A single nasal spray vaccine could protect against all coughs, colds and flus, as well as bacterial lung infections, and may even ease allergies, say US researchers.

The team at Stanford University have tested their "universal vaccine" in animals and still need to do human clinical trials.

Their approach marks a "radical departure" from the way vaccines have been designed for more than 200 years, they say.

Experts in the field said the study was "really exciting" despite being at an early stage and could be a "major step forward".

Current vaccines train the body to fight one single infection. A measles vaccine protects against only measles and a chickenpox vaccine protects against only chickenpox. The approach described in the journal Science does not train the immune system. Instead it mimics the way immune cells communicate with each other.

It is given as a nasal spray and leaves white blood cells in our lungs – called macrophages – on "amber alert" and ready to jump into action no matter what infection tries to get in.

The researchers showed this heightened state of readiness led to a 100-to-1,000-fold reduction in viruses getting through the lungs and into the body.

And for those that did sneak through, the rest of the immune system was "poised, ready to fend off these in warp speed time" said Prof Bali Pulendran, a professor of microbiology and immunology at Stanford.

Pulendran told the BBC: "This vaccine, what we term a universal vaccine, elicits a far broader response that is protective against not just the flu virus, not just the Covid virus, not just the common cold virus, but against virtually all viruses, and as many different bacteria as we've tested, and even allergens.

"The principle by which this vaccine works is a radical departure from the principle by which all vaccines have worked so far. "

The way it steers the immune system towards fighting an infection also seemed to reduce the response to house dust mite allergens – which are a trigger of allergic asthma.

"This is a really exciting piece of research," says Prof Daniela Ferreira, a professor of vaccinology at University of Oxford, who was not involved in the study.

She said it could "change how we protect people from common coughs, colds and other respiratory infections" if the results are confirmed in human studies.

"One of the strengths" of the study was a clear explanation of how this new style of vaccine was working, she added.

Ferreira said the research "could mark a major step forward" offering protection against infections that "place such a heavy burden" on us all.

The vaccine was given as a nasal spray in the experiments, but may need to be breathed in through a nebuliser to reach the depths of human lungs.

It is not known whether the same effect can be achieved in people or how long the immune system would stay in amber alert.

There are differences in the immune systems between mice and humans, including our immunity being shaped by decades of infections.

So the researchers are planning trials where one person is vaccinated and then deliberately infected to see how their body copes.

There may also be consequences to dialling up the immune system beyond its normal state – raising questions of immune disorders.

Jonathan Ball, professor of molecular virology at the Liverpool School of Tropical Medicine, said the work was undeniably "exciting" but cautioned "we have to ensure that keeping the body on 'high alert' doesn't lead to friendly fire, where a hyper-ready immune system accidentally triggers unwelcome side effects".

The research team in the US does not think the immune system should be permanently dialled up and think such a vaccine should be used to compliment rather than replace current vaccines

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