Feb 03 2011

Follow up on Pandemrix and Narcolepsy

Last year it was reported that there was a possible increase in narcolepsy, a sleep disorder characterized by excessive sleepiness, in children who had received the Pandemrix brand of H1N1 flu vaccine in Sweden, Finland, and Iceland. However a review of the data did not find a convincing connection, although concluded there was insufficient data at present and recommended further surveillance. A narcolepsy task force was formed in Finland, and now we have their preliminary report.

They conclude that the evidence suggests there is a connection:

Based on the preliminary analyses, the risk of falling ill with narcolepsy among those vaccinated in the 4-19 years age group was 9-fold in comparison to those unvaccinated in the same age group. This increase was most pronounced among those 5–15 years of age. No cases were observed among those under 4 years of age. Also, no increase in cases of narcolepsy or signs of vaccination impacting risk of falling ill with narcolepsy was observed among those above 19 years of age.

These results are intriguing, but should be considered preliminary. Epidemiology is a complex endeavor, and there are lots of wrinkles to this data. The increased risk of narcolepsy was only seen within a certain age range. In Iceland (but not Sweden or Finland) the increase in narcolepsy was also seen in those who were not vaccinated. And further, other countries that also used the Pandemrix vaccine have seen no increase narcolepsy, including Norway, the UK, Germany, and Canada.

Overall we have a very inconsistent pattern. The vaccine does not appear to be a consistent or unique risk factor for narcolepsy in these populations. The task force concludes from this that there must be another factor that is combining with the vaccine to increase the risk. This is logically possible, but until this factor X is identified it remains speculation.

One possibility is that the vaccine only increases risk within certain genetic populations. If a gene variant can be localized that renders individuals susceptible to some component in this brand of vaccine that could explain this data. Perhaps something else is also triggering the increase in Iceland, but not Finland and Sweden, to explain the rise in narcolepsy there in the unvaccinated.

Another possibility is that there is one or more confounding factors leading to the increase in narcolepsy, and the vaccines are a correlating but not causative factor.

Such is the nature of epidemiology, or observational studies. Variables are not controlled for and confounding factors are always a possibility. That does not mean that observational data is not useful or cannot be definitive – but it requires careful, thoughtful, and thorough collection and analysis of data from multiple different angles. The data we have so far from Finland is very preliminary, and generates more questions than answers. There is certainly sufficient cause for caution and further analysis. But at this point I would not be surprised by any particular outcome, since the data can be interpreted in many ways.

It will be interesting to see what the final outcome of this story is. These cases are always like mystery investigations, and the outcomes are often very interesting and can differ greatly from initial impressions. I would not be surprised if it turns out to be a real effect of the Pandemrix vaccine. Vaccines are not without risk, although over the decades the risks have proven to be very small and vastly outweighed by the benefits. Obviously it would be hugely useful to identify which ingredient was the culprit and exactly how it triggered narcolepsy in this population.

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