chain reaction (RT-PCR) test using upper respiratory specimens is considered the gold standard for confirming SARS-CoV-2 infection [17]. Nevertheless, low sensitivity of the RT-PCR test may induce false-negative SARS-CoV-2 results, widely ranging from 1% to 30% due to several factors, such as insufficient samples, inappropriate specimen type, the timing of testing since exposure, and low viral load [17,18,19]. Interestingly, unlike typical seroconversion profiles, near-simultaneous production of both immunoglobulin-M (IgM) and immunoglobulin-G (IgG) was observed after approximately 1–3 weeks of both symptomatic and asymptomatic COVID-19 infection [20,21]. IgM eventually disappears after 4 to 6 weeks of infection, while IgG remains detectable within six months afterward [22,23]. Therefore, a novel approach focusing on detecting SARS-CoV-2 anti-nucleocapsid and anti-spike protein IgG might be a potential method for a more accurate estimation of asymptomatic or subclinical infections for several months after infection [14,19]. Various subsequent studies on the seroprevalence of SARS-CoV-2 among children have been carried out worldwide but enunciated a wide range of positivity rates, depending on studies, timeframe, and geography [12,14,19,24,25]; therefore, additional studies are required.

Spiking COVID-19 infection among children during the Omicron variant surge and in the new-normal period has driven an increasing number of vaccines being authorized for use in children of 6 months and above (fda.gov accessed on 17 June 2022). However, many countries, including Vietnam, have given emergency-use vaccination only for children aged five and older; children aged six months and below remain out of protection. It is clear that the decision to immunize children, indeed, accounts for the safety and effectiveness of vaccines and prioritization to the highest-risk age groups; however, it is also necessary to synchronously consider each country’s specific epidemiological and social context to develop their relevant COVID-19 immunization policies and programs. Therefore, this study aimed to identify the serostatus of antibodies against SARS-CoV-2 and associated risk factors in children who seek medical-care unrelated to COVID-19 in a tertiary hospital in Vietnam. There are also available data providing timely recommendations for establishing national immunization campaigns for children under five years, particularly children below six months, who stay on the front line of being vulnerable to COVID-19 infection.

4. Discussion

This paper reported a sero-surveillance of anti-SARS-CoV-2 antibodies conducted on 4032 children seeking medical care in a tertiary children’s hospital in Vietnam between 13 March and 3 April 2022. Our findings highlight a substantially high seropositivity rate of IgG against SARS-CoV-2 in the studied population and raise questions about the real burden of COVID-19 on children. Additionally, we found that the hazard of SARS-CoV-2 seropositivity was considerably associated with the clustering of infections within households, geography, underlying conditions, and history of using corticosteroids or immunosuppressants. This paper provided a snapshot of the hidden burden of susceptibility to SARS-CoV-2 in the pediatric population, which expressed the need for rush-out vaccination coverage in children, particularly those under five years, fitting the context of school reopening and future emerging variants.

Earlier data about COVID-19 suggested that children are less likely to contract and have a much milder course of infection than adults [8,9,10,11], which is supported by several generated hypotheses. These potential explanations were extensively discussed from insight into specific properties of children, comprising the lower expression in receptors of angiotensin-converting enzyme 2 (ACE2), cross-protection against SARS-CoV-2 infection, and their immune system response [13,27]. With human-to-human transmitting pathways of COVID-19, given that most pediatric cases were described inside familial clusters, another theory is that children are less likely to be exposed to the virus due to fewer community interactions and outdoor activities than adults [7,27,28]. This is in line with several early studies from different places, such as Iceland, Toulouse, Geneva, Austria, and Wuhan/Shanghai, where the percentage of seropositive children was significantly lower than that of adults [12,29,30,31,32]. However, recent studies have shown that children had a similar prevalence of SARS-CoV-2 infections compared to adults but presented in the asymptomatic form in a majority; many of those were not tested at all [12]. Recently, reports from previous sero-surveillance yielded a high seroprevalence of asymptomatic status [19,33], causing a misleading exact burden of COVID-19 infection in children.To the best of our knowledge, the first published sero-surveillance for anti-SARS-CoV-2 antibodies in May 2020 reported a wide range of positive estimates, from 0.4 to 59.3% of the general population [24]. Various subsequent studies have been carried out worldwide; however, available data on the pediatric seroprevalence for anti-SARS-CoV-2 antibodies