Mexico has included the BCG vaccine in its national immunization program for decades, administering a single dose to newborns shortly after birth as protection against severe forms of tuberculosis. Coverage has historically stayed above 90%, making BCG one of the most reliably delivered vaccines in the country’s schedule. But that record has fractured in recent years, with coverage plummeting during the late 2010s and collapsing to around 21% in 2020. Behind the headline numbers lies a more layered story involving a unique vaccine strain, widening disparities among Indigenous communities, and growing scientific interest in what BCG does beyond tuberculosis.
Mexico’s Universal BCG Policy and Coverage Over Time
Mexico gives BCG as a single neonatal dose, typically within the first days of life. Among individual vaccines in the national schedule, BCG has consistently been one of the best-performing. A national survey found that timely BCG coverage in children under twelve months reached about 93%, far ahead of other vaccines in the same schedule like hepatitis B’s third dose, which lagged below 4%.1PubMed Central. Comparative estimates of crude coverage of the Mexican immunization program: Findings from a national survey Across multiple national health surveys between 2000 and 2018, BCG coverage held above 90% in children under five regardless of the age group examined.2PLoS ONE. Vaccination coverage estimation in Mexico in children under five years old: Trends and associated factors
That stability broke down starting around 2017. A 16-year analysis of Mexico’s Expanded Program on Immunizations found that coverage dropped below 90% in each of the final four years studied (2017–2020), bottoming out at roughly 21% in 2020. The researchers estimated that between about 3.9 million and nearly 5 million children went unvaccinated with BCG over the entire 2006–2021 study period.3MDPI (Vaccines). Report of 16 Years of the BCG Vaccine under the Expanded Program on Immunizations in Mexico (2006–2021) The 2020 collapse was largely driven by the COVID-19 pandemic, which disrupted health services across the country, but the slide began before the pandemic hit. Supply instability, shifting procurement decisions, and institutional reorganization within Mexico’s health system all played a role in the pre-pandemic dip.
The same 16-year study also flagged a paradox in the years when coverage was high: Mexico routinely purchased more BCG doses than it needed, leading to significant vaccine wastage. BCG is packaged in multi-dose vials that must be used within hours of opening, so any clinic session with fewer babies than expected means leftover doses get thrown away. When supply was ample, waste ran high. When supply tightened, coverage dropped. Neither end of that seesaw served the program well.
The BCG Mexico 1931 Strain
Not all BCG vaccines are the same. The original Bacillus Calmette-Guérin culture has been passaged in laboratories around the world since the 1920s, and different countries use daughter strains that have diverged genetically over time. Mexico uses BCG Mexico 1931, a strain derived from the original French Pasteur lineage but with its own distinct genomic fingerprint. Full genome sequencing showed that BCG Mexico 1931 has a circular chromosome of about 4.35 million base pairs and falls into what researchers call DU2 group IV on the BCG family tree, alongside strains like BCG Tokyo.4PubMed Central. Genomic and proteomic analyses of Mycobacterium bovis BCG Mexico 1931 reveal a diverse immunogenic repertoire against tuberculosis infection
What makes this matter practically is that BCG Mexico 1931 carries three regions of genetic difference not found in the more widely studied Pasteur and Tokyo strains, along with dozens of unique single-nucleotide changes affecting protein-coding genes.5PubMed Central. Genomic and proteomic analyses of Mycobacterium bovis BCG Mexico 1931 reveal a diverse immunogenic repertoire against tuberculosis infection These differences could influence how the vaccine stimulates the immune system, how strong a scar it leaves, and how it performs against local tuberculosis strains. Researchers have noted that BCG Mexico 1931 has a diverse set of proteins capable of triggering immune responses, which makes it a potentially rich platform for future vaccine improvements. Still, head-to-head clinical trials comparing real-world protection across BCG sub-strains remain scarce everywhere, not just in Mexico.
What BCG Does and Does Not Protect Against
The core purpose of BCG vaccination at birth is to shield infants and young children from the most dangerous forms of tuberculosis, particularly TB meningitis and disseminated (miliary) TB. On that front, the vaccine works well. The evidence is consistent that BCG reduces childhood TB illness and death.6PubMed. Vaccination strategies to prevent tuberculosis in the new millennium: from BCG to new vaccine candidates Where it falls short is in preventing the transmission of TB among adults. A vaccinated person can still become infected with Mycobacterium tuberculosis, carry it silently, and pass it on. That is why countries with universal BCG programs, Mexico included, still have significant TB burdens.
An interesting finding from a study of elderly Hispanic patients along the US–Mexico border adds nuance to the long-term picture. Among older adults, many of whom were born in Mexico and vaccinated at birth, BCG vaccination was still associated with a meaningful protective effect against active TB. Those who had a documented BCG scar had substantially lower odds of developing tuberculosis compared to unvaccinated individuals.7PLOS ONE. A prospective cross-sectional study of tuberculosis in elderly Hispanics reveals that BCG vaccination at birth is protective whereas diabetes is not a risk factor This challenges the common assumption that BCG protection fades entirely within 10 to 15 years. The same study turned up a surprising null finding: type 2 diabetes, widely considered a major TB risk factor, was not associated with active TB in that particular elderly cohort, despite being very common among the participants. The researchers did not have a definitive explanation, but it underscores how TB risk in real populations can defy textbook expectations.
