Influenza viruses--why are they such a concern?

by Tara Smith     Department: Health & Life Sciences

I wrote a bit about influenza back in October, when the season officially begins. Yesterday at Aetiology, I followed that up with an update on some interesting new research on influenza viruses.

Influenza is in a category of viruses that we refer to as "constantly emerging." This means that the virus we're exposed to is constantly changing. Influenza is an RNA virus--it has ribonucleic acid as its genome, rather than DNA as we do. RNA is much more prone to mutations than DNA is--it doesn't have a molecular "spellchecker" that DNA uses, so viruses can quickly accumulate errors. It's this accumulation of mutations that makes the formulation of new influenza vaccines necessary each season. However, what's really worrisome is the possibility of a completely novel influenza strain entering the human population--and more importantly, spreading between humans.

This is what's worried us about avian influenza infections that have turned up in humans--and particularly, infections with serotype H5N1. (The serotype just describes two of the surface proteins of the influenza virus--particular variants of the hemagglutinin and neuraminidase proteins). Humans haven't experienced infection with this particular type of virus previously--and currently, it's killing about 70% of those infected. However, except for a few potential instances, H5N1 hasn't gained the ability to spread efficiently between humans; most of the cases have been in people who caught the virus directly from its bird reservoir. For a pandemic to emerge from infection with a novel strain, it needs to be able to transmit between humans--so while we're watching H5N1 closely, it doesn't appear to be capable of causing a pandemic right now.

This doesn't mean, however, that we can rest when it comes to new influenza strains. Recent research has shown evidence of another new influenza type (H2N3) infecting not birds or humans, but swine. This is worrisome for a number of reasons. First, swine have long been seen as a potential "mixing vessel" for avian and human influenza viruses. (This is because they contain sugars on cells of their lungs that are able to bind both avian and human types of influenza viruses). Additionally, because swine are closer relatives to us than birds, this H2N3 virus is already adapted to mammals--making the jump to humans potentially easier. If that happens and the virus can be transmitted between humans (as the authors of the new research in PNAS already showed it can do between swine and ferrets, an animal model frequently used for influenza studies), that means bad news for us--especially those of us born after 1968.

Why that time point? The last time an influenza virus that carried an H2 protein circulated in the human population was in 1968. For several years prior, a H2N2 serotype influenza virus was endemic in humans, but this was replaced during the 1968 influenza pandemic (caused by a H3N2 virus). Therefore, those of us born after 1968 (and anyone born between 1957, when the H2N2 pandemic began, and 1968, and who managed to escape infection with this virus) would be highly susceptible to infection because of our lack of pre-existing immunity to this serotype.

Influenza is a scourge that has haunted us throughout history, and will remain with us for the foreseeable future. All we can do is be aware, be informed, and be vigilant (and get your yearly vaccination!).

Tags: influenza, pandemics