Post by trustyman on Oct 15, 2009 10:35:43 GMT 10
A short extract from this www.theatlantic.com/doc/200911/brownlee-h1n1 article
Public-health officials consider vaccine their most formidable defense against the pandemic—indeed, against any flu—and on the surface, their faith seems justified. Vaccines developed over the course of the 20th century slashed the death rates of nearly a dozen infectious diseases, such as smallpox and polio, and vaccination became one of medicine’s most potent weapons. Influenza virus was first identified in the 1930s, and by the mid-1940s, researchers had produced a vaccine that was given to soldiers in World WarII. The U.S. government got serious about promoting flu vaccine after the 1957 flu pandemic brought home influenza’s continuing potential to cause widespread illness and death. Today, flu vaccine is a staple of public-health policy; in a normal year, some 100 million Americans get vaccinated.
But while vaccines for, say, whooping cough and polio clearly and dramatically reduced death rates from those diseases, the impact of flu vaccine has been harder to determine. Flu comes and goes with the seasons, and often it does not kill people directly, but rather contributes to death by making the body more susceptible to secondary infections like pneumonia or bronchitis. For this reason, researchers studying the impact of flu vaccination typically look at deaths from all causes during flu season, and compare the vaccinated and unvaccinated populations.
Such comparisons have shown a dramatic difference in mortality between these two groups: study after study has found that people who get a flu shot in the fall are about half as likely to die that winter—from any cause—as people who do not. Get your flu shot each year, the literature suggests, and you will dramatically reduce your chance of dying during flu season.
Yet in the view of several vaccine skeptics, this claim is suspicious on its face. Influenza causes only a small minority of all deaths in the U.S., even among senior citizens, and even after adding in the deaths to which flu might have contributed indirectly. When researchers from the National Institute of Allergy and Infectious Diseases included all deaths from illnesses that flu aggravates, like lung disease or chronic heart failure, they found that flu accounts for, at most, 10 percent of winter deaths among the elderly. So how could flu vaccine possibly reduce total deaths by half? Tom Jefferson, a physician based in Rome and the head of the Vaccines Field at the Cochrane Collaboration, a highly respected international network of researchers who appraise medical evidence, says: “For a vaccine to reduce mortality by 50 percent and up to 90 percent in some studies means it has to prevent deaths not just from influenza, but also from falls, fires, heart disease, strokes, and car accidents. That’s not a vaccine, that’s a miracle.”
The estimate of 50 percent mortality reduction is based on “cohort studies,” which compare death rates in large groups, or cohorts, of people who choose to be vaccinated, against death rates in groups who don’t. But people who choose to be vaccinated may differ in many important respects from people who go unvaccinated—and those differences can influence the chance of death during flu season. Education, lifestyle, income, and many other “confounding” factors can come into play, and as a result, cohort studies are notoriously prone to bias. When researchers crunch the numbers, they typically try to factor out variables that could bias the results, but, as Jefferson remarks, “you can adjust for the confounders you know about, not for the ones you don’t,” and researchers can’t always anticipate what factors are likely to be important to whether a patient dies from flu. There is always the chance that they might miss some critical confounder that renders their results entirely wrong.
Public-health officials consider vaccine their most formidable defense against the pandemic—indeed, against any flu—and on the surface, their faith seems justified. Vaccines developed over the course of the 20th century slashed the death rates of nearly a dozen infectious diseases, such as smallpox and polio, and vaccination became one of medicine’s most potent weapons. Influenza virus was first identified in the 1930s, and by the mid-1940s, researchers had produced a vaccine that was given to soldiers in World WarII. The U.S. government got serious about promoting flu vaccine after the 1957 flu pandemic brought home influenza’s continuing potential to cause widespread illness and death. Today, flu vaccine is a staple of public-health policy; in a normal year, some 100 million Americans get vaccinated.
But while vaccines for, say, whooping cough and polio clearly and dramatically reduced death rates from those diseases, the impact of flu vaccine has been harder to determine. Flu comes and goes with the seasons, and often it does not kill people directly, but rather contributes to death by making the body more susceptible to secondary infections like pneumonia or bronchitis. For this reason, researchers studying the impact of flu vaccination typically look at deaths from all causes during flu season, and compare the vaccinated and unvaccinated populations.
Such comparisons have shown a dramatic difference in mortality between these two groups: study after study has found that people who get a flu shot in the fall are about half as likely to die that winter—from any cause—as people who do not. Get your flu shot each year, the literature suggests, and you will dramatically reduce your chance of dying during flu season.
Yet in the view of several vaccine skeptics, this claim is suspicious on its face. Influenza causes only a small minority of all deaths in the U.S., even among senior citizens, and even after adding in the deaths to which flu might have contributed indirectly. When researchers from the National Institute of Allergy and Infectious Diseases included all deaths from illnesses that flu aggravates, like lung disease or chronic heart failure, they found that flu accounts for, at most, 10 percent of winter deaths among the elderly. So how could flu vaccine possibly reduce total deaths by half? Tom Jefferson, a physician based in Rome and the head of the Vaccines Field at the Cochrane Collaboration, a highly respected international network of researchers who appraise medical evidence, says: “For a vaccine to reduce mortality by 50 percent and up to 90 percent in some studies means it has to prevent deaths not just from influenza, but also from falls, fires, heart disease, strokes, and car accidents. That’s not a vaccine, that’s a miracle.”
The estimate of 50 percent mortality reduction is based on “cohort studies,” which compare death rates in large groups, or cohorts, of people who choose to be vaccinated, against death rates in groups who don’t. But people who choose to be vaccinated may differ in many important respects from people who go unvaccinated—and those differences can influence the chance of death during flu season. Education, lifestyle, income, and many other “confounding” factors can come into play, and as a result, cohort studies are notoriously prone to bias. When researchers crunch the numbers, they typically try to factor out variables that could bias the results, but, as Jefferson remarks, “you can adjust for the confounders you know about, not for the ones you don’t,” and researchers can’t always anticipate what factors are likely to be important to whether a patient dies from flu. There is always the chance that they might miss some critical confounder that renders their results entirely wrong.