Epidemiology of Influenza subtypes
Here at YCPMicro, we are moving steadily into the 2011-12 seasonal influenza epidemic. Currently (5 November 2011), Pennsylvania is experiencing ‘sporadic’ influenza levels, but these are expected to reach widespread levels by late December to early January. During the 2009-10 seasonal influenza outbreak, epidemiologists were actually much more concerned about the so-called ‘swine-flu’ outbreak, which was in fact different from the predicted seasonal influenza variety. The name ‘swine-flu’ is somewhat deceptive, in that the virus is actually not particularly able to jump from pig to human. The name ‘swine-flu’ actually refers to the origin of the virus; it was a variety on influenza found in pig populations, and a random variant acquired the ability to jump into human populations. This variant then was very much able to infect humans, so it is much more appropriately referred to as ‘human-flu, originally from a pig’. Scientists were particularly worried about this outbreak, because the virus had many of the characteristics shared with the flu variant responsible for the 1918 Influenza pandemic that killed upwards of 100 million people, or about one in 20 people.
A new commentary published in mBio, the open access journal of the American Society of Microbiology, presents a hypothesis as to why season influenza outbreaks are transient. The authors from Mount Sinai School of Medicine in New York note that pandemic influenza emerges in human populations with some regularity, and that these outbreaks are coupled with the disappearance of seasonal variant, and they propose a hypothesis as to why this might occur.
Seasonal influenza vaccines typically are only effective for that year’s most prevalent influenza subtype, and offer little protection for subtypes that emerge in the future. However, unless an individual is in one of the high risk groups (children, the elderly, or the immunocompromised), many cases of seasonal influenza can be subclinical or only cause mild signs and symptoms in otherwise healthy individuals. Consequently, when coupled with a good level of vaccination compliance, seasonal influenza outbreaks typically burn themselves out as the population becomes essentially completely immune either due to vaccination or by being exposed to the virus.
What was noticed in 2009 was that the seasonal influenza outbreak was extremely minor in comparison to the more significant H1N1 ‘swine-flu’ outbreak, and that isolates from the ‘seasonal’ outbreak disappeared much more quickly than typical. The authors conjecture that pre-existing immunity in individuals exposed to earlier H1N1 outbreaks (primarily from 1957 and 1977) offered a degree of partial immunity mediated by B lymphocytes. The immunity was against antigens in conserved regions of viral proteins such as hemagglutinin, diagrammed here. The hemagglutinin antigen (the “H” part of H1N1) is variable from flu isolate to isolate, however immunity to the conserved stalk region can confer partial protection. Antibodies against these conserved regions further reduced the spread of the less virulent seasonal virus. Infection and spread occurs most prevalently in the more virulent pandemic variety than the seasonal variety, consequently the season variant quickly dies out due to inability to infect hosts.
The authors postulate for the future as novel influenza subtypes are generated in animal reservoirs that emergence of new pandemic influenza will rapidly displace seasonal variants if they are somewhat related, due to cross-reactivity of antibodies directed against conserved regions of viral antigens. If an emergent pandemic influenza subtype is very different from existing seasonal varieties, they may co-exist with the seasonal varieties without causing them to die out, such as what happened in the 1977 H1N1 outbreak, which was marked by a non-H1N1 seasonal influenza variety.
BONUS: The interaction between influenza virus and the human host is extremely complicated, and it is unlikely that we will ever eradicate pandemic influenza as a malady of humanity. Describe a reason why this is so. Remember: no repeats allowed! Offer good through the next exam, 16 Nov. 2011.