BCG Vaccination in Early Childhood and Risk of Atopic Disease: A Systematic Review and Meta-Analysis

Background Several large-scale studies suggest that Bacille Calmette–Guerin (BCG) vaccination in early childhood may reduce the risk of atopic diseases, but the findings remain controversial. Here, we aimed to investigate the potential correlation between early childhood BCG vaccination and the risk of developing atopic diseases. Methods Eligible studies published on PubMed, EMBASE, and Cochrane CENTRAL were systematically sourced from 1950 to July 2021. Studies with over 100 participants and focusing on the association between BCG vaccine and atopic diseases including eczema, asthma, and rhinitis were included. Preliminary assessment of methods, interventions, outcomes, and study quality was performed by two independent investigators. Odds ratio (OR) with 95% confidence interval (CI) was calculated. Random effects of the meta-analysis were performed to define pooled estimates of the effects. Results Twenty studies with a total of 222,928 participants were selected. The quantitative analysis revealed that administering BCG vaccine in early childhood reduced the risk of developing asthma significantly (OR 0.77, 95% CI 0.63 to 0.93), indicating a protective efficacy of 23% against asthma development among vaccinated children. However, early administration of BCG vaccine did not significantly reduce the risk of developing eczema (OR 0.94, 95% CI 0.76 to 1.16) and rhinitis (OR 0.99, 95% CI 0.81 to 1.21). Further analysis revealed that the effect of BCG vaccination on asthma prevalence was significant especially in developed countries (OR 0.73, 95% CI 0.58 to 0.92). Conclusion BCG vaccination in early childhood is associated with reduced risk of atopic disease, especially in developed countries.


Introduction
Allergy is malfunctioned immune response caused by various foreign antigens, which can increase the risk of asthma, atopic dermatitis (eczema), and rhinitis (hay fever) [1]. Atopic diseases are characterized by the incubation period between initial exposure (sensitization) and symptoms (elicitation) that will develop in subsequent exposures and may involve IgE and/or non-IgE-mediated responses. An IgE-mediated allergic reaction (sometimes called immediate-type hypersensitivity (type I)) involves the production of 2 cytokines, which initiate IgE production by B cells [2]. Increased prevalence in allergic and autoimmune diseases has been appreciated worldwide, especially for the Western countries where the prevalence is as high as 20% [3], and one in three children in these countries [4] are affected by atopic diseases. Atopic diseases also have been positioned as the most common chronic disease, which greatly compromised life quality of patient and impose great social and economic impacts on both individuals and their families. Among patients with atopic disease, pediatric patients are affected greatly as they have high prevalence [5]. A recent register-based study showed that the lifetime prevalence of asthma and allergic rhinitis at age 10 was 15.6% and 20.4%, respectively [6].
Vaccination is used worldwide for preventing infectious diseases [7]. Childhood vaccination plays a crucial role in the early development of the immune system [8]. Atopic disease usually develops in the early life of children, whose immune system is under development [9]. ere is evidence revealing the relationship between vaccination and the risk of atopic disease [10], and many publications have investigated this hypothesis [11][12][13][14][15]. Among those relationships, the most investigated one is the relationship between BCG vaccine and atopic disease, but the results remain controversial. National BCG vaccination programs are standard for childhood immunization in most countries apart from Western European counties and North American countries to prevent tuberculosis [16]. 2 immunologic response is inhibited by BCG. Antagonism of atopy by BCG has been observed in both human and animal models. Considering these characteristics, BCG is considered to be a therapeutic model for investigating the consequence of early-life stimulation of TH1 cells [17]. Recently, BCG is shown to have nonspecific beneficial effects on early immune system, reducing infant hospitalization [18]. However there were conflicting results between BCG and childhood atopic disease [19]. To clarify the relationship between BCG and childhood atopic disease, we conducted a meta-analysis of studies. We examined the association between BCG vaccine and atopic disease incidence and explored the implications of available literature data for clinical practice and future examination.

Strategy of Literature Search.
Our systematic review was performed and reported in accordance with Meta-analysis Of Observational Studies in Epidemiology (MOOSE) [20] and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [21] guidelines.
We performed a systematic literature search in PubMed/ Medline (1950 to Jul 2021), EMBASE (1980 to Jul 2021), and Cochrane CENTRAL (1950 to Jul 2021) for association between BCG vaccination and atopic disease by using relevant keywords including asthma, eczema, rhinitis, BCG vaccines, and other synonyms. e search strategy suitable for PubMed is provided in Appendix 1. e analysis was restricted to studies published in English. We screened bibliographies of relevant review articles to ensure that all relevant studies were included.

