The manufacturer, MedImmune, has changed its LAIV vaccine, FluMist, in the hope it will make this year’s vaccine more effective.

Its ineffectiveness was discovered when researchers from the US Centers for Disease Control and Prevention (CDC) tested it against the standard flu vaccine. Children given the nasal vaccine were three times more likely to contract the H1N1 flu than those given the vaccine, and protection was even less effective in smaller children aged between two and eight years, who were five times more likely to develop flu.

The researchers looked at data involving 2,703 children aged from two to 17 years, who had been vaccinated from 2010 and up to 2014.

Overall effectiveness of flu vaccines ranged between 54 per cent and 75 per cent depending on the child’s age—but the nasal spray achieved just a 17 per cent protection rate.

Nobody is sure why the nasal spray was so ineffective, but some researchers surmise that it was based on the 2009 strain, which had changed dramatically by 2014.