BCG vaccination and the incidence of lymphomas and leukaemia

Efficacy of Bacillus Calmette-Guérin in Cancer Prevention and Its Putative Mechanisms

Bacillus Calmette-Guérin (BCG) is an attenuated strain of Mycobacterium bovis. Although it was developed as a prophylactic vaccine against tuberculosis (TB), researchers have also evaluated it for preventing cancer development or progression. These studies were inspired by the available data regarding the protective effects of microbial infection against cancers and an inverse relationship between TB and cancer mortality. Initial studies demonstrated the efficacy of BCG in preventing leukemia, melanoma and a few other cancers. However, mixed results were observed in later studies. Importantly, these studies have led to the successful use of BCG in the tertiary prevention of non-muscle invasive bladder cancer, wherein BCG therapy has been found to be more effective than chemotherapy. Moreover, in a recently published 60-year follow-up study, childhood BCG vaccination has been found to significantly prevent lung cancer development. In the present manuscript, we reviewed the studies evaluating the efficacy of BCG in cancer prevention and discussed its putative mechanisms. Also, we sought to explain the mixed results of BCG efficacy in preventing different cancers.

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

INTRODUCTION

Infections may play a role in the etiology of childhood cancer and immunizations may be protective because vaccinations stimulate the immune system. Observational studies reported inconsistent associations between vaccination and risk of childhood cancer. Since a synthesis of the evidence is lacking, we conducted a meta-analysis stratified by histological and site-specific cancer.

METHODS

We performed a systematic review (CRD42020148579) following PRISMA guidelines and searched for literature in MEDLINE, Embase, and the Science Citation Index databases. We identified in three literature databases 7,594 different articles of which 35 met the inclusion criteria allowing for 27 analyses of 11 cancer outcomes after exposure to nine different types of vaccinations. We calculated summary odds ratios (ORs) and 95% confidence intervals (CIs) using random effects models.

RESULTS

We observed four inverse associations between childhood leukemia and certain vaccines as well as the number of vaccinations: OR 0.49 (95% CI = 0.32 to 0.74) for leukemia death after bacillus Calmette-Guérin vaccination; OR 0.76 (95% CI = 0.65 to 0.90) for acute lymphoblastic leukemia after Haemophilus influenzae type b vaccination; OR 0.57 (95% CI = 0.36 to 0.88) for leukemia; and OR 0.62 (95% CI = 0.46 to 0.85) for acute lymphoblastic leukemia after three or more vaccinations of any type. All other conducted analyses did not show any associations.

DISCUSSION

The results are consistent with the hypothesis that vaccinations reduce the risk of childhood leukemia. However, the robustness and validity of these results is limited due to the small number, substantial heterogeneity, and methodological limitations of available studies.

Association of BCG Vaccination in Childhood With Subsequent Cancer Diagnoses: A 60-Year Follow-up of a Clinical Trial

IMPORTANCE

The BCG vaccine is currently the only approved tuberculosis vaccine and is widely administered worldwide, usually during infancy. Previous studies found increased rates of lymphoma and leukemia in BCG-vaccinated populations.

OBJECTIVE

To determine whether BCG vaccination was associated with cancer rates in a secondary analysis of a BCG vaccine trial.

DESIGN, SETTING, AND PARTICIPANTS

Retrospective review (60-year follow-up) of a clinical trial in which participants were assigned to the vaccine group by systematic stratification by school district, age, and sex, then randomized by alternation. The original study was conducted at 9 sites in 5 US states between December 1935 and December 1998. Participants were 2963 American Indian and Alaska Native schoolchildren younger than 20 years with no evidence of previous tuberculosis infection. Statistical analysis was conducted between August 2018 and July 2019.

INTERVENTIONS

Single intradermal injection of either BCG vaccine or saline placebo.

MAIN OUTCOMES AND MEASURES

The primary outcome was diagnosis of cancer after BCG vaccination. Data on participant interval health and risk factors, including smoking, tuberculosis infection, isoniazid use, and other basic demographic information, were also collected.

RESULTS

A total of 2963 participants, including 1540 in the BCG vaccine group and 1423 in the placebo group, remained after exclusions. Vaccination occurred at a median (interquartile range) age of 8 (5-11) years; 805 participants (52%) in the BCG group and 710 (50%) in the placebo group were female. At the time of follow-up, 97 participants (7%) in the placebo group and 106 participants (7%) in the BCG vaccine group could not be located; total mortality was 633 participants (44%) in the placebo group and 632 participants (41%) in the BCG group. The overall rate of cancer diagnosis was not significantly different in BCG vaccine vs placebo recipients (hazard ratio, 0.82; 95% CI, 0.66-1.02), including for lymphoma and leukemia. The rate of lung cancer was significantly lower in BCG vs placebo recipients (18.2 vs 45.4 cases per 100 000 person-years; hazard ratio, 0.38; 95% CI, 0.20-0.74; P = .005), controlling for sex, region, alcohol overuse, smoking, and tuberculosis.

CONCLUSIONS AND RELEVANCE

Childhood BCG vaccination was associated with a lower risk of lung cancer development in American Indian and Alaska Native populations. This finding has potentially important health implications given the high mortality rate associated with lung cancer and the availability of low-cost BCG vaccines.

