It is not SARS or monkeypox or West Nile, or any of the dozens of headline-grabbing viruses that so famously travel the world.
It is the common flu, and we take it for granted at our peril.
Last year, about 120 million of those shots were distributed throughout the country. This year, public health officials estimate manufacturers produced upward of
132 million doses.
How effective those shots will be, however, depends largely upon how clever the scientists were in anticipating how far, and in which direction, the virus has evolved since last winter's flu season.
“We've had three relatively mild flu seasons in a row,” said Curtis Allen, a spokesman at the Centers for Disease Control and Prevention. “But that doesn't necessarily speak to this year. We don't know what might happen this year.”
What he does know, however, is that influenza forecasters are fortunately more often right than wrong.
The process of guessing what a flu season might look like begins, Allen said, a full year in advance.
At 118 national influenza centers in 84 countries, doctors collect influenza information for the World Health Agency and the CDC, among others. They look at the past season's strains, then duck down under, into the Southern Hemisphere, to see what's happening there, because summer here is winter there.
By February, the Food and Drug Administration has made its picks for the coming fall, the top bets for which flu strains likely will race across the countryside. Vaccine manufacturers then begin growing the strains, and it can take six months to produce a big batch.
If science has predicted virus evolution, and there's a good match between the shot and the bug, then effectiveness can approach 90 percent. If the bug beats the odds - as happened in the mismatched 2003-2004 flu season - then effectiveness is cut to perhaps 50 percent.
“But usually,” Allen said, “even in a bad year the antibodies help.”
Antibodies are how humans fight the flu. The virus attacks and the immune system responds, “learning” which specialized antibodies it must make to defeat the invader.
If the same invader returned year after year, flu would be no problem. Our bodies would “remember” how to prepare for the fight.
The trouble is, flu viruses change very quickly. Mostly, they change by a method science calls “antigenic drift,” small genetic changes over time that produce new viral strains our bodies haven't experienced before.
That's why this year's vaccine is aimed, in part, at flu type A-Solomon Islands instead of A-New Caledonia, as was last season's. (The other two elements of this year's vaccine are pretty much unchanged from the 2006-2007 season - type A-Wisconsin, and type B-Malaysia, according to the CDC.)
The genetic differences from year to year are relatively small, Allen said, but they're enough to spark an annual epidemic. At issue are a pair of proteins that sit atop the surface of the flu virus. Science calls them hemagglutinin and neuraminidase, then thankfully shortens it all to “H” and “N.”
There are, in all, 16 subtypes of H and nine subtypes of N, and humans tend to get flu types such as A-(H1N1) or
A-(H3N2). The type B, well, it's just B.
Trouble is, the slow and steady process of evolution by antigenic drift is not the only way flu viruses can change. There's also antigenic shift, which is far more troubling to public health people such as Allen.
Wild and domestic birds, for whatever evolutionary reason, carry all 16 H subtypes and all nine N subtypes of flu. Mostly, the viruses don't make the birds sick, unless they get a case of H5 or H7 or some other “avian” flu.
And humans, for the most part, don't seem to catch bird influenza (although the species barrier is not absolute.)
The real problem is pigs. They can catch bird flu, and they can catch human flu, and they can catch pig flu (think “swine flu.”)
And here's where CDC researchers say the antigenic shift happens. Inside a pig, a bird flu and a human flu can meet, and can produce a whole new virus. It might have most of the human flu's genetics, but the H or the N from the bird flu. That means it could spread from person to person, but with protein combinations our immune systems haven't seen before.
That sort of antigenic shift brought us the Asian flu pandemic in 1957, and the 1968 Hong Kong flu outbreak.
The new virus that pops up from such a mix - “zoonosis” is the word for an animal-to-human transfer - appears abruptly and out of the blue, not in the slow and steady way of antigenic drift, and consequences can be dramatic.
The Spanish flu pandemic of 1918 killed perhaps
50 million people and made an estimated 40 percent of humankind feel mighty miserable.
In a single October day in New York City, 851 people died. In some World War I camps, more than 90 percent of the soldiers fell ill, and medical historians point to that outbreak - just as Germans troops were mounting an offensive in France - as a turning point in the war.
The virus, then, marches hand in hand with humankind through history.
Flu is an old enemy. Hippocrates described it in 412 B.C., Charlemagne in 876.
It persists because it is so hard to combat, and it remains hard to combat because it can so easily hide.
A human-specific disease can often be wiped out, or at least controlled, by hunting it out of human systems. But what of an ever-changing zoonotic invader that can hole up in all the world's birds and pigs? How to root it out?
The best way, so far, is to watch, and to wait, and to build annual vaccines that, hopefully, deal with antigenic drift, if not the more rapid shift.
The flu shot, which contains a dead virus, holds two A-type flu bugs and one B, and is advised for pretty much everyone except the youngest infants. The flu spray, spritzed up the nose, contains live virus - again two A and one B - and is not for kids under five or anyone over 50, Allen said.
Both, however, work in pretty much the same way, by safely introducing the immune system to this year's predicted flu strain so antibodies can begin their arms race. By the time the real virus appears - right about this time of year and on through spring - our systems know how to wipe it out thanks to the vaccine.
Beyond that, the war against influenza falls back on what Allen calls “mother's advice” - cover your mouth when you cough, wash your hands, keep your finger out of your nose (or your mouth or your eye or whatever).
And stay home while you're sick, he said, because even though viruses have no way of moving around on their own, we're more than willing to pick up hitchhikers - only to drop them off where our friends sincerely wish we hadn't.
A sneeze, for instance, can travel several feet, and an invisible bug can live on a doorknob for a remarkably long time, waiting for a hand, then a mouth or an eye or a nose or some other open door.
“Vaccine and hygiene are two essentials,” Allen said.
Viruses have been called, by some, nature's bio-terrorists, far better at laying low entire populations than the most rabid extremist.
Last winter, when the annual epidemic spiked in February, some 80 percent of those infected came down with type-A flu, which unlike the human-specific type-B is shared with birds and pigs.
At the season's height¸ about 8 percent of all deaths in the nation were traced back to flu and its complications, such as pneumonia. Workplace productivity was hit, as it is every year, and the economy stretched to absorb all those sick days.
The CDC estimates that as many as 20 percent of Americans are infected each year, hit hard by fever and fatigue, headache, body ache, cough and sore throat.
Researchers at CDC say we're infectious a full day before we feel symptoms, and for a week after that.
Flu shots help, Allen said, and so do new anti-viral drugs aimed at inhibiting the N protein, but both rely on a good match. This year, the CDC recommends only Tamiflu and Relenza among the prescription antivirals, as the rest have been made ineffective by viral evolution and resistance.
Because ultimately, nothing stops the drift. It's why our immune systems must respond anew each season, and it's why health officials have such a hard time predicting which strain to include in vaccines.
“We had a good match last year,” Allen said. “We won't know about this year until it's over.”
That should be late spring, perhaps as late as May.
Between now and then, Allen said, “we've very optimistic. Supplies of vaccine are robust.”
Perhaps one in three Americans will receive the shot or the spray, he said, far more than in 2004-2005 when a last-minute shutdown at a vaccine manufacturing plant halved the number of available shots.
“There should be no problem with supply this year,” Allen said. “I think about 60 million doses are already out there, and more than 100 million should be delivered by the end of this month.”
As to how effective those shots will be, Allen said, “call back in the spring and we'll let you know. This virus can be very tricky to predict.”
Reporter Michael Jamison can be reached at 1-800-366-1786 or at mjamison@missoulian.com.
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