Colour-enhanced scanning electron micrograph of chlamydia, the most common bacterial sexually transmitted disease in humans – but a vaccine may not be far off. DAVID M PHILLIPS / Getty Images

Preventing the spread of chlamydia may be as simple as a vaccine sprayed up your nose.

Biologists in Canada, led by David Bulir and Steven Liang from McMaster University, developed a vaccine that protects against chlamydia in mice. And because the bits of the bacterium the vaccine uses are also found in the human version of the disease, they believe the vaccine could be used in humans too.

The work was published in Vaccine.

With around 113 million new infections each year, mostly in under-developed parts of the world, the sexually transmitted chlamydia (Chlamydia trachomatis) is one of the most dangerous bugs out there.

It’s an obligate intercellular bacteria – meaning it replicates inside the host’s cells before being expelled, or shed, to go on and infect others – but it’s sneaky, often managing this without producing any outward symptoms.

Caught early, it can be killed by a course of antibiotics. But left untreated, it can cause infertility and blindness – indeed, it is the leading cause of preventable blindness, particularly in developing countries.

Scientists have sought a vaccine for the bacterial infection since the 1950s. Early trials only provided partial and short-term protection.

But in the past few decades, researchers identified a number of suitable chlamydia antigens – a protein or toxin that, when injected into a healthy person, prompts their immune system to make antibodies against the disease.

The problem is chlamydia comes in more than 15 different flavours, and these antigens might only protect against one or two at a time.

The challenge has been to find an antigen that works across all varieties.

So Bulir, Liang and colleagues set about finding proteins that are present on all types of chlamydia. An essential protein on the surface of the bacterium is called T3SS – it’s needed for the bacterium to invade a cell.

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They concocted a vaccine, called BD584, made of three T3SS proteins called CopB, CopD and CT584 (which are found in most strains of the disease, both human and mouse).

The vaccine was sprayed it into the right nostril of mice three and six weeks before they were infected with the mouse version of chlamydia, C. muridarum.

A control group of mice wasn’t vaccinated before infection. For all mice, vaginal swabs were taken over the next month. Swabs were to ascertain the amount of bacterial shedding.

At the peak of the disease – five to seven days post-infection – the BD584-vaccinated mice had 95% less bacterial shedding than their unvaccinated counterparts, meaning fewer bacterial cells were able to replicate.

And after a month, the infection had completely cleared in those who’d been given BD584.

Next, the team will test the vaccine’s effectiveness against different strains of chlamydia and in different formulations.