The non-Hodgkin lymphomas: a review of the epidemiologic literature

The non-Hodgkin lymphomas (NHL) are a heterogeneous group of B-cell and T-cell neoplasms that arise primarily in the lymph nodes. NHL incidence rates in the US doubled between about 1970 and 1990, and stabilized during the 1990s. NHL accounts for approximately 3.4% of cancer deaths in the US. Although some of the observed patterns in NHL have been related to HIV/AIDS, these conditions cannot fully explain the magnitude of the changes; neither do changes in classification systems nor improved diagnostic capabilities. Studies of occupational and environmental exposures (e.g., pesticides, solvents) have produced no consistent pattern of significant positive associations. Inverse associations with ultraviolet radiation exposure and alcohol and fish intake, and positive associations with meat and saturated fat intake have been reported in several studies; additional studies are needed to confirm or refute these associations. Family history of NHL or other hematolympho-proliferative cancers and personal history of several autoimmune disorders are associated with increased risk of NHL, but are not likely to account for a large proportion of cases. HIV and other infectious agents, such as human herpesvirus 8 and Epstein-Barr, appear to be associated with differing types of NHL, such as some B-cell lymphomas. Future epidemiologic studies should evaluate associations by NHL type, enhance exposure information collected, and elucidate factors that may identify susceptible (or resistant) subpopulations because of genetic, immunologic or other characteristics. The extent to which the etiology of NHL types may differ is important to resolve in ongoing and future studies.

Prevention of childhood leukemia by BCG vaccination in newborns? A population-based case-control study in Lower Saxony, Germany

Data from a case-control study in Lower Saxony, Germany, were used to assess whether the risk for childhood cancer may be reduced by bacille Calmette-Guérin (BCG) vaccination in the neonatal period. There were 420 newly diagnosed childhood cancer cases from the German cancer registry and 613 controls eligible for this study. A mailed questionnaire was completed during a telephone interview with parents. Details on the perinatal history were abstracted from the birth charts by nurses blinded to the children's case-control status. Complete information was available for 259 cases and for 323 controls. A total of 85% of the controls had been BCG vaccinated in the newborn period. The adjusted odds ratios for BCG vaccination were 0.90 (95% confidence interval; 0.51-1.61) for leukemia and 0.61 (95% confidence interval; 0.25-1.50) for other cancers. Based on these data the probability of a 50% or more reduction of more reduction of the cancer risk by BCG vaccination in the newborn period is small. The statistical power of this study, however, was not high enough to rule out a smaller, still relevant reduction in cancer risk.

BCG prophylaxis against cancer: methodological evaluation of a continuing controversy

A possible protective effect of tuberculosis against cancer, together with the known immune-stimulating properties of BCG, have motivated 14 investigations of BCG prophylaxis against cancer. The results have been conflicting: 3 studies found high levels of protection, 8 found no effect, and 3 suggested that BCG may actually raise the risk of cancer. To assess whether the inconsistent results could have a methodological basis, we analyzed each study according to several basic criteria necessary for scientific validity in cause-effect research. The analyses showed two major problems: (1) None of the studies used safeguards against detection bias by ensuring that the diagnosis of cancer in vaccinees and non-vaccinees was made with equal follow-up and surveillance procedures, and in an accurate and unbiased fashion; (2) Studies alleging that BCG is carcinogenic were weakened by the inappropriate use of cancer mortality data, by the analysis of multiple BCG-cancer associations without appropriate statistical adjustment, and by post hoc creation of new cancer risk categories after inspection of the data. Since these methodologic inadequacies preclude any firm conclusions about the role of BCG in preventing cancer, a resolution of the controversy will require new research, designed and conducted with greater attention to methodologic principles necessary to ensure scientific validity.

BCG and vole bacillus vaccination in adolescence and mortality from leukaemia

In total, 28 deaths from leukaemia are known to have occurred among the 54,239 participants in the Medical Research Council tuberculosis vaccines trial (initially aged about 14 years), from its beginning in 1950 to the end of 1979. There is evidence that this total is likely to be very nearly complete. During the entire period (average 27.1 years) the leukaemia mortality per million person-years was 25 in the BCG vaccinated group, 20 in the vole-bacillus vaccinated group, and 21 in the randomly-allocated unvaccinated (control) group. Although there was neither a benefit nor a disadvantage from vaccination over the whole period, detailed figures suggested that there may have been a beneficial effect against leukaemia during the first 15 years after entry (that is between age 15 and 30 years) and a deleterious effect after longer intervals (or above the age of 30 years). Alternatively this swing may simply represent an unusual chance fluctuation. There has been no sign in recent years of any substantial decrease in leukaemia mortality in England and Wales at ages 15-29 years, although the proportion of children given BCG vaccine at about age 13 years has risen from less than 1 per cent (for the cohort aged 15-19 years in 1951-1955) to more than 70 per cent 25 years later. It has therefore been provisionally concluded that chance is the explanation for the apparent swing from prevention to enhancement, and that BCG vaccination at age 13-14 years in Great Britain does not affect subsequent mortality from leukaemia.