The Bacillus Calmette-Guerin (BCG) vaccine, used to treat tuberculosis for the last 100 years, is not effective against SARS-CoV-2, the virus behind COVID-19 infections, according to results of an international trial.

The trial, led by Murdoch Children's Research Institute in Australia, involved 3,988 healthcare workers from Australia, the Netherlands, the UK, Spain, and Brazil.

It tested whether the BCG vaccine could protect healthcare workers against SARS-CoV-2 in the first six months after vaccination.

The result, published in the New England Journal of Medicine, found that the BCG vaccine did not reduce the risk of developing COVID.

The research, however, could not determine whether the vaccine reduced hospitalisations or death due to the low number of participants with severe COVID-19.

BCG was originally developed to prevent tuberculosis and is still given to more than 130 million babies worldwide each year for that purpose.

In 2021, a study led by researchers from the ICMR-National Institute for Research in Tuberculosis and the ICMR-National Institute of Epidemiology and published in the journal Science Advances suggested that BCG could potentially act to inhibit the biomarkers of inflammation by the immune cells in the blood, and may help prevent the risk of COVID infections in older adults.

A Murdoch Children's led study, published in Clinical & Translational Immunology last year, also showed that the BCG vaccine did provide an immune response consistent with protection against severe COVID-19.

Further, the latest research showed that the risk of symptomatic COVID-19 was 14.7 per cent in the BCG group and 12.3 per cent in the placebo group during the first six months after joining the trial.

Symptomatic COVID-19 being observed slightly more frequently in the BCG group might be explained by stronger immune responses induced by the vaccine, said Professor Nigel Curtis, Chief Principal Investigator of the trial, from Murdoch Children's and the University of Melbourne.

"When we analysed the immune cells of our health care workers, we saw that the BCG vaccine altered the immune response to COVID-19. Symptoms reflect the immune system by working hard to fight the virus. A stronger response induced by BCG could be beneficial in killing the virus more rapidly and protecting against progression to more severe disease. There was some evidence of this in trial participants over the age of 60, in whom COVID-19 symptoms were shorter in the BCG-vaccinated group," Curtis said.

He noted that the trial data analysis was ongoing with further results on the effect of BCG expected later this year, including the impact of the vaccine on other infections, such as respiratory illnesses and the effect on COVID-19 vaccine responses.