Tuesday, 15 February, 2005

Do flu shots work?

A study published yesterday in the Archives of Internal Medicine indicates that flu shots might not save the lives of the elderly. According to the study:"We could not correlate increasing vaccination coverage after 1980 with declining mortality rates in any age group." Media coverage has reduced the findings to a quite spectacular headline: "Flu shots don't work for old people," which is a pretty narrow reading of the findings. First, the study itself says "we could not rule out some benefit from the vaccine, but it is less efficacious than we thought," meaning that there might very well be some benefit. Secondly, the article states "And we only looked at deaths, not other complications." So although there's no evidence that flu shots save lives, it's quite possible that the flu shots prevent people from getting sick.  Read more about the study in this Yahoo article.

The article goes on to state that doctors have known for years that vaccines in general are less effective in older people because their immune systems are not as efficient at creating antibodies. This research finding is news not because it discovered anything new (doctors have known this for some time), but rather because it debunks the popular theory among laypeople that flu shots are some kind of magic elixer for the elderly.

Most importantly, the findings only apply to elderly people. The jury is still out on the effectiveness of flu shots on other segments of the population. Some studies show that vaccinations are effective in younger people. Conversely, vaccination rates have increased tremendously since 1980 and flu deaths have remained essentially flat. Does that mean that influenza vaccinations are ineffective? That's a hard question.

As we all know, the flu vaccination works by introducing a flu virus into the body in a measured dose so that the immune system can recognize it and create antibodies. When the real virus comes along, the body can make quick work of defeating it before the virus takes hold and creates major complications. The complicated nature of the immune system results in this being less than an exact science, but in general it works. The primary benefit, as far as the person being vaccinated is concerned, is that he doesn't get sick, or he doesn't get as sick as he would have without the vaccination. But society gains an even bigger benefit: that vaccinated person can't spread the disease to anybody else. That's a huge benefit when you consider that a person with the flu can potentially infect everybody with whom he comes into contact.  That's more important than it sounds.

What many people don't realize is that the influenza virus mutates, as does HIV. It adapts to its human host and sometimes (as happened in 1918), those mutations are incredibly virulent and can sweep through a population like wildfire. But it takes time for the virus to develop that virulence. In 1918, the virus presented itself as "just the flu" in a large part of the population over a short period of time. Then there was a lull--a plateau--during which the virus mutated inside those who were infected. Just when public health officials thought the wave of illness was over, the mutated virus flew through the population, killing over 100,000,000 people worldwide in a single year.  In the United States it accounted for about 675,000 excess deaths (0.6% of the population), comparable to about 1,750,000 today.  And it's quite likely that the number would be even higher than that today because a larger percentage of the population lives in crowded cities.

What we don't know, and can't really determine, is whether the increased influenza vaccination rate has prevented such a mutation from forming. Considering the potential consequences of another 1918-type mutation, I'd say we're better off continuing the vaccinations if there's any credible evidence that they're effective.

Read John Barry's The Great Influenza about the 1918 influenza epidemic before you make up your mind on this one.