Vaccination and the Risk of Childhood Cancer-A Systematic Review and Meta-Analysis

Causes of Childhood Cancer: A Review of the Recent Literature: Part I-Childhood Factors

PURPOSE

To review the childhood risk factors for pediatric cancer (diagnosis before age 20).

METHODS

We conducted literature searches using Ovid Medline and Scopus to find primary research studies, review articles, and meta-analyses published from 2014 to 3 March 2021.

RESULTS

Strong evidence indicates that an array of genetic and epigenetic phenomena, structural birth defects, and chromosomal anomalies are associated with an increased risk of various childhood cancers. Increased risk is also associated with prior cancer, likely due to previous treatment agents and therapeutic ionizing radiation. Convincing evidence supports associations between several pediatric cancers and ionizing radiation, immunosuppression, and carcinogenic virus infection both in healthy children and in association with immune suppression following organ transplantation. Breastfeeding and a childhood diet rich in fruits and vegetables appears to reduce the risk of pediatric leukemia but the evidence is less strong. Childhood vaccination against carcinogenic viruses is associated with a lower risk of several cancers; there is less strong evidence that other childhood vaccinations more broadly may also lower risk. Ultraviolet (UV) radiation is associated with increased melanoma risk, although most melanomas following childhood UV exposure occur later, in adulthood. Evidence is weak or conflicting for the role of body mass index, other childhood infections, allergies, and certain treatments, including immunomodulator medications and human growth therapy.

Association between medically diagnosed postnatal infection and childhood cancers: A matched case-control study in Denmark, 1978 to 2016

Although the association between infection and childhood cancer has been long investigated, there is limited information on rarer cancers. This article aimed to explore the association between postnatal infection and childhood cancers in the Danish population. A matched case-control study was conducted using Danish nationwide registries from 1978 to 2016. Each childhood cancer case was matched 1:25 with controls by birth date within a week and sex. Postnatal infections were identified from the Danish National Patient Registry, which lists diagnoses seen in hospital, specialist or emergency care services. Multivariable conditional logistic regression was used to estimate adjusted odds ratios (adj.OR) and 95% confidence intervals (CI). Specific types of infections and the number of infection episodes were also considered. The study included 4125 childhood cancer cases and 103 526 matched controls with ages ranging from 0 to 19 years. Medically diagnosed postnatal infections were positively associated with many types of childhood cancer including acute lymphoblastic leukemia (adj.OR = 1.42; 95% CI: 1.23-1.63), acute myeloid leukemia (adj.OR = 1.80; 95% CI: 1.28-2.52), non-Hodgkin lymphoma (adj.OR = 1.53; 95% CI: 1.19-1.97) and central nervous system tumors (adj.OR = 1.57; 95% CI: 1.39-1.77). A higher number of infection episodes were also associated with an increased risk of these cancers. Specific infections such as viral, enteric and urinary tract infections were also strongly associated with specific types of cancer. In conclusion, children who later develop cancer appear to have adverse reactions to infections necessitating referral to specialized health care services, perhaps indicating dysregulated immune function.

BCG vaccination policy, natural boosting and pediatric brain and CNS tumor incidences

Bacille Calmette-Guérin (BCG) vaccination supposedly imparts and augments "trained immunity" that cross-protects against multiple unrelated pathogens and enhances general immune surveillance. Gradual reductions in tuberculosis burden over the last 3-5 decades have resulted in the withdrawal of BCG vaccination mandates from developed industrialized countries while reducing to a single neonatal shot in the rest. Concurrently, a steady increase in early childhood Brain and CNS (BCNS) tumors has occurred. Though immunological causes of pediatric BCNS cancer are suspected, the identification of a causal protective variable with intervention potential has remained elusive. An examination of the countries with contrasting vaccination policies indicates significantly lower BCNS cancer incidence in 0-4-year-olds (per hundredthousand) of countries following neonatal BCG inoculations (n=146) vs. non-BCG countries (n=33) [Mean: 1.26 vs. 2.64; Median: 0.985 vs. 2.8; IQR: 0.31-2.0 vs. 2.4-3.2; P=<0.0001 (two-tailed)]. Remarkably, natural Mycobacterium spp. reexposure likelihood is negatively correlated with BCNS cancer incidence in 0-4-year-olds of all affected countries [r(154): -0.6085, P=<0.0001]. Seemingly, neonatal BCG vaccination and natural "boosting" are associated with a 15-20-fold lower BCNS cancer incidence. In this opinion article, we attempt to synthesize existing evidence implying the immunological basis of early childhood BCNS cancer incidence and briefly indicate possible causes that could have precluded objective analysis of the existing data in the past. We draw the attention of the stakeholders to consider the comprehensive evaluation of immune training as a potential protective variable through well-designed controlled clinical trials or registry-based studies as feasible for its potential applications in reducing childhood BCNS cancer incidence.

"Trained Immunity" from Mycobacterium spp. exposure (BCG vaccination and environmental) may have an impact on the incidence of early childhood leukemia

Preventive variables for childhood leukemia incidence (LI) remain unknown. Past assertions that childhood vaccinations, especially BCG, may be potentially protective have remained disputed for over five decades because of the lack of a unifying framework to explain variable outcomes in different studies. An examination of the early childhood LI for 2020 in European Region countries with supposedly similar underlying confounders but differential childhood vaccination coverage displays negative covariation with prevailing Mycobacterium spp. exposure in BCG-vaccinated children. The childhood LI in 0-4-year-old populations with >90% childhood BCG vaccination coverage is found to be strongly but negatively correlated with prevailing tuberculin immunoreactivity [r(24): -0.7868, p-value: < 0.0001]. No such correlation existed for the LI in 0-4-year-old populations without BCG vaccinations, though weak associations are hinted at by the available data for MCV2, PCV3, and DTP3 vaccinations. We hypothesize that early childhood BCG vaccination "priming" and subsequent "trained immunity" augmentation by "natural" boosting from Mycobacterium spp. exposure play a preventive and protective role in childhood LI. The non-consideration of prevailing "trained immunity" could have been a cause behind the conflicting outcomes in past studies. Exploratory studies, preferably performed in high-burden countries and controlling for the trained-immunity correlate and other potential confounders, would be warranted in order to establish a role for BCG vaccination and early-life immune training (or lack thereof) in childhood LI and help put the current controversy to rest.

Germline De Novo Mutations as a Cause of Childhood Cancer

Germline de novo mutations (DNMs) represent one of the important topics that need extensive attention from epidemiologists, geneticists, and other relevant stakeholders. Advances in next-generation sequencing technologies allowed examination of parent-offspring trios to ascertain the frequency of germline DNMs. Many epidemiological risk factors for childhood cancer are indicative of DNMs as a mechanism. The aim of this review was to give an overview of germline DNMs, their causes in general, and to discuss their relation to childhood cancer risk. In addition, we highlighted existing gaps in knowledge in many topics of germline DNMs in childhood cancer that need exploration and collaborative efforts.