A widely used tuberculosis vaccine protected people with Type 1 diabetes from COVID-19, according to a study that demonstrates the potential of multiple doses of the Bacillus Calmette-Guerin (BCG) preventive against SARS-CoV-2 and other viruses.

The study, published in the journal Cell Reports Medicine on August 15, was conducted on 144 patients with type 1 diabetes at the start of the pandemic, much before COVID-specific vaccines were available.

Researchers at Massachusetts General Hospital (MGH) in the US found that 12.5 per cent of placebo-treated individuals and 1 per cent of BCG-treated individuals had confirmed COVID-19, yielding vaccine effectiveness of 92 per cent.

The BCG-vaccinated group also displayed protective effects against other infectious diseases, including fewer symptoms, lesser severity and fewer infectious disease events per patient. No BCG-related systemic adverse events occurred, according to the researchers.

BCG's broad-based infection protection suggests that, in addition to COVID-19, the vaccine may potentially provide protection against new SARS-CoV-2 variants and other pathogens, they said.

The researchers are hoping the results will pave the way for a large-scale study of the effects of the BCG vaccine in patients with type 1 diabetes, considered among the most vulnerable groups to COVID-19.

The BCG vaccine is an avirulent tuberculosis strain Mycobacterium Bovis historically given to protect against tuberculosis and, since its introduction in 1921, has been the most widely administered vaccine in the history of medicine.

Considered to be extremely safe, BCG is on the World Health Organization's List of Essential Medicines and is given to roughly 100 million children per year globally, the researchers said.

BCG is also one of the most affordable medicines, costing less than a dollar a dose in many parts of the world, they said. "Multiple studies have shown that adults with type 1 diabetes who are diagnosed with COVID-19 are at increased risk of severe illness," said Denise Faustman, director of the Immunobiology Laboratory at MGH.

"We found that three doses of BCG administered prior to the start of the pandemic prevented infection and limited severe symptoms from COVID-19 and other infectious diseases," Faustman said. The researchers noted that unlike the antigen-specific vaccines currently in use to prevent COVID-19, BCG's mechanism of action is not limited to a specific virus or infection.

The participants in the COVID trial had previously enrolled in a clinical trial testing the effectiveness of the BCG vaccine for type 1 diabetes. Participants in the test group had received multiple vaccinations prior to the onset of the pandemic in early 2020.

"This data set is unique and exciting because the patients were all vaccinated with multiple doses of BCG prior to the onset of the epidemic," said Hazel Dockrell, from the London School of Hygiene & Tropical Medicine, who was not involved in the study.

"Prior to the trial, they had no known exposure to tuberculosis or prior BCG vaccination. This eliminates the major confounding factors that have limited other trials," Dockrell said. The results support the idea that BCG needs time to have a clinical effect, but its effects may then be very lasting and durable, the researchers said.

The 144 adult type diabetics (96 BCG treated and 48 placeboes) analysed in the COVID-19 trial were part of an ongoing Phase IIb clinical trial testing BCG as a treatment for adults with established type 1 diabetes. Patients were followed for COVID-19-related outcomes for 15 months, the researchers said.

Outcomes for the COVID-19 trial included: COVID-19 infection rate, COVID-19-related symptoms, reduction in overall infections disease and SARS-CoV-2 antibody-level presence and intensity, they added.