More “striking” evidence has emerged that the BCG vaccine given to counter TB may provide protection against Covid-19 and significantly reduce death rates in countries with high levels of vaccination.

A study of 178 countries by an Irish medical consultant working with epidemiologists at the University of Texas in Houston shows countries with vaccination programmes – including Ireland – have far fewer coronavirus cases by a factor 10, compared to where BCG programmes are no longer deployed.

This translates into a death rate up to 20-times less, according to urologist Paul Hegarty of the Mater Hospital, Dublin.

Their “correlation” study, expected to be published shortly by PLOS journal, is largely a statistical one and comes with caveats because of possibility of confounding factors. But it is more comprehensive than an initial one conducted in New York, which prompted a scaling up of clinical trials on people with Covid-19.

To reduce the possibility of error, the researchers re-evaluated cases during the course of the pandemic and made country-by-country comparisons including between Ireland and the UK, Mr Hegarty said. “We did not expect to see such a marked difference.”

He trained in Houston but continued to work with colleagues there as BCG vaccine is used to treat bladder cancer and reduces its recurrence – another indication it has broader benefits beyond TB.

A global shortage of BCG vaccine prompted the Department of Health to end blanket immunisation in 2015 though it is believed there is a high degree of immunity within the Irish population. It is still widely used in developing countries, where it prevents infant deaths from a variety of causes. The UK ran a more modest BCG vaccination programme to Ireland’s, starting in 1953 and ending in 2005.

BCG-vaccinated older people experience decreased respiratory infections while in bladder cancer patients BCG boosts immunity, reduces tumour size and decreases mortality.

Mr Hegarty said they were heartened by similar results to the New York study and were in discussion about a clinical trial in the US. This would be targeted at healthcare workers experiencing “shocking” rates of infectivity – accounting for one in four cases in Ireland.

As the pandemic unfolds clinical trials were critical, given a coronavirus vaccine “is expected to take a minimum of 12 to 18 months to develop. In the meantime, repurposing existing and safe vaccines that induce non-specific immune benefits may be an additional tool”.

On Monday, scientists in Melbourne, Australia, started administering BCG vaccine or a placebo to thousands of healthcare workers. A clinical trial of 1,000 healthcare workers started recently in the Netherlands, said Dr Mihai Netea of Radboud University Medical Centre.

He said he expected results within three to six months. He did not advocate giving the vaccine to populations until that was completed because of the possibility of side effects as a novel virus was involved, and because not enough vaccine was available – though production could be scaled up quickly.

Immunologist Prof Luke O’Neill of Trinity College, who has worked on the vaccine for years, confirmed at least seven trials had begun or were about to begin but stressed the need for physical distancing and hardwashing proven ways to curb transmission.

The non-specific immune benefits of BCG are known for decades. Introduced to Ireland in 1937, it has a strong safety record. Recent studies show revaccination is safe.

US virologist Robert Gallo of the Institute of Human Virology in Maryland has confirmed he is working with a team who will make an announcement shortly that will have “a major effect” on global efforts to tackle Covid-19.

Best known for his role in the discovery of HIV as the infectious agent responsible for AIDS, Prof Gallo did not go into detail other than to indicate it involves deployment of “an adjusted existing vaccine” that will be available within months.