One group of researchers have suggested that the severity of Covid-19’s impact is greater in countries that do not have a history of Bacillus Calmette–Guerin (BCG) vaccination programmes, including Italy and the US. BCG vaccines contain a live attenuated strain of Mycobacterium bovis, the causative agent of TB. The BCG vaccination has been suggested to induce heterologous immunity against non-mycobacterial pathogens.

On the 12 April, WHO stated in a press briefing that there is currently no data-based evidence that the BCG vaccination provides protection against Covid-19. The organisation is currently monitoring the progression of two clinical trials that are investigating the effect of the BCG vaccination in healthcare workers to determine if the vaccine is able to offer a level of innate immunity against Covid-19.

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