A study claims that rates of narcolepsy, a condition that causes sufferers to fall asleep without warning, peak five to seven months after the highest levels of H1N1 infections and colds.
The finding goes against recent concerns that a jab widely used in epidemics, Pandemrix, was to blame for children in Finland developing the sleeping disorder.
Scientists at Stanford medicine school in California say their study suggests that restrictions on the vaccine, intended to protect people from narcolepsy, could actually lead to higher rates of infection.
Emmanuel Mignot, an expert on narcolepsy, writes in a new paper: “Together with recent findings, these results strongly suggest that winter airway infections such as influenza A (including H1N1), and/or Streptococcus pyogenes are triggers for narcolepsy.
“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.”
After the global outbreak of swine flu – the H1N1 strain of the virus – in 2009, some 30million people across Europe were given the vaccine Pandemrix including 6m in Britain.
But its safety was questioned by health officials in Finland, and in total 335 cases of narcolepsy have now been reported in people vaccinated with Pandemrix.
In July the European Medicine Agency said that as a precautionary measure, the vaccine should only be given to the under-20s if they are at risk of contracting swine flu and alternative jabs are not available.
Its analysis found that for every 100,000 adolescents who are given the injection, up to seven are likely to develop narcolepsy.
However Britain’s drug watchdog said the recommendations were not binding and that Pandemrix would not be restricted in this country.
Now further doubt has been cast on the link between the vaccine and narcolepsy, a condition suffered by 3m people worldwide that is characterised by daytime drowsiness and a sudden loss of muscle strength.