The Incidence of Neutralizing Antibodies for Swine Influenza Virus in the Sera of Human Beings of Different Ages

Abstract

Sera from a very high proportion of the human adults and new-born infants studied neutralized swine influenza virus; sera from children below the age of 12 years seldom exerted such an effect. The results of neutralization experiments with human sera and the virus of swine influenza have been compared with the outcome of similar tests with the virus of human influenza, and it seems evident that the presence of antibodies neutralizing swine influenza virus cannot be deemed the result of repeated exposures to the current human type of virus. From the known history of swine influenza and the similarity of its etiologic virus to that obtained from man it seems likely that the virus of swine influenza is the surviving prototype of the agent primarily responsible for the great human pandemic of 1918, as Laidlaw has already suggested. The presence in human sera of antibodies neutralizing swine influenza virus is believed to indicate a previous immunizing exposure to, or infection with, an influenza virus of the 1918 type.

Characterization of the 1918 “Spanish” influenza virus neuraminidase gene

“…Two human strains, A͞Weiss͞43 (H1N1) and A͞FM͞1͞47 (H1N1), were included to bridge the gap between the early human strains of the 1930s and the next available sequence from 1954. A͞Swine͞Iowa͞30 (H1N1) was included as it is known to be closely related antigenically to the 1918 virus (13). A͞Duck͞ Alberta͞35͞76 (H1N1) and four duck strains from Ohio were added to provide strains representative of those found in wild birds; previously, the only avian N1 sequences available were from domesticated birds.…”

Origin and evolution of the 1918 “Spanish” influenza virus hemagglutinin gene

“…It has been known since the 1930s that survivors of the 1918 influenza had antibodies that neutralized classic swine influenza virus (33), and A͞Sw͞Iowa͞30 is very similar to the 1918 strains at the antigenic sites. Because individual swine do not live long enough to exert immune selective pressure on the virus, influenza drifts more slowly in swine than in humans (30).…”

Molecular events leading to the creation of a pandemic influenza virus

“…Initially it was thought that the infl uenza virus cannot break the interspecies barrier but the myth was cleared when the prototype strain of SIV was recovered in 1930, which was genetically and antigenically similar to the 1918 pandemic H1N1 virus [16]. Infl uenza viruses are endemic to wild aquatic birds, waterfowl and ducks, hence exposure to them gives more opportunities for the introduction of an infl uenza virus into a swine herd, which acts as the much needed carrier in nature to bridge the gap between the two specieshuman and avian.…”

“…Infl uenza in pigs was fi rst detected in 1930 and was considered of same lineage as 1918 human pandemic strain [16]. Termed as classical swine H1N1 virus it remained antigenically stable and continued to circulate in pigs of Asia and North American in 20th century and until 1980s in Europe, where it was later replaced by a novel avian origin swine H1N1 virus [19].…”