Less than one-fourth of those who got a flu shot last season received any protection from it, but scientists believe they are on the right track to improving next season’s version.

Last season's vaccine was largely ineffective because the influenza viruses in circulation had mutated in ways researchers did not account for when the vaccine was developed, say researchers at the Wistar Institute in Philadelphia.

Each year, the flu sickens some 3 to 5 million people worldwide and is responsible for up to half a million deaths.

The vaccine elicits antibodies that respond to specific strains of the flu virus. If the antibodies match the strain that attacks a vaccinated patient, the person should be protected from the flu.

However, viruses are able to mutate to avoid detection, a phenomenon known as “drift” and thus create new viruses for which the vaccine is ineffective.

The scientists say they have detected 10 different viral mutations that rendered the past season’s vaccine less effective, and say this knowledge should allow them to produce a more effective formulation for the coming season.