Determining Existing Human Population Immunity as Part of Assessing Influenza Pandemic Risk

Emerg Infect Dis. 2022 May;28(5):977-985. doi: 10.3201/eid2805.211965.


Zoonotic influenza infections continue to threaten human health. Ongoing surveillance and risk assessment of animal viruses are needed for pandemic preparedness, and population immunity is an important component of risk assessment. We determined age-stratified hemagglutinin inhibition seroprevalence against 5 swine influenza viruses circulating in Hong Kong and Guangzhou in China. Using hemagglutinin inhibition seroprevalence and titers, we modeled the effect of population immunity on the basic reproduction number (R0) if each virus were to become transmissible among humans. Among 353 individual serum samples, we reported low seroprevalence for triple-reassortant H1N2 and Eurasian avian-like H1N1 influenza viruses, which would reduce R0 by only 18%-20%. The smallest R0 needed to cause a pandemic was 1.22-1.24, meaning existing population immunity would be insufficient to block the spread of these H1N1 or H1N2 variants. For human-origin H3N2, existing population immunity could suppress R0 by 47%, thus reducing pandemic risk.

Keywords: China; Hong Kong; hemagglutination inhibiting antibody; hemagglutinin; influenza; neuraminidase; pandemics; population immunity; reproduction numbers; risk assessment; seroprevalence; swine; swine flu; viruses; zoonoses.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Hemagglutinins
  • Humans
  • Influenza A Virus, H1N1 Subtype*
  • Influenza A Virus, H1N2 Subtype
  • Influenza A Virus, H3N2 Subtype
  • Influenza A virus*
  • Influenza, Human*
  • Orthomyxoviridae Infections*
  • Reassortant Viruses / physiology
  • Seroepidemiologic Studies
  • Swine
  • Swine Diseases* / epidemiology
  • Zoonoses


  • Hemagglutinins