First Flu Exposure Impacts Lifelong Susceptibility

The study team behind new research published in Clinical Infectious Diseases has provided evidence that the first strain of influenza virus a person is exposed to in early childhood impacts their ability to fight flu for life. The phenomenon is known as antigenic imprinting.

The investigators examined the Canadian flu season of 2018-19, which was unusual in that both strains of influenza A dominated at different points within the seasons. During the winter of 2019 in Canada, H3N2 viruses progressively replaced H1N1 viruses as the dominant subtype.

The study team employed sentinel data from the Institut National de Santé Publique du Québec (INSPQ). INSPQ reports weekly counts of positive test samples for influenza A and B strains by age group, which allowed investigators to assess whether people born in different years might be susceptible to different flu strains.

In order to estimate weekly rates of influenza A by age group, the total cases for each period under observation were divided by total population for each group in Québec.

The results showed that even without direct data on subtype dominance, the progressive shift could be captured by looking at the distribution of influenza A cases by age, with a shift in age-specific incidence as the subtype dominance changed observed. Younger individuals displayed increased susceptibility to H1N1 relative to H3N2, and H3N2 disproportionately targeted older individuals.

“These results are consistent with the antigenic imprinting hypothesis: during the 2017-18 and 2018-19 seasons, individuals aged 40-49 years were born in 1968-78 and in 1969-79, respectively, and were thus most likely imprinted by H3N2,” the study authors wrote.

It appears that individuals are less susceptible to influenza during seasons which feature the same predominant strain that they were first exposed to.

Future research may be able to investigate transmission dynamics within households to examine how imprinting may affect the transmission of strains within families. The results of the recent study could have useful public health implications down the line.

“Understanding how their prior immunity either leaves them protected or susceptible is really important for helping us to identify the populations who are most at risk during seasonal epidemics and new outbreaks," said study author Matthew Miller, PhD, associate professor at the Michael G. DeGroote Institute for Infectious Disease and the McMaster Immunology Research Centre in a press release.