WASHINGTON – People may differ in their susceptibility to different influenza subtypes based in part on the year when they were born and the flu strains that circulated during their birth year, according to infection patterns during a recent U.S. flu season.
“Our findings may indicate protection against H1 [influenza] viruses in age groups with early exposure to H1N1pdm09 during the 2009 pandemic or to older, antigenically similar H1N1 viruses,” Shikha Garg, MD, said at an annual scientific meeting on infectious diseases. If results from further studies confirm this relationship it could have implications for flu vaccine effectiveness in various age groups and influence the composition of flu vaccines based on the ages of the people who will receive them, said Dr. Garg, a medical epidemiologist with the Centers for Disease Control and Prevention in Atlanta.
The analysis she reported using data collected by the CDC’s Influenza Hospitalization Surveillance on 18,699 people hospitalized for influenza infection during the 2018-2019 season, Oct. 1, 2018–April 30, 2019. The database provides a representative sampling of patients hospitalized for influenza at more than 250 acute care hospitals in 13 states. During the season studied, both the H1N1 and H3N2 subtypes circulated and caused similar cumulative rates of infections, with H1N1 causing about 32 confirmed cases per 100,000 people and H3N2 causing about 29 cases/100,000.
But a more granular analysis that divided the hospitalized patients by their birth year showed an excess of H1N1 infections in two demographic slices: those born during 2010-2019 (corresponding to children 0-9 years old), in whom H1N1 accounted for roughly 60% of cases; and also in those born during 1948-1995 (people aged 24-70 years old) in whom H1N1 caused roughly 70% or more of all infections in some for some birth-year groups in this demographic range. In contrast, infection with the circulating H3N2 strain in the 2018-2019 season dominated among those born during 1996-2009 (people aged 10-23), as well as in those born in 1947 or earlier (those who were at least 71 years old). Some age groups within those born in 1996-2009 had H3N2 infection rates that made up 70% or more of all flu infections, and among nonagenarians well over three-quarters of flu infection were by the H3N2 subtype.
Dr. Garg also showed a similar pattern of predominant flu subtype by age using U.S. influenza hospitalization data for the 2017-2018 season, as well as for all types of 2018-2019 U.S. influenza infections that underwent strain typing including outpatients as well as in patients. All of these findings support the hypothesis and extend the data published earlier this year by Dr. Garg and several of her CDC colleagues that described a pattern of “antigen imprinting” that appeared caused by influenza exposure during the first year of life (). However, more data are needed to better assess time trends for children who were first exposed to H1N1 influenza during the 2009 pandemic, Dr. Garg said.
SOURCE: Garg S. ID Week 2019, Abstract .