A separate genomic study of drug-resistant TB patients in Mexico found that only about half showed evidence of prior BCG vaccination based on the characteristic scar, raising questions about whether gaps in vaccination coverage contribute to the persistence of TB, including resistant strains.8PLOS ONE. Genomic epidemiology analysis of drug-resistant Mycobacterium tuberculosis distributed in Mexico
Coverage Gaps Among Indigenous Communities
Mexico’s national averages for BCG coverage mask deep inequalities. A nationally representative survey examining vaccine uptake found that Indigenous-language-speaking populations had vaccination rates up to 30 percentage points lower than non-Indigenous populations for several vaccines, BCG among them.9PLOS Global Public Health. COVID-19 vaccine uptake and barriers among Indigenous language speakers in Mexico: Results from a nationally representative survey The same data showed that in 2018, only about 18.5% of one-year-old children nationally had full vaccination coverage across all scheduled doses, a number that was even lower in Indigenous communities.
The reasons are layered. Geographic isolation plays a role: many Indigenous communities are in rural or mountainous areas where clinics are sparse and cold-chain infrastructure for vaccine storage is unreliable. Language barriers between healthcare workers and families create additional friction. Institutional trust also matters. Mexico’s public health system underwent a major restructuring in recent years, transitioning from the Seguro Popular model to INSABI and then to IMSS-Bienestar, and those transitions disrupted service delivery in regions that were already underserved. For a vaccine like BCG that must be given within the first days or weeks of life, even brief disruptions in clinic availability can mean a missed window that is hard to recover.
Safety of BCG in Mexican Children
BCG is a live attenuated vaccine, meaning it contains a weakened but living form of Mycobacterium bovis. In healthy newborns it is considered safe, but because it is alive, it can occasionally cause local or systemic complications. A study at a Mexican children’s hospital identified 22 adverse events among roughly 15,000 vaccinated children, yielding a rate of about 1.5 per thousand doses. The most common reaction was lymphadenopathy, or swollen lymph nodes near the injection site, followed by persistent pustules, nodules, and ulcers.10Boletín Médico del Hospital Infantil de México. Adverse reactions in association with Bacillus Calmette-Guerin immunization in Mexican children Four of the affected children had underlying immunosuppression, which is the main risk factor for serious BCG complications.
In immunodeficient patients, particularly those with severe combined immunodeficiency or advanced HIV, BCG can cause localized disease (sometimes called BCGitis) or, in the worst cases, disseminated infection throughout the body (BCGosis), which can be fatal.11PubMed Central. BCG: a vaccine with multiple faces This is why screening for immune deficiencies before BCG vaccination is clinically important, though in practice, most severe immunodeficiencies are not yet diagnosed at the time of neonatal vaccination. In countries like Mexico where BCG is given universally at birth, a rare case of disseminated BCG disease is sometimes the first signal that a child has an undiagnosed immune disorder.
Trained Immunity and Effects Beyond Tuberculosis
One of the more fascinating developments in immunology over the past decade has been the recognition that BCG does more than just protect against TB. The vaccine appears to reprogram certain innate immune cells so that they respond more vigorously to a range of unrelated pathogens. Researchers call this phenomenon trained immunity. It involves long-lasting changes in how monocytes, macrophages, and natural killer cells behave, driven by modifications to the chemical tags on their DNA packaging (epigenetic reprogramming) and shifts in how those cells generate energy (metabolic reprogramming).12PubMed Central. BCG-induced trained immunity: history, mechanisms and potential applications
Studies in both mice and humans have shown that after BCG vaccination, monocytes ramp up a specific metabolic pathway, glycolysis, and that this metabolic shift is essential for the epigenetic changes that underlie trained immunity. Blocking the enzymes involved in glycolysis blocks the trained immunity effect.13Cell Reports. Immunometabolic Pathways in BCG-Induced Trained Immunity More remarkably, BCG vaccination appears to reprogram not just mature immune cells circulating in the blood but also the stem cells in bone marrow that produce those cells. Three months after vaccination, bone marrow stem cells showed lasting changes in gene activity that biased them toward producing myeloid cells, the frontline responders of the innate immune system.14Cell Host & Microbe. Persistent Epigenetic Reprogramming of Human Hematopoietic Stem Cells after BCG Vaccination That stem-cell-level reprogramming helps explain why BCG’s non-specific protective effects can last for months or years rather than fading within days.
