[Withdrawn] Pandemic flu vaccine and narcolepsy
Published 4 April 2023
Applies to England
© Crown copyright 2023
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In 2009, a new H1N1 strain of flu emerged in Mexico. Because the human population had very low levels of immunity against the new strain, this was able to cause a global pandemic. Between 2009 and 2011, in response to the fast spread and the large number of serious cases of flu in the UK, 2 pandemic influenza vaccines were used.
One of those vaccines, called Pandemrix, was associated with a very rare risk of a condition called narcolepsy. The cases were mainly reported in teenagers and young adults. Around one case was reported for every 170,000 doses of the vaccine given to adults (aged 18 to 59 years) in the UK, with no cases in individuals aged 60 years or over.
Narcolepsy
Narcolepsy is a rare condition that causes excessive sleepiness. It can be very debilitating, with people falling asleep in the middle of normal activities; it is often made worse by stress. It is caused by an abnormality in the brain cells that produce a hormone called hypocretin.
Hypocretin acts as a messenger in the brain to help regulate sleep and appetite. When the brain cells cannot produce enough hypocretin it results in the symptoms of narcolepsy.
The condition only seems to occur in people with a particular genetic profile. Around 16% of the UK population have this genetic make-up but the vast majority of these people will never develop narcolepsy. So, it is clear that another trigger is required before the condition can develop, although the full range of triggers is unknown.
Pandemrix
Pandemrix was a bespoke influenza vaccine produced in response to the emergence of the new H1N1 (2019) strain of flu. Unlike the existing flu vaccines, which cover 3 or 4 strains of flu, this product only contained the pandemic H1N1 strain.
In order to rapidly increase the level of immunity in those people who were vulnerable to flu, the vaccine also included an adjuvant, called AS03.
Adjuvants
An adjuvant is an ingredient added to vaccines to make them work more effectively. This may be particularly important for certain individuals, such as young babies or older people.
It also means that the vaccine will work even with a lower amount of antigen (the part that stimulates the immune response). This allows the manufacturer to make more vaccine in response to a pandemic and means that we can protect more people more quickly.
Adjuvants used in vaccines
The most common vaccine adjuvants are aluminium salts. These have been used safely in many of our routine infant and adult vaccines for more than 70 years. The salts slow down the take-up of the vaccine antigen, which helps to make sure the vaccine is better tolerated and that it provides good, long-term protection.
Newer adjuvants have now been developed which target different aspects of the immune system, to help produce a stronger and broader immune response. Some of these are particularly important for older people, as their immune systems may respond less well and more slowly.
Many of these adjuvants are based on lipids (oils or fats). Adjuvants based on lipids are now part of the influenza and shingles vaccines which have been given to tens of millions of older people.
AS03 is an adjuvant system that was developed specifically for pandemic vaccines. It contains squalene (made using fish oil) and vitamin E, combined with a compound commonly used in vaccines (polysorbate) that helps to make an oil-in-water emulsion. AS03 has now been used in vaccines given to over 90 million people. This adjuvant is also part of VidPrevtyn Beta, one of the coronavirus (COVID-19) vaccines being offered to older people this spring.
The cause of the narcolepsy
The precise reason why the Pandemrix vaccine triggered narcolepsy is unclear, although the lead theory is that it is due to the body’s immune response to a component of the H1N1 (2009) influenza antigen used in the vaccine. This is supported by laboratory tests that suggest that when cells of the immune system are stimulated by the H1N1 flu antigen they can also attach to cells that produce hypocretin, reducing the amount of this hormone and resulting in symptoms of narcolepsy.
Laboratory studies of COVID-19 vaccine antigens have not shown this effect and there is also no association between COVID-19 infection and narcolepsy. Other than Pandemrix, no other vaccines have been associated with an increased risk of narcolepsy. So, there is no reason to suspect that any COVID-19 vaccines could cause narcolepsy.
Further information
Narcolepsy
Symptoms of narcolepsy on NHS.UK.
Vaccine ingredients
Why vaccines are safe and important (vaccine ingredients section at the bottom of the page) on NHS.UK.
Vaccine ingredients, Vaccine Knowledge Project.