A swine flu vaccine has been linked to higher rates of developing narcolepsy among children and adolescents in England who received the injection, says a new study.

The children in the study received the swine flu vaccine between 2009 and 2010. The vaccine, Pandemrix, was administered to approximately 30 million people in Europe by was not used in the United States. According to researchers, the vaccine has not been given to young people since July 2011.

The research findings, published in the British Medical Journal this week, are similar to those of previous sleep disorder studies conducted in Finland and Sweden. Those studies found a link between narcolepsy in children and the 2009 Pandemrix swine flu vaccine, and suggested that the association might be generalized to other countries.

The pandemic influenza A (H1N1) virus spread very quickly in 2009, and by March 2010 about 24% of healthy children in England under 5 and 37% below the age of 15 received the swine flu vaccine.

The authors of the study stress that the risk may be overestimated, since they only show an association between narcolepsy and the swine flu vaccine, and cannot prove that Pandemrix causes narcolepsy. Also, since so many people were given Pandemrix and narcolepsy is so rare to begin with, the chances that a child will develop narcolepsy after vaccination are very small, at about 1 in 55,000.

Narcolepsy is a nervous system sleep disorder that causes extreme sleepiness during the daytime and frequent "sleep attacks" caused by temporary loss of muscle tone that causes falling down, says PubMed. It can also include hallucinations and sleep paralysis while falling asleep or waking.

The exact cause of narcolepsy is unknown, but it tends to run in families and typically occurs first between ages 15 and 30.

Based on the Scandinavian research on the connection between Pandemrix and narcolepsy, British researchers began examining the swine flu vaccine's effects on sleep disorders in England in early 2011.

A research team led by Dr. Elizabeth Miller, an epidemiologist at the United Kingdom's Health Protection Agency, analyzed information on 245 children and teens who visited sleep centers in England. Seventy-five of the children had developed narcolepsy after January 2008, and 11 of them had been vaccinated with Pandemrix before their symptoms started.  Seven had received the swine flu vaccine within 6 months of their first symptoms.

After analyzing the data, the researchers found that children with narcolepsy were 14 times more likely to have been vaccinated with Pandemrix than other British children of the same age. The British results are similar to those of a 2012 study from Finland, which found a 13-fold increase in the risk of narcolepsy after children aged 4-19 received the swine flu vaccine.

Narcolepsy is often hard to identify, and a person with symptoms may go for years without getting a doctor's diagnosis. It's possible that British parents were more concerned about signs of sleep disorders in their children after hearing news of the Scandinavian research on the swine flu vaccine, which made them more likely to seek treatment. Researchers said that in that case, the risk of narcolepsy after swine flu vaccination in England may be overestimated.

The researchers concluded:

"The increased risk of narcolepsy after vaccination with ASO3 adjuvanted pandemic A/H1N1 2009 vaccine indicates a causal association, consistent with findings from Finland. Because of variable delay in diagnosis, however, the risk might be overestimated by more rapid referral of vaccinated children."

Dr. Andy Pavia, chief of the University of Utah's Division of Pediatric Infectious Diseases, who was not involved in the study, told MyHealthNewsDaily  that "the evidence is adding up to suggest that there is a small increased risk of narcolepsy," after swine flu vaccination. Pavia said that earlier research shows that narcolepsy risk is higher among people with a particular genetic marker, so it is possible that some ingredient in Pandemrix makes people with that genetic risk marker more likely to develop narcolepsy.

Ingredients called adjuvants are usually added to vaccines to boost their effects, and the Pandemrix swine flu vaccine contains a particular adjuvant called ASO3. Other vaccines contain ASO3, but none of them have yet been linked to narcolepsy. At this point, Pandemrix, which mixes ASO3 with the 2009 pandemix H1N1 vaccine, is the only one that has been linked to the sleep disorder.

Dr. Paul Offit, chief of the Division of Infectious Diseases at the Children's Hospital of Philadelphia, told MyHealthNewsDaily that a link the swine flu vaccine and narcolepsy exists only if three factors converge: the vaccine contains the 2009 pandemic H1N1 flu strain; the vaccine contains ASO3, and the child being vaccinated has a genetic risk marker for narcolepsy.

Because these three factors coming together is so rare, Offit believes that it's very unlikely for someone to develop narcolepsy from the swine flu vaccine.

"The risk of being hospitalized or killed by [flu] is greater than the risk of narcolepsy," he said.

British officials agree that the risk of the pandemic swine flu was greater than the risk for developing sleep disorders like narcolepsy.

"Pandemrix may have triggered an immune reaction against the sleep centre cells in those children who were genetically predisposed to develop narcolepsy," a Department of Health spokesman told the Daily Mail. "Pandemrix was developed specifically for use in a flu pandemic when the number of lives lost and serious cases could have been enormous.

"The decision to recommend that children got this vaccine during the flu pandemic was based on evidence available at the time, along with the advice from the European Medicines Agency which approved its use.

"We keep all emerging evidence under review and that's why use of Pandemrix in those less than 20 years old was stopped in the UK in 2011."