During the COVID-19 pandemic, these non-specific effects drew worldwide attention. Researchers hypothesized that countries with universal BCG programs might see some degree of protection against severe COVID-19, and several ecological studies explored the association. One global assessment noted that BCG is already known to protect against respiratory infections and bladder cancer in addition to TB, and investigated whether similar mechanisms could blunt COVID-19 severity.15ScienceDirect / Journal of Clinical Tuberculosis and Other Mycobacterial Diseases. Variances in BCG protection against COVID-19 mortality: A global assessment The evidence remained inconclusive: ecological correlations between national BCG policies and COVID outcomes are confounded by many variables, and randomized trials launched during the pandemic produced mixed results. Still, the hypothesis spurred renewed interest in BCG’s broader immunological potential, and several clinical trials of BCG revaccination for elderly and at-risk populations are ongoing globally.
The Diagnostic Wrinkle of Prior BCG Vaccination
One practical headache that BCG vaccination creates is confusion when screening for latent tuberculosis infection. The tuberculin skin test, which involves injecting a small amount of purified protein derivative under the skin and measuring the resulting bump, can come back positive in someone who was vaccinated with BCG as an infant and has never been infected with TB. This false-positive concern has driven debate for years about which screening tool to use in populations with high BCG coverage.
A prevalence study in Guadalajara, Mexico, compared tuberculin skin test results with an interferon-gamma release assay (a blood test unaffected by prior BCG vaccination) in children. Despite 83% of the children having a history of BCG vaccination, the researchers found no cases where the skin test was positive while the blood test was negative. They attributed this to the fact that most children were over six years old and had been vaccinated in the first three months of life; by more than five years after vaccination, a positive skin test result likely reflects genuine latent infection rather than a vaccine artifact.16PLoS ONE. A prevalence study in Guadalajara, Mexico, comparing tuberculin skin test and QuantiFERON-TB Gold In-Tube This is a useful insight for clinicians in Mexico: in older children and adults vaccinated as newborns, a positive skin test should not be casually dismissed as a BCG effect. It probably means actual TB exposure.
BCG as a Cancer Treatment
Beyond its role as a vaccine, BCG has a second life as a therapy for bladder cancer. For non-muscle-invasive bladder cancer, BCG is instilled directly into the bladder after surgical removal of visible tumors. The live bacteria provoke a strong local immune response that helps destroy remaining cancer cells and reduces the likelihood of recurrence. This use has been standard in urology worldwide since the 1970s.
A clinical study at a Mexican hospital followed patients with non-muscle-invasive bladder cancer treated with intravesical BCG and found a recurrence rate of about 51% and a progression rate of about 39%, with a median progression-free survival of 4.7 years. Patients with multiple lesions, high-grade tumors, or deeper-stage disease had worse outcomes. The most common side effects were bladder inflammation and mild fever.17SciELO – Scientific Electronic Library Online. Evolución clínica de pacientes con cáncer de vejiga no músculo-invasor tratados con bacilo Calmette-Guérin Global shortages of BCG for bladder cancer use have periodically affected Mexican hospitals, since the therapeutic-grade product requires careful manufacturing and quality control that not every supplier can guarantee. When BCG supply runs short, oncologists face difficult choices about treatment alternatives.
The overlap between BCG as a childhood vaccine and BCG as a cancer drug sometimes confuses patients. The formulations differ in concentration, preparation, and route of administration, and a person’s neonatal BCG vaccination does not substitute for intravesical therapy. Still, the biological principle is related: in both contexts, a weakened mycobacterium is being used to stimulate immune cells that would otherwise remain quiescent.
Where Mexico’s BCG Program Stands Now
Mexico’s BCG program faces a recovery challenge. The sharp coverage drop in 2020 means that an entire birth cohort has largely missed the vaccine, and catching up is harder with BCG than with most other vaccines because it is ideally given in the neonatal period. Vaccinating older children is possible but less common in practice, and the immunological benefit may differ when the dose is delayed. The millions of children estimated to have missed BCG between 2006 and 2021 represent an accumulating gap in population-level protection against childhood TB.18MDPI (Vaccines). Report of 16 Years of the BCG Vaccine under the Expanded Program on Immunizations in Mexico (2006–2021)
At the same time, TB has not gone away. Mexico reported over 20,000 new TB cases per year before the pandemic, and disruptions in both vaccination and case detection during 2020–2021 may have widened the gap between actual infections and reported ones. Drug-resistant strains circulate in several Mexican states, and among patients with resistant TB, only about half show evidence of having received BCG in infancy.19PLOS ONE. Genomic epidemiology analysis of drug-resistant Mycobacterium tuberculosis distributed in Mexico Whether improving BCG coverage would meaningfully reduce drug-resistant TB is debatable, given that the vaccine’s main contribution is preventing severe childhood disease rather than blocking transmission. But it would at least protect the youngest and most vulnerable children from forms of TB that are otherwise devastating.
Meanwhile, the global pipeline for improved TB vaccines is more active than it has been in decades. Several candidates aim to either replace BCG or boost its effects through revaccination strategies in adolescents and adults, the age groups where BCG’s protection is weakest. If any of these candidates succeed in late-stage trials, Mexico’s large and relatively well-organized immunization infrastructure could position it for faster-than-average adoption, assuming the procurement and supply-chain problems of recent years are resolved.