Vaccines are one of the great success stories of modern medicine. Because of them we are no longer vulnerable to smallpox or polio or measles. The flu vaccine, however, is a different story. Its effectiveness varies from patient to patient, from population to population, and from year to year. It needs to be updated each season, and even in a good year is usually no more than 50% effective. We may rely on it to avoid catching the flu, but its story demonstrates how far we still are from a reliable vaccine.
Because the influenza virus can mutate so quickly, nailing the exact recipe is challenging. In some seasons the match is close to perfect, but this is not always the case. If the virus drifts after the February meeting of the WHO, there will be a mismatch between vaccine and virus. The greater the mismatch, the less effective the vaccine. In a good year, we might expect the vaccine to be 50-60% effective. In the 2004-5 flu season, that figure was only 10%, meaning that the vaccine was a big misfire. We also botched the 2014-5 season, when new strains hadn’t been included in the vaccine. That season the vaccine was a measly 19% effective, compared to over 50% in the previous year. Early on in the 2017-8 influenza season, there were near-record numbers of hospitalisations, and though the latest vaccine’s performance improved throughout the season, overall effectiveness is estimated to have been 40%.
Even if the vaccine hits the bullseye, different demographics have different reactions to it. Children have a very good response to the vaccine. The situation is more complicated with elderly patients, who have weaker immune systems overall but also have a lifetime buildup of natural immunity. After withstanding many flu seasons, their immune systems are wiser, you might say, than those of the young.
The US and most other developed countries strongly recommend that older people receive a flu vaccine. One study compared 18 different groups over 10 influenza seasons and found that the vaccine reduced the overall winter mortality rate in older people by an astonishing 50%.
The bottom line is this: even if elderly people are vaccinated, they are still the population most likely to die from influenza.
Around one billion people in the USSR were vaccinated using the live but weakened flu virus, and it was still in use at the end of the 20th century. Although it appeared to be successful, the live-flu vaccine was never tested in a rigorous way, and it remained a constant danger. Since it used a live virus, it could cross with other strains and morph into a more virulent version.
Vaccine researchers therefore turned their attention to creating a vaccine containing what they called “inactive” strains. The virus was still grown in chick embryos, but this time it was rendered inactive by dunking it in a bath of formalin disinfectant. Although a higher dose of the inactive vaccine was needed to produce an immune response, there was no concern about the virus replicating.
For the first several years the influenza vaccine contained only one strain, the influenza A virus, because, as far as anyone knew, that was the only kind of influenza out there. In 1940 influenza B was identified, which kicked off the perpetual task of calibrating vaccines to deal with multiple evolving strains. By the 1950s we had a vaccine that was effective against both A and B, but the virus, as always, was outpacing us. By the late 1970s we had to make a vaccine to hit three strains. For the 2016-7 flu season, most of the vaccine doses manufactured in the US targeted four different strains. The past 100 years have been a ceaseless arms race against an enemy with whom we cannot negotiate.
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