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

Area of Science:

• Virology
• Epidemiology
• Public Health

Background:

• Zoonotic influenza viruses pose a continuous threat to human health.
• Pandemic preparedness requires ongoing surveillance and risk assessment of animal viruses.
• Population immunity is a key factor in assessing the pandemic potential of emerging viruses.

Purpose of the Study:

• To determine age-stratified hemagglutinin inhibition seroprevalence against circulating swine influenza viruses in Hong Kong and Guangzhou.
• To model the impact of population immunity on the basic reproduction number (R0) for swine influenza viruses if they become human-transmissible.
• To assess the pandemic risk posed by specific swine influenza virus strains based on existing population immunity.

Main Methods:

• Collected and analyzed 353 individual serum samples.
• Performed age-stratified hemagglutinin inhibition assays to determine seroprevalence and antibody titers.
• Modeled the effect of population immunity on the basic reproduction number (R0) using seroprevalence data.

Main Results:

• Low seroprevalence was observed for triple-reassortant H1N2 and Eurasian avian-like H1N1 influenza viruses.
• Existing population immunity would only reduce the basic reproduction number (R0) by 18%-20% for these H1N1 and H1N2 variants.
• For human-origin H3N2, population immunity could suppress R0 by 47%, indicating a reduced pandemic risk compared to the swine-origin viruses.

Conclusions:

• Existing population immunity is insufficient to block the potential human-to-human spread of certain swine influenza H1N1 and H1N2 viruses.
• The basic reproduction number (R0) required for a pandemic (1.22-1.24) could be surpassed by these swine influenza variants.
• Human-origin H3N2 poses a lower pandemic risk due to higher existing population immunity, highlighting the importance of strain-specific risk assessment.