July 6 (UPI) -- The first case of flu a person experiences as a child might impact the ability to fight off other strains of the virus later in life, according to a study published Monday in the journal Proceedings of the National Academy of Sciences.
Infection with H1N1, the strain of influenza that caused a global pandemic in 2009, resulted in immunity against the more common H3N2 strain later in life, the researchers said.
Infection with H3N2 as a child, however, did not offer protection against H1N1, they said.
Most people experience their first flu infection by age 5, the researchers said.
"The specific types of influenza virus that an individual encounters in childhood may affect susceptibility to different influenza virus strains later in life," study co-author Scott E. Hensley told UPI.
"Influenza virus infections in childhood establish memory immune responses that can be recalled later in life," said Hensley, an associate professor of microbiology at the University of Pennsylvania.
This is a concept called "Original Antigenic Sin," according to Hensley and his colleagues.
It means that the immune system's antibody response to a first influenza infection will be boosted with each subsequent infection with a distinct influenza virus strain over the course of person's lifetime, they said.
Such "memory immune" responses have particular relevance now in light of the ongoing new coronavirus pandemic, Hensley said. He and his team already are working on similar research with the new coronavirus, SARS-CoV-2, which causes the disease COVID-19.
For the influenza study, Hensley and his colleagues used ferrets and blood samples from 69 children 2 to 6 years old. They infected both with H1N1 and H3N2 at separate times to see how each virus impacted immunity against the other, they said.
In general, their findings indicate that seasonal influenza viruses are highly transmissible to those who have never had them, the researchers said. They could aid future research into flu vaccines, they said.
"Our study suggests that humans have immune responses that are biased toward viruses that they encountered early in life," Hensley said.
"It may be possible to [create] universal influenza virus immune memory by designing new vaccines that delivery many influenza virus antigens early in life."