Instant Solutions
Cracking the Flu Code
Over the past few months you’ve certainly heard and seen plenty of news about the “swine flu.” Opinions on the matter vary widely. Some people believe that when the flu breaks into humans it will be the beginning of the apocalypse. Others say that it’s all a bunch of hooey, blown out of proportion by people who have other agendas.
I’m somewhere between those two camps. Evidence so far is that the virus isn’t very infective (yet). Only a few thousand people in Mexico have come down with this particular flu, plus a few hundred others scattered in a dozen or so countries around the world. The concern isn’t really over what this flu is, but over what it has the potential to become.
For a flu virus to cause widespread infection, it has to have three characteristics. First, it has to be a subtype new to humans. Second, it has to be able to infect humans. And third, it has to be transmissible from person to person. These events don’t come together very often—as evidenced by the fact that there have been only three pandemics in the last hundred years.
The virus currently under discussion is referred to as the H1N1 subtype. The letters refer to proteins on the surface of the virus: H for hemagglutinin (which allows the virus to penetrate a host cell in the respiratory system) and N for neuraminidase (which allows it to escape from the host cell and move on to infect other cells). The numbers refer to which variety of each protein is present. There are 16 varieties of the H protein, and 9 varieties of the N.
Influenza viruses are also categorized by the core proteins, either A, B, or C. The swine flu under question comes from a virus that has the A proteins. So, properly speaking, it’s Type A H1N1.
Theoretically, the annual flu vaccine could address the swine flu. Each year’s formulation contains antibodies against two Type A viruses and one Type B, with the decisions based on what showed up in the Far East the year before. However, flu viruses undergo what’s called “antigenic drift,” which means that minor genetic changes slightly alter a particular subtype from year to year. The differences mean that some who came down with the flu last year could catch it again this year. However, because of the similarities that remain, many people will still have antibodies in their system to fight the virus.
Trouble comes when viruses undergo “antigenic shift”: substantial changes that result from a reassortment of genes within the virus. Birds are susceptible to type A viruses only. Obviously, though, other creatures are too—including humans. The reassortment happens most often in intermediate hosts (such as pigs) that are susceptible to both avian and human viruses—which is exactly what’s happened with this go-round of the swine flu. When an animal happens to be infected with an avian virus and a human virus at the same time, the two can trade genetic material. The finished product, you might say, is one or more viruses with properties that allow it to infect humans but that have a protein coat that we haven’t encountered before—which means humans have no resistance. Interestingly, all the flu pandemics of the 20th century have come from Type A viruses: the Spanish flu (1918–1919), subtype H1N1; Asian flu (1956–1957), subtype H2N2; and the Hong Kong flu (1968–1969), subtype H3N2.
The current swine flu is causing so much concern because epidemiologists have observed an antigenic shift. They’re afraid that the virus is so much different from what’s been seen before that few people will have adequate resistance to it. And a pandemic is born.
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