Could your future Covid-19 or flu shot be swallowed or sprayed?

For those wary of needles, that’s not such a far-out idea – and New Zealand scientists have just identified a promising new target that could make “mucosal” vaccines effective against respiratory infections like coronavirus.

While the concept of inhalable or sprayable vaccines isn’t new, these shots remain relatively uncommon in medicine, given some enduring limitations.

Increasingly, however, scientists have been finding compelling reasons to make mucosal vaccines an option for people – and not just because they don’t need to be jabbed.

Dr Theresa Pankhurst, of Wellington’s Malaghan Institute for Medical Research, explained these vaccines worked in a similar way to injectables in that they contained weakened, killed or parts of a pathogen that prompted the immune system to build a response against it.

But, given in forms like pills or nasal sprays, they were instead administered into the mucosal tract, via our digestive or respiratory system.

“The aim of giving a mucosal vaccine is to build up a strong immune response against the pathogen throughout the body but, importantly, at the locations in the body where pathogens can invade - that is through the mucosal system,” Pankhurst said.

“Vaccines injected into the arm give strong immune responses that circulate around the body, but can be limited in generating protective immune responses in those more vulnerable mucosal sites.”

This gave the pathogen slightly more time and opportunity to infect us.

In the case of SARS-CoV-2, the coronavirus happened to mainly affect the respiratory system – just where mucosal vaccines had been shown to induce strong immune responses.

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“This is because mucosal vaccines may be more effective at preventing transmission of the virus as immune responses in the respiratory tract can block viral entry into our bodies.”

Specifically, they prompted our bodies to make a specialised type of antibody – called IgA – that’s secreted on to mucosal surfaces, such as the respiratory tract.

“When a pathogen enters the body, IgA antibodies bind to the pathogen, preventing it from infecting the body and reduces the chances that the pathogen will be transmitted to others.”

Worldwide, there were several mucosal vaccines for Covid-19 in early development, but more research was needed to determine their safety and effectiveness before they could be rolled out.

Dr Theresa Pankhurst and colleagues have identified a promising new target that could make 'mucosal' vaccines effective against respiratory infections like coronavirus. Photo / Supplied

Scientists were also moving away from those that contained live or weakened strains, and toward “subunit” ones that included only a portion of the pathogen, giving them better safety profiles.

But there were barriers.

“One of the main challenges is that subunit vaccines may not be able to produce a strong enough immune response at the mucosa, and therefore require the addition of an adjuvant,” said Kaitlin Buick, a PhD student at Victoria University and Malaghan.

Adjuvants are ingredients added to vaccines to help boost the immune response.

“Another challenge is that subunit vaccines may be rapidly degraded or eliminated from the body before it has had the chance to induce an immune response,” Buick added.

“It’s also important to ensure that the vaccine does not produce too strong of an immune response that could damage the delicate structure of the lung – thus a careful balance needs to be achieved.”

In their just-published study, Pankhurst, Buick and colleagues Dr Lisa Connor and professors Gavin Painter and Ian Hermans pin-pointed a promising avenue to overcome those issues.

That was in harnessing the power of a specific type of immune cell found in our lungs - and called a mucosal-associated invariant T (MAIT) cell – to give mucosal vaccination that extra push.

“MAIT cells can be targeted and activated by an ingredient found in the production of vitamin B,” Buick said.

“When we added the MAIT cell activating ingredient to a subunit vaccine, we found that the activation of MAIT cells provided help to other immune cells in the lung and ultimately stimulated antibody-producing B cells to make antibodies - including IgA antibodies.

“Importantly, these vitamin B ingredients could withstand the mucosal environment and did not induce inflammation – providing that important balance needed for a successful mucosal vaccine.”

The study, published in the scientific journal Cell Reports, concluded that MAIT cells could make a potential target for tomorrow’s mucosal vaccines.

“It’s exciting for us to get this work out there and published in the scientific community.”