For some reason that I have never quite fathomed, immunization against infectious diseases has from its very inception in Jenner’s time been one of the most viscerally feared and bitterly opposed of all medical techniques. Perhaps people felt that to immunize was to interfere sacrilegiously with the course of nature, and that people, especially children, had the duty to die of infectious diseases just as Nature “intended.” Perhaps they felt that, if it worked, it would allow the survival of the unfittest. At any rate, few medical procedures have been as persistently, minutely, and fervently examined for harmful effects as immunization has.

In general, the results have been disappointing for those who wished to show that immunization was invariably followed by Nature’s retribution, particularly in the neurological sphere. Scare has succeeded scare without ever being confirmed, though those who hold to the anti-immunization faith refuse to abandon it. Now, at last, there seems to be evidence of a genuine association between a certain type of immunization and a neurological condition.

That association is that between the immunization of children with an anti-influenza virus and narcolepsy, a condition characterized by chronic, excessive daytime sleepiness and a tendency to cataplexy, that is to say a loss of muscular tone triggered by strong emotion. It was first observed in Finland and Sweden; subsequent studies in other European countries and in Canada failed to find an association, but a further study, this time in England, and reported in the British Medical Journal, confirmed that the Finnish and Swedish findings.

In October 2009, children at risk of pulmonary complications during a pandemic of influenza were immunized against it with a vaccine against the causative virus. Most of the children immunized suffered from asthma (interestingly, one of the theories to account for the recent rise in the proportion of children suffering from asthma and other allergic conditions is that, having been immunized against all the common childhood infectious diseases, their immune systems have not developed as Nature “intended”).

The researchers found that children who had been immunized were 14.4 times as likely to develop narcolepsy as were similar children who had not been immunized. The figure of 14.4 is very similar to that found in Finland, where it was 13 times. The association might not be causative, of course: for example, if the immunization merely brought forward the development of the narcolepsy in children that would have developed in any case at a later stage in the children’s lives. Nor can we yet say whether narcolepsy developing within a few months of immunization with this influenza vaccine will last the rest of the child’s life, remit, or get worse.

The relative shortness of the follow-up did not allow for the possibility to be excluded that the immunization only brought forward the development of narcolepsy, but the authors nonetheless felt — for what such a feeling is worth — that the relationship was indeed a causative one.

The increased risk of narcolepsy, even if the association were a causative one, does not settle the issue of whether the vaccine should continue to be used. Even though the relative risk was very high, the absolute risk — estimated at about one per 55,000 doses — is small. And since the vaccine has proved very effective against the type of flu it is supposed to protect against, which can itself kill or severely harm the vulnerable, it might still save life or disability. In medicine, as in life in general, there is no gain without pain.

There is little doubt, however, that the findings will be grist to a mill that has now been grinding for more than 200 years, and that (remarkable to record) gave rise to mass protest movements in the nineteenth century in Britain and America.

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