| Summary: | Background: Seasonal epidemics of influenza are characterized through national or international surveillance efforts to determine vaccine efficacy and vaccine strain selection, but they do not provide detailed information about local variations in factors that can influence influenza cases and disease severity. Methods: Surveillance for influenza like illness was performed in Emergency Medicine Departments in Taipei, Taiwan and Baltimore, Maryland during the winter of 2015–16. Detailed demographic and clinical data were obtained. Nasal swabs or washes were collected for influenza virus diagnosis, sequencing and isolation. Serum was collected to determine neutralizing antibody levels. Results: H1N1 viruses dominated both sites, but more influenza cases occurred in Taipei compared to Baltimore. H1 HA clade diversity was greater in Taipei. Vaccination rates were lower in Taipei than Baltimore, but vaccination was associated with an increase in serum neutralizing antibodies to recent H1N1 strains in Taipei, but not Baltimore. There was a higher level of preexisting immunity to circulating H1N1 strains in Baltimore. Conclusions: Regional differences in preexisting immunity and H1N1 strain circulation may have contributed to the vastly different 2015-16 influenza seasons in Taipei and Baltimore and suggest immune responses to vaccination can be affected by the degree of preexisting immunity in the population.
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