The BCG vaccine has been used for nearly a century to protect against tuberculosis, a bacterial disease that affects the lungs. Tuberculosis is caused by a bacterium called Mycobacterium tuberculosis.
BCG is short for Bacillus Calmette-Guérin, as it was created by Léon Charles Albert Calmette and Jean-Marie Camille Guérin in the early 1900s.
To make the vaccine, they used Mycobacterium bovis, a bacterium found in cows and closely related to Mycobacterium tuberculosis. They grew it on a nutrient-rich jelly in the lab for nearly 13 years. The bacterium adapted to this comfortable lifestyle by losing elements in its DNA it no longer needed, including elements that cause disease.
This process is called attenuation and it results in a live but weakened microbe that can be given to humans as a vaccine.
Could BCG Vaccine protect against COVID-19?
We don’t know yet whether BCG will reduce the severity of COVID-19, but the vaccine has some interesting features.
First, BCG is a potent stimulator of the immune system. Currently, it’s used alongside other therapies to treat bladder cancer and melanoma, because it can stimulate immune cells to attack the tumour.
BCG also seems to benefit lung immunity. As we mentioned, children who have had the vaccine appear to get fewer respiratory infections.
There’s a study underway in Melbourne looking at whether BCG can reduce symptoms of asthma in children.
And finally, BCG has been shown to limit viral infection. In one study, human volunteers were given BCG or a placebo one month before being infected with a virus. Volunteers who received BCG had a modest reduction in the amount of virus produced during infection compared to those who received the placebo.
If BCG is shown to be effective, we’ll face other challenges. For example, supply of the vaccine is currently limited. Further, there are many different strains of BCG and they might not all provide the same protection against COVID-19.
Protection would likely start to wane relatively quickly. When trained immunity was tracked in humans after BCG, it started waning from three to 12 months after vaccination.
So BCG would be most helpful for people at high risk of exposure, but it wouldn’t replace a traditional vaccine based on immune memory.