enunciates a high uncertainty. Remarkably, seroprevalence considerably varied among studies, timeframe, and geography [12,14,19,25]. Our results also indicated that 2385 of 4032 patients were seropositive, corresponding to a high overall seroprevalence (59.2%) of immunity against SARS-CoV-2 (Table 1). Our seropositivity rate was significantly higher than that of previous studies, such as in Canada (5.8%) [34], the United Kingdom (6.9%) [19], and Croatia (2.9–8.4%) [33], which were mainly conducted during the second wave of the pandemic. In our context, during the fourth wave of COVID-19, in which the dominant Omicron variant had higher transmissibility than the original variants, the high seroprevalence was consistent with the more rapid increased daily case incidence [35]. Similarly, serologic data at the timeframe of Omicron variant predominance in South Africa showed that both seroprevalences of children 12 to 17 years of age and those younger than 12 years of age were high, with 73.8% and 56.2%, respectively [36]. The younger children are, the more they frequently depend on their caregivers and are exposed to the potential risk of exposure to pathogens; not to mention, this population presented asymptomatically in the majority, which is likely contributable to a “chain joint” in viral transmission over time. Additionally, most children with multisystem inflammatory syndrome (MIS-C) are between the ages of 3 and 12 years old, with an average age of 8 to 9 years old [37]. Consequently, the U.S. Food and Drug Administration (FDA) recently authorized the emergency use of the Moderna and the Pfizer-BioNTech COVID-19 Vaccine for the prevention of COVID-19 to include children down to 6 months of age (fda.gov accessed on 17 June 2022). Despite the latest data about the safety and effectiveness of COVID-19 vaccines on mitigating future contraction rates, hospitalizations, long-term influence, and deaths in children [38,39,40,41,42], worldwide officials raise controversies around immunization benefits and vaccine safety in such age groups, particularly children under five years. For many countries, including Vietnam, the vaccines were approved only for children aged five and older, while few countries, such as the United States, Brazil, and Costa Rica, have vaccinated children from six months old [38]. Our results showed that children aged ≤ 12 months were likely equal to be seropositive compared to children aged 36 to 60 months (59.2% vs. 57.5%, OR = 0.93, 95%CI = 0.75–1.14, p = 0.49) and those aged ≥ 144 months (59.2% vs. 65.5%, OR = 1.31, 95%CI = 0.87–1.96, p = 0.16) (Table 2). Remarkably, our results expressed that the highest seroprevalence rate was reported in children aged 13 to Table 2), who currently remain out of vaccination coverage in Vietnam and most countries. Given the potential ongoing reservations and sources of newly emerged variants [39], we reinforce that a “rush-out” national immunization for the pediatric population, particularly children aged less than three years, should begin now. Basically, the decision to immunize children, indeed, accounts for the safety and effectiveness of vaccines and prioritization to the highest-risk age groups; however, it is also necessary to synchronously consider each country’s specific epidemiological and social context to develop their relevant COVID-19 immunization policies and programs.As expected, there was considerable variation in the seroprevalence of anti-SAR-CoV-2 antibodies according to geographics. Hanoi capital had significantly higher seroprevalence than all other provinces, at 62.6% and 53.3% (OR = 1.46, 95%CI = 1.29–1.67, p = 0.0001) (Table 2). This point complied with the densely populated urban areas of Hanoi, regarded as the epicenter of the COVID-19 pandemic in Vietnam for six months prior to our study period. Of note, local epidemiological data are crucial for guidance on preventing SARS-CoV-2 pandemic expansion, such as the priority of vaccination roll-out to these high-risk regions.Our results confirm that the seroprevalence of the group with at least one household member presenting COVID-19 infection was higher than their counterpart (65.8% vs. 48.5%, OR = 2.04, 95%CI = 1.79–2.33, p = 0.0001) (Table 2), in accordance with other studies [7,27,43]. Young children tend to depend significantly on their caregivers, so appropriate quarantine and separation from other household members can be particularly challenging in this population. Although the early closing of all schools, ranging from kindergarten to universities, might reduce the number of social contacts, the potential transmission of the virus still exists in a confined space [43]. According to Wu et al., most children contract the SARS-CoV-2 virus during contact with family members (85.2%) instead of the community [44]. Another study noted an uncommon correlation between outbreaks in educational settings and the incidence of COVID-19 in the community [45]. These findings might provide benchmark worldwide debates around the true efficacy of school closures in minimizing the SAR-CoV-2 burden in possible future outbreaks. Hence, now more than ever, the ongoing vaccination campaigns in this new-normal period will