Uh-oh, there’s a flu virus going around and you’re worried that you will get sick. There may be a simple way to know if you are actually susceptible. Researchers from the Stanford University School of Medicine have found a biomarker that is very accurate in predicting whether someone will become infected with a flu virus or not.

From analyzing thousands of immune cell samples and using an experimental approach, the researchers found, for the first time, an important biomarker that holds across multiple strains of influenza. The biomarker is a gene called KLRD1 and it is connected to the presence of immune cells that fight off flus. The more cells with this biomarker that a person has, the less likely that they are to get the flu.

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The researchers analysed data collected from over 150 studies that monitored the gene expression of immune cells. They looked at 20 immune cell types to see if any of them showed a consistent response to the H1N1 or H3N2 flu. If they did, then the researchers looked at their genes as well. Additionally, there were two experimental studies, one at Harvard University and one at Duke University in which, crazily enough, 52 participants volunteered themselves to be exposed to influenza. Their immune cells were then quantified.

“We found that a type of immune cell called a natural killer cell was consistently low at baseline in individuals who got infected,” said lead author Erika Bongen. The participants with a higher proportion of natural killer cells had a better immune system and were less likely to get sick.

The gene KLRD1 was a good proxy for these natural killer cells. It expresses a receptor on the surface of the natural killer cells and thus helps count how many of these immune cells that there are in the body. People whose immune cells consisted 10-13% of natural killers did not become infected, while those with less than 10% got sick. This was true in all of the cases.

The researchers do emphasize that the link between KLRD1 levels and influenza is just an association for now. They don’t know exactly how both are linked. The next step in the research is to find the mechanism for why this is the case.

“It will be crucial to understand the role of natural killer cells’ protection so that we can potentially leverage that in designing better flu vaccines,” said Purvesh Khatri, Ph.D., associate professor of medicine and of biomedical data science at the Stanford University School of Medicine. “Since we see that natural killer cells are protective across different strains, maybe that would be a path to a universal flu vaccine.”

In addition to helping to develop a vaccine, these findings can also help by identifying who is the most susceptible to the flu and should take preventative measures.

Bongen et al. 2018. KLRD1-expressing natural killer cells predict influenza susceptibility. Genome Medicine.