|Target disease||Pandemic H1N1/09 virus|
|(what is this?)|
The vaccine is one of the H1N1 vaccines approved for use by the European Commission in September 2009, upon the recommendations of the European Medicines Agency (EMEA). The vaccine is only approved for use when an H1N1 influenza pandemic has been officially declared by the World Health Organization (WHO) or European Union (EU). The vaccine was initially developed as a pandemic mock-up vaccine using an H5N1 strain.
In August 2010, The Swedish Medical Products Agency (MPA) and The Finnish National Institute for Health and Welfare (THL) launched investigations regarding the development of narcolepsy as a possible side effect to Pandemrix flu vaccination in children, and found a minimum 6.6-fold increased risk among children and youths, resulting in a minimum of 3.6 additional cases of narcolepsy per 100,000 vaccinated subjects.
Thiomersal (thimerosal) is added as a preservative. Being manufactured in chicken eggs, it contains trace amounts of egg proteins. Additional important non-medicinal ingredients are formaldehyde, sodium deoxycholate, and sucrose.
While other 2009 H1N1 vaccines have been developed, the use of a proprietary immunologic adjuvant is claimed to boost the potency of the body's immune response, meaning that only a quarter of the inactivated virus is needed. 
Professor David Salisbury, Head of Immunisation at the UK Department of Health said the vaccines with adjuvants offer good protection even if the virus changes over time; "One of the advantages with adjuvanted vaccines is their ability to protect against drifted (mutated) strains. It opens the door for a whole new strategy in dealing with flu."
The vaccine is supplied in separate vials, one containing the adjuvant, and the other the inactivated virus, which require mixing before intramuscular injection. Originally it was thought that two doses given 21 days apart would be required for full efficacy. Subsequent testing has allowed the UK programme to consist of just a single dose for most people, with a two-dose schedule for children under the age of 10 years and immunocompromised adults.
As of 3 December 2009, 11.2 million doses of Pandemrix had been delivered to health services in the UK, where the vaccine forms the bulk of the government's mass vaccination programme.
The Marketing Authorisation from the European Medicines Agency expired in August 2015 when GSK Biologicals did not apply for renewal of it citing lack of demand for the vaccine.
According to GlaxoSmithKline's Patient Information Leaflet, the following side effects may occur (sorted by rate of occurrence):
In the summer of 2010, the Swedish Medical Products Agency (MPA) and the Finnish National Institute for Health and Welfare (THL) received reports from Swedish and Finnish healthcare professionals that narcolepsy was a suspected adverse drug reaction to the Pandemrix flu vaccination. The reports concern children 12 to 16 years old, whose symptoms occurred one to two months after vaccination. The symptoms were later confirmed to be compatible with narcolepsy. Consumer reports describing similar symptoms were also received. Both organizations, in consultation with external experts, have assessed the possible relationship between the vaccination and the reported reactions. MPA and THL have been in contact with other EU member states to enquire whether there have been any similar reports in other countries.
Right now, THL is recommending that further Pandemrix vaccinations be discontinued pending further investigation into 15 cases of recently vaccinated children who developed narcolepsy in late 2009 and early 2010. THL later raised this figure to 17; the expected average annual occurrence is 6 cases. In Sweden, MPA has discovered 12 confirmed cases and another 12 suspected cases. Additionally, MPA says it is aware of individual case reports from France, Norway and Germany.
On August 27, 2010, the European Medicines Agency announced that the agency's Committee for Medicinal Products for Human Use would be launching a review of Pandemrix in light of the "limited number of cases" reported in Finland and Sweden, so as to "determine whether there is evidence for a causal association".
In August 2010 the Swedish MPA issued a statement which included the following: "An investigation is ongoing, but any relationship between the vaccination and the reported symptoms can not be concluded." 
In February 2011, THL concluded that there is a clear connection between the Pandemrix vaccination campaign of 2009 and 2010 and the narcolepsy epidemic in Finland. The probability of developing narcolepsy was determined to be nine times higher in those who received the Pandemrix vaccination than those who didn't. A total of 152 cases of narcolepsy have been found in Finland during 2009–2010, and ninety percent of them had received the Pandemrix vaccination. Authorities believe that the number of cases may still increase.
At the end of March 2011, an MPA press release stated: "Results from a Swedish registry based cohort study indicate a 4-fold increased risk of narcolepsy in children and adolescents below the age of 20 vaccinated with Pandemrix, compared to children of the same age that were not vaccinated." The same study found no increased risk in adults who were vaccinated with Pandemrix. While cautioning that the increase in risk for children is still uncertain in magnitude, it recommends they not be vaccinated.
A study by the Stanford University School of Medicine examined the incidence of narcolepsy in relation to upper airway infection and a H1N1 vaccine (not Pandemrix) in Chinese patients. Their principal conclusion was that an increased incidence of narcolepsy was seen following a wave of upper airway infections (such as H1N1 influenza). They found no correlation between vaccination and narcolepsy. According to the authors, "the new finding of an association with infection, and not vaccination, is important as it suggests that limiting vaccination because of a fear of narcolepsy could actually increase overall risk." Since narcolepsy is now believed to be an autoimmune disease, the authors suspect that these upper airway infections trigger an immune response which leads ultimately to narcolepsy in susceptible individuals. Pandemrix contains two adjuvants designed to provoke a stronger immune response; they were not in the vaccine used in China, however.
In 2013, the New Scientist reported that "part of a surface protein on the pandemic virus looks very similar to part of a brain protein that helps keep people awake". However, the original scientific article claiming that HA protein in both the virus and the vaccine could, in some people, trigger an immune reaction against hypocretin, was recently retracted because the data could not be reproduced. However, further investigations indicated that "antibodies to influenza nucleoprotein cross-react with human hypocretin receptor 2".
In 2014, a Finnish group published results that showed Pandemrix contained a higher amounts of structurally altered viral nucleoproteins than Arepanrix, a similar vaccine not associated with narcolepsy.
In 2015 it was reported that the British Department of Health was paying for Sodium oxybate medication for 80 patients who are taking legal action over problems linked to the use of the swine flu vaccine, at a cost to the government of £12,000 per patient per year. Sodium oxybate is not available to patients with narcolepsy through the National Health Service.