Bacillus Calmette-Guérin (BCG) vaccines are given to more than 100 million children every year, but there is considerable debate regarding the effectiveness of BCG vaccination in preventing tuberculosis and death, particularly among older children and adults.
The most extensive study ever conducted on the efficacy of the BCG vaccine for protection against tuberculosis, stratified by age and history of previous tuberculosis, was published in September 2022 in. The study, which comprises a systematic review and meta-analysis, analyzed individual-level data from 26 case-contact cohort studies published over the past 20 years. The studies included data from 70,000 participants. The primary outcome was a composite of prevalent (diagnosed at or within 90 days of baseline) and incident (diagnosed more than 90 days after baseline) tuberculosis in contacts exposed to tuberculosis. Secondary outcomes were pulmonary tuberculosis, extrapulmonary tuberculosis, and mortality.
Participants were characterized as having been exposed to tuberculosis if they were reported to have been a close contact (either living in the same household or having substantial interaction outside the household) of a person with microbiologically or radiologically diagnosed pulmonary tuberculosis. Previous tuberculosis was defined as a positive interferon-gamma (IFN-gamma) release assay or tuberculin skin test, also known as PPD or Mantoux test.
Most studies included in the analysis were conducted in the past 10 years in countries with a high tuberculosis burden. Those countries included India, South Africa, China, Vietnam, Indonesia, Uganda, the Gambia, and Brazil.
The study’s main findings included the following:
- The overall effectiveness of BCG vaccination against all forms of tuberculosis was 18% (adjusted odds ratio, 0.82; 95% CI, 0.74-0.91).
- Stratified by age, BCG vaccination only significantly protected against all tuberculosis in children younger than 5 years (aOR, 0.63; 95% CI, 0.49-0.81).
- There was no protective effect among those whose previous tests for tuberculosis were negative unless they were younger than 5 years (aOR, 0.54; 95% CI, 0.32-0.90).
- Among contacts who had a positive tuberculin skin test or IFN-gamma release assay, BCG vaccination significantly protected against tuberculosis among all participants (aOR, 0.81; 95% CI, 0.69-0.96), participants younger than 5 years (aOR, 0.68; 95% CI, 0.47-0.97), and participants aged 5-9 years (aOR, 0.62; 95% CI, 0.38-0.99).
- BCG vaccination was protective against pulmonary tuberculosis (19% effectiveness), but this effect was only seen in children younger than 3 years (42% effectiveness) when stratified by age.
- Protection against all tuberculosis and pulmonary tuberculosis was greater among female participants than male participants.
“This is a definitive BCG protection study because it involves a significant number of individuals evaluated using this meta-analysis. Protection is clearly lost with age. From as early as age 5, no protective effect can be observed. Protection, including against pulmonary tuberculosis, can be observed up to 3 years of age,” stated study author Julio Croda, MD, PhD, chair of the Brazilian Society of Tropical Medicine.
Dr. Croda emphasized that the findings from their study indicate that BCG vaccine protects against pulmonary tuberculosis and that those results differ from results of some previous studies.
“Every physician believes the BCG vaccine protects against serious forms of tuberculosis up to age 5. That fact is not surprising at all,” Dr. Croda remarked. “However, the fact that it protects against pulmonary tuberculosis, especially in children younger than 3, was surprising. In medical practice, we did not believe in this protection.”
Currently, 1.2% of new tuberculosis cases in Brazil occur among those younger than 5. Nevertheless, these cases represent 40.1% of new diagnoses recorded among those younger than 15, highlighting the importance of protection for this age group. An increase in extrapulmonary tuberculosis cases was recently observed in patients younger than 5.
Isabella Ballalai, MD, PhD, is deputy chair of the Brazilian Society of Immunizations. Although she did not participate in this study, she commented on its findings. “All publications are welcome; they help us think,” she explained. She emphasized that the BCG vaccine is not optimal. “There are studies indicating 80% efficacy and others indicating 0%. So, what we can look at is decades of effectiveness in practice.”
Dr. Ballalai explained that the BCG vaccine could keep severe forms of tuberculosis, meningitis, and miliary tuberculosis at bay. She shared her experience of caring for several patients with tuberculous meningitis shortly after she had graduated. “Today, thanks to the BCG vaccine, we don’t see it anymore.” However, she pointed out that the vaccine›s efficacy and effectiveness against pulmonary tuberculosis are low and that pulmonary tuberculosis remains the most significant problem among adults.
Dr. Ballalai also emphasized a few shortcomings of the study. “One is the definition of ‘vaccinated’ and ‘unvaccinated,’ which was based on the presence or absence of a mark on the arm. Today, we know that the absence of a mark does not indicate that the child has not been vaccinated, nor that the vaccine has not been effective. Therefore, several vaccinated participants may have been included amongst the unvaccinated participants.”
The authors emphasized that the definition of “vaccinated” and “unvaccinated” was based on a scar and on vaccination records, and they recognized that participants who did not have a scar on their arm could have been misclassified. Regardless, it is still considered a sensitive indicator. “Few vaccinated children from various settings do not show a scar years after vaccine administration,” they stated in their article.