play a key role in the opportunity for sustaining safe in-person schooling, extracurricular activities, and sports for children.Interestingly, our results also showed that patients without underlying conditions or without using corticosteroids or immunosuppressants had significantly higher seroprevalence and protective titer levels of SARS-CoV-2 antibodies than their counterparts (Table 2 and Table 3). This is possibly explained by the fact that the immunocompromised are vulnerable individuals with a higher risk of infection, hospitalization, and mortality [46]; therefore, they tend to comply precautiously with the preventive measures to alleviate the risk of infection. Additionally, we provoke hypotheses that if the immunogenicity of both groups is comparable, should a serological test be utilized to determine the exact incidence of infection in the immunocompromised population? A sparse number of studies verified that immunosuppressive adults have effective immunogenicity-producing antibodies to SARS-CoV-2 infection, though a slightly delayed response compared to the immunocompetence [47]. However, these data on the pediatric population were not well understood and required further investigation.Seroprevalence in children under 12 months old also depends on several factors, including vertically transferred immunity with a maternal history of COVID-19 vaccination or infection or breastfeeding, misinterpreting the actual burden of SARS-CoV-2 in this population [48]. However, previous data showed that vertical transmission rates of SARS-CoV-2 during pregnancy are low, estimated at around 2–3% [49], inducing an insignificant change in fetal seropositivity. Our result also showed that the history of COVID-19 vaccination and breastfeeding was not associated with the hazard of seropositivity (Table 5). However, as presented in Table 5, infants aged ≤ 12 months old with a maternal history of COVID-19 infection during the pregnancy/labor period were more likely to be seropositive than their counterparts. This finding could be explained by the indispensable dependence of children aged 12 months and younger on their caregivers, accumulating more potential risk of seropositivity by exposure to pathogens from their infected mothers after birth. In line with this point, our result showed that among 316 mothers who reported having COVID-19 infection during the pregnancy/labor period, the majority of infected mothers (287 cases, 91.7%) recorded after delivery, while only 1 (0.3%) case and 25 (8.0%) cases were infected before pregnancy and during pregnancy, respectively. Another potential cause that may play a role in transmitting SARS-CoV-2 infection from the mother to the fetus is vaginal delivery [50]. Several studies also suggested the possibility of viral transmission through the placenta and breastfeeding; nonetheless, the evidence on that issue has not been fully understood [49]. We expressed that the differences in the ability of robust immunity responses to SARS-CoV-2 and the sustaining of long-lasting immunity between infants under 12 months and other age groups should be clarified in further studies.Our study also has some limitations. First, the patients were recruited from a single center in a tertiary hospital, which is not fully representative of all settings in Vietnam and other countries. However, as the largest referral tertiary children’s hospital in the north of the country, patients visiting the hospital were from different geographic regions. Second, IgG is only detectable within six months of infection afterward and reflected approximately estimated cases of COVID-19 infections during the previous six-month period. The timeframe in our study regarding the context of a new wave of the Omicron BA.2 variant that emerged in Hanoi, Vietnam, was during late 2021 and early 2022; thus, the data could not be generalized for different times. Third, due to potential cross-reactivity between SARS-CoV-2 antigens and seasonal coronaviruses [51], false-positive results for the assay may occur, and our seroprevalence rates might be overestimated. Additionally, testing performance may vary depending on the variants circulating [26], such as newly emerging strains of the Omicron compared to original variants. Fourth, missing the clinical manifestations and biochemical data in most cases is a limitation in our study, so that we could not analyze their relationship with other factors. Due to a lack of data on the specific numbers of contaminated members in households, we could not compare the risk of seropositivity between clusters with different numbers of infected family members. Hence, further investigations involving well-designed studies are needed to clarify this issue. Despite limitations, there is an extensive sample size in this study about the seroprevalence of anti-SARS-CoV-2 antibodies in children to the best of our knowledge. This paper provided a snapshot of a high burden of COVID-19 infection in the pediatric population, those generally presenting with mild or asymptomatic infections. In the meantime, this finding is a benchmark, supporting public-health providers and policymakers to roll out appropriate disease-management strategies.