The BCG (Bacillus Calmette–Guérin) vaccine, discovered by Albert Calmette and Camille Guérin, is one of (if not) the most widely used vaccine worldwide. Next year will mark the 100th anniversary of the first time this live, attenuated (weakened) version of a virulent bovine strain of tubercle bacillus was administered to a person.

The BCG vaccine protects against tuberculosis, also known as TB, a serious infection that affects the lungs and sometimes other parts of the body, such as the bones, joints and kidneys. The World Health Organization (WHO) recommends BCG vaccination as soon as possible after birth in countries with a high incidence of tuberculosis

This century-old vaccine is in the spotlight the past weeks because of a few ecological studies (pre-prints, not peer-reviewed at this stage) that claim a strong correlation between BCG vaccination and protection against SARS-CoV2, the coronavirus causing COVID-19.

Ecological studies are inherently limited since they take aggregate data and try to make inferences at the individual level. Such ecological studies are prone to significant bias from many confounders, including differences in national demographics and disease burden, testing rates for COVID-19 virus infections, and the stage of the pandemic in each country.

Some of these analyses were done over a month ago. Since then, COVID-19 cases and deaths have spiked up in many low and middle-income countries where BCG vaccination is administered to all newborns. Many of the countries with BCG in their national immunization programs seriously under-test for COVID-19.

There is experimental evidence from both animal and human studies that the BCG vaccine has non-specific effects on the immune system. However, these effects have not been well characterized and their clinical relevance is unknown.

There are two registered protocols for clinical trials — BCG-CORONA (The Netherlands) and BRACE (Australia) — both of which aim to study the effects of BCG vaccination given to health care workers directly involved in the care of patients with COVID-19. Results from these studies will provide more information whether or not the BCG vaccine indeed protects against COVID-19.

BCG vaccination prevents severe forms of tuberculosis in children and diversion of local supplies may result in neonates not being vaccinated, resulting in an increase of disease and deaths from tuberculosis.

Researchers and journalists need to be responsible not to raise false hopes based on weak evidence.  In the absence of clear and robust evidence, WHO does not recommend BCG vaccination for the prevention of COVID-19. WHO continues to recommend neonatal BCG vaccination in countries or settings with a high incidence of tuberculosis.