Study Selection.
Each study was first selected based on their titles and abstracts (when available) by two independent investigators at the same time. en, they retrieved full texts and performed further screening when studies were deemed eligible. Studies had to be cohort studies with information of authors, year, geographical area, study design, sample size, exposure (age of BCG vaccination, method of assessment), and outcome (confirmed diagnosis of specific atopic disease). Case reports, preclinical studies, and some studies without confirmed diagnosis of a specific atopic disease will not be used for this study. Disagreements were resolved by discussion and, if necessary, in consultation with a third senior investigator.

Quality Assessment.
Quality of all included trials was assessed by two authors independently by using the STROBE checklist for cohort study and JADAD scale for RCT study. e risk of bias in each domain was judged as low, high, or unclear. e overall risk of bias in a study was classified as low if all domains had low risk; as high if one or more domains had high risk; or as unclear otherwise. Based on these standards, we defined the studies into the following three grades: A, high quality and low risk of bias (scored ≥66.6%); B, moderate quality and moderate risk of bias (scored 33.3-66.6%); and C, low quality and high risk of bias (<33.3%). Discrepancy in quality assessment between the reviewers was resolved by discussion with involvement of a third senior investigator if necessary.

Data Extraction.
Two authors extracted data independently using a standard data extraction form. e following baseline characteristics were extracted from the included studies: first author, year of publication, study design, and location in which the study was performed, number of included participants, and diagnosis of atopic disease. Studies were excluded if any of the above information is not available.

Statistical Analysis.
We used R (version 3.6.3) to perform the data analysis; odds ratios (ORs) and their associated 95% confidence intervals (CI) were used to assess the strength of association between BCG vaccination in early life and the risk of getting atopic disease. Statistical significance was defined at p < 0.05 [22]. I 2 statistic were used for investigating heterogeneity, and statistical heterogeneity was defined at I 2 > 50%. We used random-effects modeling to perform the meta-analysis for all pooling. In case heterogeneity exists, analysis to investigate whether the heterogeneity is related to the participant's race will be performed. Different ethnic background in different continents may be considered potentially important to heterogeneity because of living habit diversity. Funnel plots were used to display the publication bias graphically, both specifically and officially with Egger's test.
Four studies were conducted in developing countries, and sixteen studies were in developed countries. None of the included studies were at low risk of bias (rated A) as all trials had an element of pragmatism in using different methods. Fifteen studies including 87% participants were deemed to be at moderate risk of bias (rated B). We judged five studies including only 13% participants to have high risk of bias (rated C) in the field of study participation or statistical reporting.

Association of BCG Vaccination in Childhood with
Incidence of Atopic Disease. In the pooled analysis, we found that participants receiving BCG in childhood associated with a lower risk of atopic disease than that of the non-BCG group (OR � 0.87, 95% CI 0.77 to 0.99; Figure 2). Nineteen studies involving 80922 participants reported the relationship between BCG vaccination and the risk of asthma. Compared with non-BCG group, individuals received BCG in early childhood were associated with a significantly reduced risk of asthma (OR � 0.77, 95% CI 0.63 to 0.93; Figure 2). irteen studies including 72646 participants reported the association between eczema and BCG vaccine, and twelve studies including 69360 participants studied rhinitis and BCG vaccine. Compared with the control group, BCG vaccine showed no significant effect on preventing eczema and rhinitis (OR � 0.94 and 0.99, 95% CI 0.76 to 1.16 and 0.81 to 1.21, respectively; Figure 2).

Association between BCG Vaccination and Demographic
Factors on the Risk of Atopic Disease. We evaluated the association between BCG vaccination and prevalence of asthma in terms of participants' demographics. Participants from developed countries were associated with a significantly lower risk of developing asthma when administered BCG vaccine in early childhood (pooled OR � 0.73, 95% CI 0.58 to 0.92; Figure 3). In contrast, in developing countries, participants who received BCG vaccine in early childhood were not associated with a significantly reduced risk of allergic disease (pooled OR � 0.86, 95% CI 0.69 to 1.07; Figure 3).
BCG vaccine was not associated with the risk of eczema in the subgroup analysis of different continents ( Figure 4). Similar results were obtained in participants with rhinitis ( Figure 5). ese results of our study did not support an association of BCG vaccination with reduction in the risk of eczema or rhinitis in both developed and developing countries.  Canadian Respiratory Journal

Publication Bias.
e total publication bias was outlined in the funnel plot ( Figure 6). Based on the funnel plots and Egger's test [42], it suggested that publication bias did not impact our estimates (p � 0.074).

