Mounting evidence of a link between GlaxoSmithKline's Pandemrix flu shot and a spike in narcolepsy cases among children in Europe is putting one of the vaccine's key ingredients, AS03, under intense scrutiny.
The ingredient is one of a class of pharmaceuticals known as adjuvants, or boosters, designed to turbo charge the potency of a vaccine and the body's immune response to it.
AS03 was widely used in Europe during the 2009-2010 H1N1 flu pandemic and is also contained in a GSK adjuvanted flu vaccine which in November last year became the first of its kind to be recommended for approval by the normally adjuvant-wary United States Food and Drug Administration (FDA).
There's little doubt AS03 does its job well.
Pandemrix, the flu vaccine under suspicion for the narcolepsy link, needs only a fraction of the antigen - the part that actually does the immunization - of other flu shots to give sufficient protection.
This means the manufacturer - in this case GSK - can produce multiple times the number of vaccine doses without needing to spend valuable time making large amounts of the antigen.
In a pandemic situation in which the lives of millions around the world are under threat, that could make the difference between having enough vaccines to protect entire populations, or having to decide how to ration them out.
Yet some experts believe the turbo-charged immune response AS03 generates, or the ingredients that drive it, may also be the answer to why almost 800 cases of the incurable sleeping disorder narcolepsy have been linked to Pandemrix's use in Europe during the 2009/2010 H1N1 pandemic.
"The adjuvant in the Pandemrix vaccine is very potent and we think it may have played a role," says Markku Partinen, a neurologist at the Helsinki Sleep Clinic in Finland who has spent the past few years investigating what might be behind the link between the vaccine and narcolepsy.
If it is the reason for the spike in the disease, Partinen and other researchers think it may be because the adjuvant's strength boosted not just a good immune response but possibly an adverse one too.
Partinen was one of the first scientists to pick up a signal that something might be going awry in children in Finland vaccinated with Pandemrix in a nationwide immunization campaign against the H1N1 flu.
Since then studies in Sweden, Finland, Ireland and now Britain have found the risk of developing narcolepsy is between seven and 13 times higher in children who were immunized with Pandemrix than in those who were not.