REMEMBER the 1999 controversy about the efficacy of the BCG vaccine in preventing all forms of TB, including those during childhood, when the Indian Council of Medical Research (ICMR) published the results of its 15-year follow-up study in Chingleput district on people vaccinated at birth with BCG conducted during 1971-1987?
The study, which was carried out by the Tuberculosis Research Centre (TRC), Chennai, had concluded that the vaccine provided no protection at all in adults but a low level of overall protection in children against pulmonary TB, but advocated its continued use as a public health measure for childhood vaccination (Frontline, December 10, 1999; https://frontline.thehindu.com/static/html/fl1625/16251180.htm).
BCG (a weakened form of the bacterium Mycobacterium bovis), is the only clinically approved TB vaccine, which is administered at birth in India. While it is effective in preventing extra-pulmonary forms of TB such as TB meningitis in children, its effects rarely persist beyond 15-20 years. About a quarter of the world’s population carries the bacterium M. tuberculosis, but with no disease symptoms (latent TB). They have a 5-15 per cent chance of getting the disease in their lifetime. Young adults are particularly at risk. Now a new Indian Institute of Science (IISc), Bengaluru, study has found that when young Indian adults, who have been vaccinated at birth, are once again given the BCG vaccine, their immune response against TB improves. The study, which has been published in the journal JCI Insight, was led by scientists at the Centre for Infectious Diseases Research (CIDR) of the IISc in collaboration with clinicians and researchers in India, the United States, Europe and the United Kingdom.
According to a IISc release, the study shows that revaccination boosts the numbers and response of a specific subset of white blood cells called Th17 cells that are involved in fighting TB. “We are able to demonstrate for the first time that revaccination is immunogenic in the Indian context,” says Annapurna Vyakarnam of the CIDR who has been quoted in the release.
The researchers recruited 200 young adults from Madanapalle in Andhra Pradesh, home to one of the oldest TB sanatoriums in the country, for the study. All the volunteers had been given BCG at birth. They were split into two groups: those who had latent TB—determined using the standard QuantiFERON-TB Gold test—and those who did not carry the bacteria. Within each group, half of the individuals were revaccinated with BCG. Blood samples were extracted and analysed over nine months, and detailed flow cytometry investigations were carried out to check for the presence of up to 256 immune cell subsets, says the IISc release.
Compared with unvaccinated individuals, revaccinated individuals were found to have more than twice as many immune cells belonging to specific subsets called CD4 and CD8 T-cells. These cells produce signalling proteins called cytokines that rally the body’s immune responses against TB. Revaccination also boosted the numbers of innate immune cells that form the first line of defence against TB.
“Our study also showed that it is putatively safe to revaccinate people already infected with M. tuberculosis but [are] disease-free,” says Annapurna Vyakarnam.