an allergic reaction, but this is rare. Furthermore, the common side effects and risks are mild and temporary when compared to the risks and severe health effects of the annual influenza epidemic. Guillain–Barré syndrome Although Guillain–Barré syndrome had been feared as a complication of vaccination, the CDC states that most studies on modern influenza vaccines have seen no link with Guillain–Barré. Infection with influenza virus itself increases both the risk of death (up to one in ten thousand) and the risk of developing Guillain–Barré syndrome to a far higher level than the highest level of suspected vaccine involvement (approximately ten times higher by 2009 estimates). Although one review gives an incidence of about one case of Guillain–Barré per million vaccinations, a large study in China, covering close to a hundred million doses of vaccine against the 2009 H1N1 "swine" flu found only eleven cases of Guillain–Barré syndrome, (0.1 per million doses) total incidence in persons vaccinated, actually lower than the normal rate of the disease in China, and no other notable side effects. Egg allergy Although most influenza vaccines are produced using egg-based techniques, influenza vaccines are nonetheless still recommended as safe for people with egg allergies, even if severe, as no increased risk of allergic reaction to the egg-based vaccines has been shown for people with egg allergies. Studies examining the safety of influenza vaccines in people with severe egg allergies found that anaphylaxis was very rare, occurring in 1.3 cases per million doses given. Monitoring for symptoms from vaccination is recommended in those with more severe symptoms. A study of nearly 800 children with egg allergy, including over 250 with previous anaphylactic reactions, had zero systemic allergic reactions when given the live attenuated flu vaccine. Vaccines produced using other technologies, notably recombinant vaccines and those based on cell culture rather than egg protein, started to become available from 2012 in the US, and later in Europe and Australia. Other Several studies have identified an increased incidence of narcolepsy among recipients of the pandemic H1N1 influenza AS03-adjuvanted vaccine; efforts to identify a mechanism for this suggest that narcolepsy is autoimmune, and that the AS03-adjuvanted H1N1 vaccine may mimic hypocretin, serving as a trigger. Some injection-based flu vaccines intended for adults in the United States contain thiomersal (also known as thimerosal), a mercury-based preservative. Despite some controversy in the media, the World Health Organization's Global Advisory Committee on Vaccine Safety