A vaccine designed to prevent one of the most widespread common cold types has just delivered promising results in the latest set of clinical trials, and the developers now think it could actually reach the market in just a few years.

The cold, known as respiratory syncytial virus (RSV), is so common, more than 90 percent of kids contract it by the age of two. In fact, this dangerous and sometimes deadly infection is the leading cause of serious lower respiratory diseases in children worldwide, and we still don't have a working vaccine to prevent it.

While Bavarian Nordic, the German company that owns this particular vaccine - known as MVA-BN-RSV - hopes it will become available in 2024, the medicine still has to pass a third clinical trial before the US Food and Drug Administration (FDA) would approve it for general use.

The first two clinical trials of a vaccine are usually limited to examining its safety and optimal dosage. The results of these phases might also give some indication of effectiveness, but the size and breadth of such trials are usually not enough to determine immunity.

So far, it appears as though a single dose of this new vaccine safely induces a broad immune response to RSV in most of the 420 adults over the age of 55 that were enrolled in the study.

In this randomised, placebo-controlled trial, the immune response from T cells, which hunt and destroy infections, and antibodies, which recognise foreign invaders, persisted for at least six months.

When followed up with a booster shot at 12 months, there was an even better immune response.

Those who received either one or both doses showed higher antibody levels at 56 weeks compared to the placebo group, "thus demonstrating persistence of MVA-BN-RSV induced immune responses for up to one year."

However, after a single, high dose of the vaccine, the T cell response had pretty much maxed out.

"Peak T cell responses following the booster vaccination were lower than peak responses following the initial vaccination, suggesting that activation of T cells may be regulated by pre-existing levels of antigen-specific T cells," the researchers write in a study summarising the results.

"This is consistent with the observation that a second vaccination did not induce further T cell responses."

In other words, if a bunch of T cells are already around, then a booster shot isn't going to induce a further response.

It's an interesting explanation, but more research will be needed to confirm those results and figure out the mechanism of action; given the challenges RSV keeps presenting for vaccine development, it's clear we're not quite there yet.

Not only is RSV good at hiding from the immune system, its presence does not induce long-lasting immunity, like chicken pox or measles might, which means we can keep on getting sick with the same thing over and over again, even as adults.

While usually mild cold and flu symptoms occur, older people and those with weakened immune systems are particularly vulnerable to RSV.

A vaccine could potentially stop 33 million serious respiratory infections a year, saving the lives of nearly 60,000 children annually. That would be a huge deal, and while there's reason to be hopeful, it's important not to get ahead of ourselves. There are many almost-vaccines out there, on the brink of hitting the market in the next five years.

The third clinical trial is set to start in 2021 and will include more than 12,000 adults. Hopefully that will be enough to answer some of these remaining questions.

Given "the broad immune response" already elicited by the vaccine, experts at Bavarian Nordic think there is more than enough promise here to merit a phase 3 efficacy trial. Watch this space.

The study was published in The Journal of Infectious Diseases.