Early exposure to seasonal flu lowers susceptibility later in life

A new study published in eLife reports that individuals who come down with influenza A have a lower risk of infection later in life. The research also shows that early exposure to the seasonal flu has an influence on the effectiveness of flu vaccines.

The findings may ultimately help to predict age-specific risks of contracting the flu.

The seasonal flu is an acute respiratory infection that has three types – A, B and C. Influenza A viruses are classified into subtypes, and the A(H1N1) and A(H3N2) are the subtypes that are currently in circulation. 

Each year in the United States, the flu causes around 100,000 to 600,000 hospitalizations and 5,000 to 27,000 deaths.

The seasonal flu evolves rapidly, and the emergence of new strains leaves more people susceptible to infection. However, it has been difficult for experts to determine how susceptibility arises and changes over time.

Study first author Philip Arevalo is a postdoctoral researcher in the Department of Ecology and Evolution at the University of Chicago.

“Since the risk of influenza infection in a given age group changes over time, factors other than age may affect our susceptibility to infection,” said Arevalo. “We wanted to see whether these differences can be explained in part by the protection gained from childhood flu infection, which has lasting impacts on the immune response to future infections and the protection against new influenza A subtypes.”

The researchers set out to measure the effects of early flu exposure and to understand how this exposure may influence the effectiveness of vaccines.

The investigation was focused on data from the 2007-2008 to 2017-2018 flu seasons in the Marshfield Epidemiologic Study Area (MESA) in Marshfield, Wisconsin. During the study, individuals were recruited and tested for flu when seeking care for acute respiratory infection. 

The team applied statistical models to the data to analyze the documented flu cases along with vaccine effectiveness.

The analysis showed that despite the large-scale evolution of influenza A subtypes H1N1 and H3N2 over the study period, early infection reduced the likelihood of an individual seeking medical attention for the same subtype later in life. This effect was found to be stronger for H1N1 compared to H3N2. 

The study also revealed that the effectiveness of a flu vaccine varies with both age and birth year, suggesting that flu vaccinations are also influenced by early exposure.

“We hope the findings from our study will improve our understanding of influenza epidemiology and the low and variable effectiveness of the seasonal flu vaccine,” said study senior author Sarah Cobey. “This would lead to better forecasting and vaccination strategies to help combat future flu seasons.”

The study is published in the journal eLife.

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By Chrissy Sexton, Earth.com Staff Writer