Influenza infection could more accurately predict if an individual would develop symptoms than current methods which primarily rely on antibody levels, according to a study.

The study found certain immune cells were associated with increased protection, while other immune cells were associated with increased susceptibility to developing symptoms after catching the virus.

“We’ve been struggling for decades, if not centuries, with why some people get sick with infections and some don’t,” said Richard Webby, from St. Jude Children’s Research Hospital's Department of Host-Microbe Interactions.

“This is one of the best attempts to try and figure that out for influenza. We were able to measure many different immune parameters from a single blood draw and correlate them with protection from, or susceptibility to, infection symptoms.”

In the study, published in the journal Nature Immunology, the researchers found that having a more functionally diverse set of immune cells was correlated with increased protection from flu symptoms.

The group identified these cells by comparing the immune cells present in the blood of patients who had symptoms of flu infection to those who were asymptomatic or uninfected.

The blood samples, taken up to six months before that flu season, showed very different sets of immune cells in the two groups.

Those without symptoms not only had a more functionally diverse set of immune cells but those cells were also associated with an influenza-specific long-term response, sometimes called the memory response.

Patients with symptoms tended to have a more similar set of inflammatory immune cells, which are more likely to be involved in a nonspecific, functionally narrow and short-term response.

Further, the study showed that those vaccinated for the flu generally had increased protective anti-flu immune cells, improving their chance of avoiding symptoms.

Those rarer individuals who were unvaccinated and avoided symptoms seemed to have a set of immune cells that mimicked the functions of the protective cells in the vaccinated population.

This may explain why some people are less affected by the flu, even when unvaccinated, than others, but it still suggests vaccination creates the best chance of avoiding symptoms. One way to encourage this vaccine uptake is to determine the inherent risk of staying unvaccinated accurately.

“Our results reemphasise that vaccination prevents influenza symptoms, and now we can point to the increased levels of those immune cells correlated with that protection,” said Paul Thomas from the St. Jude Department of Immunology. “Get your annual flu vaccine.”