Dating the emergence of pandemic influenza

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Mean estimates of the time of most recent common ancestor also suggest that the H2N2 and H3N2 pandemic strains may have been generated through reassortment events in unknown mammalian hosts and involved multiple avian viruses preceding pandemic recognition. In the 20th century, 3 influenza viruses caused major pandemics: the 1918 H1N1 virus, the 1957 H2N2 virus, and the 1968 H3N2 virus.

The possible generation of pandemic strains through a series of reassortment events in mammals over a period of years before pandemic recognition suggests that appropriate surveillance strategies for detection of precursor viruses may abort future pandemics. These pandemics were initiated by the introduction and successful adaptation of a novel hemagglutinin subtype to humans from an animal source, resulting in antigenic shift.

They are now known to represent 3 different antigenic subtypes of influenza A virus: H1N1, H2N2, and H3N2, respectively.

Not classified as true pandemics are 3 notable epidemics: a pseudopandemic in 1947 with low death rates, an epidemic in 1977 that was a pandemic in children, and an abortive epidemic of swine influenza in 1976 that was feared to have pandemic potential.

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Similar observations were made on the other side of the world and reported in a little-known paper in the National Medical Journal of China ().

Nevertheless, although diagnostic virology was not yet available, bacteriology was flourishing and many careful postmortem examinations of patients by academic bacteriologists and pathologists disclosed bacterial pathogens in the lungs ().

This information is important in considering the question of "will there ever be another 1918." To the degree that secondary bacterial infection may contribute to influenza death rates, it should at least be partially controllable by antimicrobial agents, as indeed was the case in 1957.

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