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Updated: Aug 10, 2026
06:34
An Optimized Hemagglutination Inhibition (HI) Assay to Quantify Influenza-specific Antibody Titers
Published on: December 1, 2017
Naturally acquired immunity to influenza type A. Lessons from two coexisting subtypes
The Medical Journal of Australia
|January 21, 1985
Summary
Influenza A virus H1N1 subtype infections were limited to younger individuals in Sydney after 1977. Older adults showed protection against H1N1, likely due to prior exposure, not just antibody levels.
Area of Science:
- Virology
- Epidemiology
- Immunology
Background:
- Influenza A virus H1N1 subtype re-emerged in 1977 after a 20-year absence.
- H1N1 displayed different epidemiological behavior compared to the co-circulating H3N2 subtype.
Purpose of the Study:
- To investigate the incidence of influenza A subtypes (H1N1 and H3N2) in unvaccinated volunteers.
- To determine age-specific attack rates and identify factors contributing to protection against H1N1.
Main Methods:
- Studied 287 unvaccinated volunteers in Sydney over multiple winter seasons.
- Measured serum antibody titres before and after winter to detect clinical and subclinical infections.
- Monitored infections during H3N2 (1977) and H1N1 (1979, 1981) epidemics.
Main Results:
- H3N2 infected all age groups during the 1977 epidemic.
- H1N1 epidemics in 1979 and 1981 primarily affected individuals born after 1950.
- Older participants exhibited protection against H1N1, suggesting long-term immunity.
Conclusions:
- Age-dependent susceptibility to H1N1 was observed, with younger individuals being more vulnerable.
- Homologous protection in older adults against H1N1 was likely acquired from past exposures predating 1977.
- This protection was independent of strain-specific hemagglutination inhibition antibody levels.
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