Narcolepsy after swine flu jab studied
The swine flu vaccine may have been responsible for a sudden increase in cases of the sleep disorder narcolepsy, The Independent has today reported. The rare disorder causes people to feel drowsy or to spontaneously fall asleep and sometimes experience loss of muscle power.
The paper’s story was based on two studies from Finland, where there have been high-profile reports that the Pandemrix swine flu vaccine may have triggered new cases of child narcolepsy after its use in 2009. The authors of the studies found that, in Finland, the incidence of narcolepsy among children under the age of 17 years was about 17 times higher in 2010 than in the previous eight years. A further study found that the incidence of narcolepsy among children aged 4-19 who had been vaccinated was nearly 13 times higher than incidence among unvaccinated children.
These findings, which are based on comprehensive data taken from the entire Finnish population, raise some concerns and the issue is reportedly being investigated by UK officials. However, at present there is no proven association between the swine flu vaccine and narcolepsy in children. As the researchers point out, awareness of narcolepsy has increased in Finland in recent years and this may explain the rise in cases rather than any effect from the vaccine.
It is worth noting that among vaccinated children the “vaccine attributable risk” of narcolepsy was still extremely small – a one in 16,000 chance. The “attributable risk” is the difference in the incidence of narcolepsy between children who had been vaccinated and those who had not. One possibility mentioned by the authors is that the vaccine might have triggered or exacerbated an existing genetic predisposition to narcolepsy in some children.
What is the basis for these current reports?
The news is based on a new study looking for potential links between use of the swine flu vaccine and narcolepsy, which is a rare neurological sleep disorder that usually occurs where there is a strong genetic predisposition. Narcolepsy not only causes sudden daytime sleepiness but is also associated with cataplexy, which is loss of muscle power.
In 2009–2010 there was a global flu pandemic involving a relatively new strain of influenza that became known as swine flu. It is also referred to as H1N1 influenza (because it is caused by the H1N1 strain of virus). Once suitable vaccines were created many countries began vaccinating at-risk groups, including certain children. Shortly after vaccination, around early 2010, doctors noticed excessive daytime sleepiness and cataplexy in an exceptionally large number of Finnish children. Finnish children had been given the vaccine Pandemrix from 2009 onwards, causing some people to suggest a possible link.
To look at any potential link between Pandemrix and narcolepsy the researchers carried out a systematic analysis of the incidence of narcolepsy in Finland between the years 2002 and 2010. To further evaluate the potential connection between the vaccine and narcolepsy, researchers analysed data on vaccinations and childhood narcolepsy for all children born between 1991 and 2005.
What did the research find?
Today’s news coverage is based on two related studies looking at the potential relationship between Pandemrix and narcolepsy. The first paper was a study based on the Finnish national hospital discharge register. In this paper looking at narcolepsy rates, researchers found that during 2002–2009, 335 cases (all ages) of narcolepsy were diagnosed in Finland, giving an annual incidence of 0.79 cases per 100,000 inhabitants (95% confidence interval [CI] 0.62 to 0.96).
Among children under 17 years of age, the average annual incidence during this period was 0.31 cases per 100,000 inhabitants (95% CI 0.12 to 0.51).
In 2010, 54 children under the age of 17 were diagnosed with narcolepsy, equating to 5.3 cases per 100,000 children. This represents a 17-fold increase in incidence compared with the average for the previous eight years. Some 34 of the 54 children diagnosed in 2010 carried a gene variant associated with an increased risk of narcolepsy.
Fifty of the 54 children had received the Pandemrix vaccination between 0 and 242 days before onset of the disorder (42 days on average). In these 50 children, symptoms of excessive daytime sleepiness (EDS) had started abruptly. Forty-seven (94%) had cataplexy, which started at the same time or soon after the onset of EDS. Psychiatric symptoms were also common.
Among adults of 20 years or over, the incidence rate in 2010 was 0.87 per 100,000 population, only very slightly raised above previous years.
The second research paper was a retrospective cohort study that analysed rates of narcolepsy in children before and after vaccination. In this study, researchers found that 75% of the children analysed had been vaccinated against swine flu. There were 67 confirmed cases of narcolepsy, of which 46 vaccinated and seven unvaccinated cases were included in the primary analysis.
This second study looked at cases of narcolepsy within “person years”. A person year represents one person exposed for a year and is a way of correcting for the various lengths of time the children were in the study before they were vaccinated or after vaccination. For example, one child studied for 18 months and another studied for six months would provide two person years of data altogether.
The incidence of narcolepsy among the vaccinated children was 9.0 cases per 100,000 person years, compared with 0.7 cases per 100,000 person years in unvaccinated children, which means those in the former group were 12.7 times more likely to be diagnosed with narcolepsy than those in the latter (95% CI 6.1 to 30.8).
The researchers calculated that the “vaccine attributable risk” of developing narcolepsy within about eight months after vaccination was 1 in 16,000. (95% CI 1:13,000 to 1:21,000). An “attributable risk” is the difference in the incidence of a condition between an “exposed” and an “unexposed” group – or in this case, the difference in incidence between children who had been vaccinated and those who had not.
Were there any limitations to the studies?
Although these findings are of concern, it is too early to conclude that there is a clear association between Pandemrix and a higher risk of narcolepsy. For example, the authors of the first study say:
- It is possible that the increase in incidence of narcolepsy in 2010 could be explained by an increased awareness of the disease, triggered by “intensive public discussion” about the possible association of narcolepsy with the Pandemrix vaccine. However, they say that this took place in August but that most patients had reported symptoms before this high-profile discussion.
- The diagnosis of onset of the disease and symptoms is based on pieces of information from a number of groups including parents, patients, school nurses and GPs, rather than from a single expert source, which they say could lead to error.
- They also say that it is possible that infectious agents such as H1N1 itself, seasonal flu and various viruses could have contributed to the increase in narcolepsy rates for 2010.
The second study is based on confirmed diagnoses of the disorder, but the authors say:
- It is possible that parents may have been “tempted” to recall the onset of symptoms as occurring after their child received the vaccine rather than before, although the authors tried to take this into account.
- The study does not take into account other factors that may have influenced whether or not a child developed narcolepsy, such as the presence of infection or environmental, social or psychological factors.
What should I do now?
The H1N1 flu pandemic is officially over, however the H1N1 flu virus is thought to be one of the main flu viruses in circulation and has been included in the 2011-2012 seasonal flu vaccine. In the UK this is currently recommended for people in high risk groups, such as pregnant women and those who have lung disease or a poor immune system. It is not at present recommended for all children.
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