This century-old vaccine is suddenly in the news, thanks to a flurry of ecological studies (pre-prints, at this stage) that claim a strong correlation between BCG vaccination and protection against COVID-19. These studies have used the BCG World Atlas that my team developed nearly a decade ago, and updated in 2017. The Atlas is neither perfect nor complete, but it is the only such database.
As a TB researcher, I would be thrilled if BCG worked against COVID-19. But, these ecological studies have serious limitations (which I will cover later) which most media reports have ignored. In fact, BCG has been talked up to be a ‘silver bullet’ and a ‘game-changer’. We need to tone down the hype and focus on getting stronger trial evidence, because the hypothesis is definitely worth chasing. The good news is that rigorous trials are getting underway, to settle the issue, one way or another.
BCG is easily the most widely used vaccine worldwide. Next year will mark the 100th anniversary of the first human administration of this vaccine, a live, attenuated version of a virulent bovine strain of tubercle bacillus.
BCG is inexpensive and quite safe. But, it also the most mysterious and controversial vaccine, with at least 10 BCG substrains used across countries, variable vaccine efficacy, and huge variations in vaccination practices across the world. In fact, TB scientists like me spend hours fighting over what exactly the vaccine does and how it works.
Kids born in India today still get this vaccine. Take vaccine efficacy, for example. In clinical trials, the efficacy of the BCG vaccine against pulmonary TB in adults has been reported to be 0–80%. The largest vaccine trial was done in south India, and the efficacy of BCG was estimated to be 0%.
Ecological studies on BCG and COVID-19
At least half a dozen studies now exist, all exploring the link between BCG vaccination policies in various countries, and incidence of COVID-19 cases and deaths. While data on BCG vaccination policies is derived from the BCG Atlas, data on COVID-19 cases are taken from WHO or other public databases tracking the pandemic.
The first such ecological study used COVID-19 data as of 21st March 2020, and concluded that “the correlation between the beginning of universal BCG vaccination and the protection against COVID-19 suggests that BCG might confer long-lasting protection against the current strain of coronavirus.” This study got lots of uncritical media attention, even before peer-review. Clearly, it inspired many other similar studies. While some studies have been careful and cautious, others could have used some epidemiological input.
Reference & Source information: https://www.forbes.com/