Of Mice and Men: H1N1 Immunity in the USA; Effects of H1N1 Vaccines Past and Present

A recent study published in PLoS Pathogens found that vaccines developed from “classical H1N1 viruses (Sw/30 or NJ/76), 1918 virus-like particles, and a human H1N1 virus isolated in 1943 (Wei/43) protected against death from 2009 pandemic H1N1…” when tested in vivo in mice.

Another study finding was that the “H1N1 virus underwent little antigenic drift in pigs, as shown by the ability of the NJ/76 strain to induce protective immunity in mice against the 2009 H1N1 virus.” The fact that H1N1 influenza virus proteins are antigenically frozen in pigs, make them natural resorvoirs for future pandemics.

This research suggests that the much-maligned 1976 swine flu vaccination effort, which had an uptake of around 40 million Americans, may have conferred some protective effect on the population, in particular for persons aged 35 and up. This is one of the conclusions drawn from a review by Professor Ranaciello in Virology.

Quantifying the extent of this protective effect (in terms of pre-existing immunity) gets a little complicated. An exchange with Professor York, who pens Mystery Rays from Outer Space, (MRFOS) suggests some protective effect from the 1976 vaccines exists, but that it should not be overstated.

A study published last year in the New England Journal of Medicine (NEJM)assessed levels of pre-existing immunity to 2009 H1N1 to be 4% in children, 6% in young adults and 34% in older adults (those born before 1950). Assuming the 1976 vaccine was proportionally distributed by age, rough calculations suggest 7% (13 million persons) of the present day young adult cohort (aged 19-64) could have pre-existing immunity from the 1976 vaccine, close to the 6% estimate in NEJM.

However, the hard part is reconciling similar patterns of immunity to H1N1 observed elsewhere in the world among populations who had no access to the 1976 vaccine.

MRFOS estimates that current immunity to H1N1 in the US is in the range of 49-52%. These projections take into account estimated H1N1 vaccine uptake (80 million) and H1N1 infection rates (55 million) and adjusts for potential overlap between both. This level of immunity augurs well against a winter wave of H1N1.

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CDC: Emerging Infectious Disease Volume 15, Number 11–November 2009, Letter, Preexisting Immunity to Pandemic (H1N1) 2009
MRFOS: How many Americans are immune to H1N1?
NEJM: Cross-Reactive Antibody Responses to the 2009 Pandemic H1N1 Influenza Virus
PLoS Pathogens: Protection of Mice against Lethal Challenge with 2009 H1N1 Influenza A Virus by 1918-Like and Classical Swine H1N1 Based Vaccines
Virology: Protection against 2009 influenza H1N1 by immunization with 1918-like and classical swine viruses

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