Flu has been spreading in ways we never even knew about as minor strains race between humans

Minor strains of flu, which are not targeted by vaccines, spread quicker than scientists previously believed. Vaccines aim to attack the dominant strains of influenza as they infect more people, but minor strains, which sneak into the body with dominant strains, replicate quickly and are unaffected by vaccines.

The research, from New York University, builds upon our knowledge that some forms of the influenza virus have large genetic diversity, so can be troublesome to remove from the body. These viruses can be broken down into two general categories: dominant and minor strains of the virus. Vaccines target dominant strains because it infects more people than the minor strains. However, this new research shows that the minor strains of flu can spread between individuals quicker than believed – rivalling the dominant strains.

"A flu-virus infection is a mix of strains that gets transmitted as a swarm in the population," said Elodie Ghedin, researcher on the study. "Current vaccines target the dominant strains. But our findings reveal an ability of minor strains to elude these vaccines and spread the virus in ways not previously known."

In the study, the researchers wanted to find out how many minor strains of flu were transmitted into, and throughout the body with dominant strains. They also aimed to investigate how they move between individuals.

Scientists used data from the 2009 Hong Kong H1N1 outbreak – more commonly known as swine flu. They used carried out genome deep sequencing of upper nasal-cavity swabs from individuals with a confirmed case of swine flu, and their household. The results, published in Nature Genetics, showed that, unsurprisingly, most carried the dominant strain of the virus. What was surprising, however, was that they all carried the minor strains too, suggesting that these strains spread similarly to dominant forms of the virus.

Ghedin said: "What stood out was also how these mixes of major and minor strains were being transmitted across the population during the 2009 pandemic -- to the point where minor strains became dominant."

The findings could encourage future treatments to target minor strains of the influenza virus. This could reduce the number of global influenza cases from five million per year.