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 in The Lancet Global Health. 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 (IFNγ) 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. In Brazil, Julio Croda, MD, PhD, Eduardo Martins Netto, MD, PhD, Antonio Carlos Lemos, MD, and Anna Cristina C. Carvalho, MD, PhD, participated in the study.

Primary Outcomes

The study’s main findings included the following:

• The overall effectiveness of BCG vaccination against all forms of tuberculosis was 18% (adjusted odds ratio [aOR], 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 IFNγ 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 Croda, chair of the Brazilian Society of Tropical Medicine and advisor to Medscape Portuguese Edition.

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,” 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 was invited by Medscape Portuguese Edition to comment 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.”

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.

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 emphasize that the definition of “vaccinated” and “unvaccinated” was based on a scar and on vaccination records, and they recognize 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 state in their article.

Adults Unprotected

Ballalai also shared her concerns regarding the lack of protection for older individuals. “We know those older than 60 are at greater risk for complications of tuberculosis. Individuals in this age group naturally have a lower immunity, and they usually have comorbidities. From this study, I can only conclude what was already expected: that adults who received a BCG vaccine as infants are not clear of pulmonary tuberculosis.”

Croda agreed that it was already evident that the BCG vaccine administered at birth did not provide protection for adults. “In the past, even with 80% to 90% vaccine coverage, there were numerous tuberculosis cases in adults in Brazil,” he affirmed.

Are Boosters Needed?

The authors concluded that immunoprotection needs to be boosted in older populations, as vaccination at birth is ineffective for adolescents and adults. They have also discussed whether children older than 10 years and adults could benefit from a booster shot.

Croda emphasized that there is no indication for this, because there are no data regarding protection with a booster dose during adulthood. However, he cited a study conducted in South Africa in which the BCG vaccine was compared with another vaccine, and another study, which is being conducted in India, is assessing whether a BCG booster offers protection against pulmonary tuberculosis. “There are few studies. Perhaps the revaccination of more vulnerable groups could be of interest, but additional studies are needed first.”

Croda intends to assess revaccination in those deprived of liberty, in which the incidence of tuberculosis is very high. From 2015 to 2021, many new cases were recorded in this population in Brazil. The number rose from 5860 to 6773 during that period.

“However, BCG revaccination carries a significant risk of patients presenting with serious adverse events,” Ballalai pointed out. He noted that several years ago, to extend protection, Brazil adopted a booster program for persons 10 years of age or older, but the program was discontinued owing to the numerous adverse events reported and the absence of evidence of benefit from increased protection against tuberculosis.

“The adult groups at greater risk for severe tuberculosis manifestations normally presented with an underlying disease, particularly in immunocompromised patient groups. The [administration of the] BCG [vaccine] is contraindicated for those who are immunocompromised. And, for the older population, we do not have data on [vaccine] safety,” she emphasized.

Nonspecific Immune Protection

One of the study’s secondary outcomes regarded mortality. Four studies in the meta-analysis followed up tuberculosis contacts for death. In these studies, which evaluated 20,000 participants, BCG vaccination was shown to be significantly protective against death for participants younger than 15 years.

However, the authors urge caution in interpreting these data. They emphasize that they were unable to identify specific mechanisms by which BCG vaccination might have reduced mortality, and there are possible study biases that could have led to an overestimation of mortality benefit. Moreover, given the observational nature of the included studies, vaccinated children might have had higher socioeconomic status and greater access to healthcare, and they may have been more likely to have received other vaccinations, compared with children who did not receive BCG vaccines.

Nevertheless, previous experimental and observational studies have found that BCG vaccination might provide nonspecific or off-target immune protection against an array of other pathogens.

“In small studies conducted in Africa, those younger than 5 were protected not only against tuberculosis but also against other respiratory diseases,” Croda affirmed. “However, these are small studies, and for now, there is no recommendation for using BCG vaccination to prevent other respiratory infections.”

A long-awaited, critical study on the impact of the BCG vaccine on COVID-19, in which Brazilian researchers participated, will be published in The New England Journal of Medicine.

New Vaccines Needed

The BCG vaccine is one of the oldest vaccines, but there are still several crucial unanswered questions about its use.

Previously published studies that examined the protective effect of BCG vaccination only considered low-burden settings and the historical literature before 1950. These studies need updating, but doing so has not been a simple task. To answer their questions, individual-level participant data for a prespecified list of variables, including the characteristics of the exposed participant (contact), the index case, and the environment, were requested from authors of all eligible studies.

Much of the data used in the published research were found through discussions with authors and experts in the field, as well as through data deposited in data storage repositories, conference abstracts, dissertations, and even direct requests to the authors. “The Pan-American Health Organization helped with this data collection and contacting some authors,” said Croda.

With the new data, the authors confirmed that infant BCG vaccination, although important to young children who are at high risk for tuberculosis, does not prevent adult-type cavitary tuberculosis and is therefore insufficient to impede the tuberculosis epidemic. “Novel vaccines are urgently needed,” they concluded. “We need to develop novel, more effective vaccines, which, when administered during infancy, would ensure lifelong protection,” added Croda.

Croda and Ballalai reported no relevant financial relationships.

Roxana Tabakman is a biologist, freelance reporter, and writer. She was born in Argentina and currently resides in São Paulo, Brazil. She is the author of the books A Saúde na Mídia, Medicina para Jornalistas, Jornalismo para Médicos (in Portuguese), and Biovigilados (in Spanish). Follow her on Twitter: @roxanatabakman.

This article was translated from the Medscape Portuguese edition .

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