Adults unprotected
Dr. 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.”
Dr. 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%-90% vaccine coverage, there were numerous tuberculosis cases in adults in Brazil.”
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.
Dr. 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.”
Dr. 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 5,860 to 6,773 during that period.
“However, BCG revaccination carries a significant risk of patients presenting with serious adverse events,” Dr. Ballalai pointed out. He noted that several years ago, to extend protection, Brazil adopted a booster program for persons aged 10 years 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 urged caution in interpreting these data. They emphasized 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 health care, 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,” Dr. 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.