Can BCG vaccination protect against COVID-19?

Covid-19 mortality rates are apparently much lower in some parts of the world. Might the TB jab you had as a teenager be the reason?

That’s one interpretation of an investigation by Aaron Miller and his colleagues at the New York Institute of Technology. Their study, released so far on the preprint medrxiv platform, compares COVID-19 death rates in countries with and without universal BCG vaccinations for tuberculosis (TB).

The study considered countries with populations of over one million people and took into account differences in standards of medical care by classifying countries according to per capita income. Deaths per million inhabitants in each country were used to determine whether BCG vaccination was associated with Covid-19 mortality. Low-income countries were excluded to avoid biases caused by suspected underreporting of COVID-19 deaths in those locales.

According to the analysis, middle and high income countries without a universal BCG vaccination programme turn out to have significantly higher COVID-19 related death rates compared with those that vaccinate routinely. There are also 4 times the number of coronavirus infections reported in countries that don’t routinely offer the BCG.

The protective effect of prior BCG vaccination on COVID-19, the team suggest, is manifest mainly among the elderly. Age-related deaths are higher, they say, in countries late to adopt BCG vaccination.

The inference is that BCG vaccination across a population might, the authors speculate, reduce COVID-19 carriage and contribute to slowing spread. That said, they acknowledge that we do not know if BCG vaccination directly causes COVID-19 resistance. And even if it does, it is also unclear how long this immunity takes to develop. Regrettably, these questions may well take longer to answer than the pandemic is expected to last.

The findings do, however, provide one explanation to account for the differences between countries like Italy - which have robust and strict social isolation policies yet high death rates - and those with less stringent social distancing but low mortality, like Japan: Italy has never had a universal BCG Policy, whereas Japan’s has been in place since 1947.

On the flip side, healthcare inequalities across the countries analysed could have biased the findings, as could differences in the implementation of social distancing polices and access to effective hospital care, which is not comparable even between middle-high and high income countries.

Overall, this is an interesting correlation worth further exploration but we should interpret the findings with caution. Further work needs to be done looking at low-income countries with universal BCG coverage to establish if this relationship exists in those with severe disease there too. It would also be interesting to learn how a history of BCG vaccination impacts other known risk factors for COVID severity, like diabetes, smoking, and male gender.