A vaccine used to treat tuberculosis has also been shown to prevent lethal infection from Covid-19, a pre-clinical study has revealed.
In findings published last month by the National Institutes of Health (NIH) in the USA and Wellington's Malaghan Institute of Medical Research, the tuberculosis vaccine BCG has been shown to protect against lethal Covid-19 infection in mice when administered intravenously.
International Research Fellow Dr Kerry Hilligan, who is part of the Immune cell biology team at the Malaghan Institute, has been researching this vaccine at the NIH in the US for the past several years, in Dr Alan Sher's laboratory.
Their findings in mice models have suggested BCG (Mycobacterium bovis bacille Calmette Guérin) - a live, or weakened virus used as a vaccine to protect against childhood tuberculosis - also offered protection against Covid-19 infection.
"We found that when BCG was administered intravenously it protected our mice from weight loss, disease and death to Covid-19 by accelerating the removal of the virus in the lungs – minimising the amount of damage the virus could cause," Hilligan said.
"This finding is striking because BCG is completely unrelated to the Covid-19 virus, yet it offers this protection.
"Importantly, BCG was only effective when administered intravenously – through the veins which stimulate the immune system in the lung, rather than via the skin."
First developed over 100 years ago, the BCG vaccine is one of the most widely used vaccines in the world and is administered through the skin within the first few days of life.
"BCG is particularly intriguing because it appears to stimulate both specific immunity (towards TB) and non-specific immunity against completely unrelated microorganisms, but we don't fully understand why," she said.
"It has limited use in New Zealand – because TB is not common here – but is used widely throughout Africa, Asia and South America."
Hilligan has been researching BCG since 2018, assessing how different routes of administration impacted immune responses in the lung. From the onset of pandemic they had suspected it could be useful in protecting against Covid-19.
"We noted that intravenous administration of BCG had a profound effect on the composition and activation state of lung resident immune cells," she said.
"When Covid-19 came along we thought that this so-called "priming" of lung immune cells could be beneficial and prevent the viral taking hold."
"While we thought that intravenous BCG may have a protective effect, we were very surprised at the level of protection we were seeing.
"It was close to what you would see with a Covid-specific vaccine."
Hilligan said their research had shown that it matters where in the body a virus enters its host. As an airborne pathogen, Covid-19 usually enters the body through nasal airway passages and infects the lungs before spreading to the rest of the body.
"Priming" immune cells in the lung can effectively work as stopping the virus "at the door" before it is able to do lasting damage.
Intravenous administration of the vaccine enables the lungs to be "primed" in this way, Hilligan said.
"The lung contains lots of blood vessels and so intravenous administration gives the vaccine direct access to the lung," she said.
"Therefore intravenous administration is activating immune cells at Covid-19's point of entry."
Unfortunately they had not so far found the same immune response from administering the vaccine through the skin.
While the team is not advocating for the BCG vaccine to be used against Covid-19, the study has highlighted potential new methods for fighting Covid-19 by boosting immune protection in the lungs.
The findings could have an impact on how future Covid-19 vaccines are designed or delivered, Hilligan said.
"We can use intravenous BCG as a model to determine what confers protection against Covid-19," she said.
"This knowledge can then be incorporated into the design of future vaccines and therapies for Covid-19."
The study was published in the Journal of Experimental Medicine in December.