Immune memory vs trained immunity
There are two main types of cells within our immune system: innate cells, which respond rapidly to microbes that cause disease, and adaptive cells, which initially respond quite slowly.
Some live attenuated vaccines, such as BCG, can trigger trained immunity, which can enhance early control of other infections.
This raises the tantalising possibility that BCG could train innate cells to improve early control of the SARS-CoV-2 virus, to reduce COVID-19 disease or even prevent infection.
Importantly, because it's a live bacterium, it can spread from the vaccine site and cause disease, called disseminated BCG, in people who are immunodeficient, like people with HIV.
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.