Discussion
In this systematic review and meta-analysis, we revealed that BCG vaccine might be effective in preventing atopic disease, especially for asthma among developed countries. However, compared with asthma, the risk of developing atopic disease was not reduced for both eczema and rhinitis. In developing countries, there was no association between BCG vaccine and atopic diseases. As far as we know, this systematic review and meta-analysis is the latest study investigating the association between BCG vaccination and atopic diseases.
In this study, we found that receiving a BCG vaccine in early childhood especially reduced the risk of asthma. According to other studies on BCG vaccination and asthma, BCG vaccine, which was proved to prevent the inflammation caused by mycobacteria, has been tested to inhibit allergeninduced airway inflammation in a murine model of atopic asthma [43]. e immune balance was adjusted by BCG vaccine during 1-like activity and decreasing the IL-4 and IL-10 production. It was not clear how IFN-c influences 2 responses, but it may involve macrophage activation and inhibition of 2 lymphocytes development or change antigen presentation directly [44]. e mechanism of BCG vaccine on asthma was not as simple as changes in the 1/ 2 balance [45], and we speculated that early childhood BCG vaccination accelerates the conversion from a 2 to a 1 type, therefore restraining the expression of atopy.
In developed countries, the association between BCG vaccination and reduction of the risk of asthma is more significant than the other developing countries of the world.
is could be attributable to the fact that, in the regions other than developed countries, helminth infections and tuberculosis are endemic; therefore, the effects of BCG vaccine on reducing prevalence of allergic diseases were compromised [46]. e protection level of BCG vaccination is correlated to the gradient of exposure to environmental mycobacteria, a gradient from lower protection in countries close to the equator towards higher protection with increasing distance from the equator [47]. It is also reported that early-life events or diseases like perinatal circumstances or early allergen exposure would increase the prevalence of atopic diseases [48]. However, due to the limited number of studies included, heterogeneity might not be shown in the subgroup analysis of subjects at high risk or of developing countries. Based on this meta-analysis, the positive protective role of BCG vaccine in atopic disease requires further investigation, especially more cohort studies on children from high-risk areas.
Our meta-analysis has several strengths. Compared with a similar meta-analysis [19], six new articles were included in this paper. Each study sample size in this meta-analysis was assessed by two independent authors, so the result can be more accurate than others with low quality [50]. Furthermore, we followed the recommendations of the Cochrane Collaboration and PRISMA statement, including a priori protocol. Comprehensive assessment of the study quality was achieved by using STROBE checklist for the cohort studies and JADAD scale for the RCT study.
As with all systematic reviews, we may have failed to identify some studies, especially those with negative results, several studies are missing in this meta-analysis, including RCTs concerning neonatal BCG vaccination and wheeze and other possibly atopic diseases. erefore, this may have influenced our findings. Finally, the length of time that the early effective protection from BCG vaccination lasts remains unanswered. e age of participants in this review may partially explain the result of the protective effect of BCG vaccination. In Linehan's study, it was shown that any benefits of BCG vaccine are likely to be transient [51]. erefore, a large proportion of the protection from BCG vaccination may not be attributed to a reduction in the risk of atopy. Nonetheless, it can be confirmed that BCG vaccination in early childhood does reduce the risk of developing asthma in early life.

Conclusion
Our results provide evidence that BCG protects against the risk of atopic diseases with the most protective effects on asthma occurrence from multiple epidemiologically different settings. Our results also suggest that that BCG     Table 1 for reference citations. SE � standard error. p � 0.074. vaccination in early childhood is associated with reduced risk of atopic disease, especially in developed countries.

BCG:
Bacille Calmette-Guerin CI: Confidence intervals OR: Odds ratios PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses CENTRAL: Cochrane Central Register of Controlled Trials MOOSE: Meta-Analysis of Observational Studies in Epidemiology IL: Interleukin TH: T-helper cell.

Data Availability
All data generated or analyzed during this study are included in this published article and its supplementary information files.

Conflicts of Interest
e authors have no conflicts of interest.