Richard Gayle
Get Your Flu Shots October 29, 1999
I noticed in the elevator the other day that Immunex is sponsoring flu shots for its employees. On the same day, I happened to see some information in New Scientist discussing a new strain of flu that had turned up in Hong Kong. The new virus seems to have jumped from pigs. Now most of us who are of an age remember the "swine flu" debacle during the late '70s. So why should this be news at all? To understand this, I'll provide some historical information and detail some of the peculiarities of the "flu" virus.
The influenza virus has a long history of causing large outbreaks called pandemics. One of the reasons for this comes from its unusual genomic structure (For more information, see links below). Essentially, an infectious influenza virus contains RNA, not DNA. And the RNA is not on 1 long strand but on 8 separate ones, each of which is required for infectivity. The separation of its genome into segments increases the potential for "recombinants" to emerge from cells simultaneously infected by different types of virus. It is this recombination that presents the greatest danger from this virus.There are 3 main strains of influenza: A, B, and C. B and C are found in humans but A can be found in birds and in pigs. The ability for the virus to recombine amongst these 3 species is why influenza A can be so dangerous. RNA strands for a virus found in birds can move through pigs into humans, potentially creating a novel, virulent strain.
The outside of the virus has 2 major proteins that react with our immune system. They are hemagglutinin (H) and neuramididase (N). Each form of H and N is given a number and defines the variant of influenza A. These variants will undergo minor changes under natural selection (called antigenic drift) every few years. These are the "new" versions we see every year and for which we get flu shots. However, every so often, the virus undergoes a major change, called antigenic shift, in which a totally new H or N molecule, sometimes from a pig or bird host, becomes associated with the virus in humans. When this happens, the human immune system is unprepared and a pandemic can occur.
Ask any of your grandparents if they have ever heard of the Spanish flu. This broke out in 1918, after the end of WWI. The returning soldiers spread it throughout the world and the close quarters they found themselves in helped spread it faster. It killed 500,000 people in the US (more than all the Americans killed by all the wars in this century), and over 20 million worldwide! As the first strain identified, it was designated H1N1––It came from pigs.
The Spanish flu (H1N1) underwent antigenic drift and stuck around until an outbreak of the Hong Kong flu (H2N2) in 1957, when it totally disappeared from the scene. That is, until its reappearance in the mid-70s in the US. This explains the real desperation in the health community at that time, since this "world killer" which had not been seen for 20 years had reappeared. Few people living had been exposed to it, so natural immunity would be lacking. Luckily, for some reason, this form was not as deadly as before.
However, a pandemic is still possible. New forms of the virus that have never been seen can come from any areas that contain birds and pigs in close proximity to humans. As it turns out, Asia has a very long history of raising poultry and pigs, in close quarters, so it is not surprising that most new flu forms come from there. The CDC feels that a new pandemic could be spread much faster than ever before due to rapid air travel. They estimate that over 200,000 people in the US could die from a flu pandemic. That is why they look so closely at Asia.
In 1997, a new version of influenza (H5N1) was found containing a version of hemagglutinin only seen in chickens, without any transition through pigs. This chicken flu was confirmed in 18 people, 6 of whom who died. A 33% mortality rate is terrifying for any infectious disease. To stop its spread, millions of chickens were destroyed. This appeared to work, since no more cases of transmission from chickens to humans was seen.
The recent news dealt with a brand new form that appears to come from pigs. The H5N1 form seen 2 years ago, was contained because it did not seem to be transmitted well from human to human. That is, lots could be exposed but few would become sick. This new form also does not appear to be very dangerous. What scares the people at WHO and the CDC is the combination of a virus having the lethality of the chicken flu with another having the infectivity of swine flu. This is why there is a large international effort underway to track flu outbreaks. It is why you should be concerned anytime you hear about a new outbreak. I know I will be from now on.
I am planning to get a flu shot.
Links
Everything about the spread and control of the virus
Background
From the CDC (This has a great picture of the virus and of its life cycle)
The Influenza Virus (Very nice info on the virus)