Clinical & Experimental Allergy

Swine flu, a new subtype of influenza A virus H1N1, not previously detected in pigs or humans has arrived in the United Kingdom in recent months with hundreds and thousands of cases already confirmed in both adults and children. The World Health Organisation (WHO) has raised the threat level to Phase 6 (pandemic) to reflect the ongoing community outbreaks of swine flu throughout the world. So far in the United Kingdom, most cases have been mild but clustering of cases shows infectivity and with an anticipated increase in cases towards the autumn, a potential winter crisis looms. International data show the age profile of affected subjects is predominantly in children especially young children and young adults up to age 30 years. The WHO has reported that fatalities have occurred in pregnant women, those with significant comorbidities and also in previously healthy young individuals. The Department of Health (DoH) is planning a mass vaccination programme as soon as the pharmaceutical industry produces sufficient quantities of an effective vaccine. Swine flu vaccine is likely to be manufactured in a similar manner to the currently available influenza vaccines by inoculating embryonated chicken eggs with the virus. The vaccine will then be extracted from the extraembryonic fluid by chemically inactivating the virus that is then further treated and purified. Consequently influenza vaccines contain measurable quantities of egg protein [1] with both fatal and non-fatal anaphylaxis reported following administration [2, 3]. Therefore, there is a potential risk of allergic reaction for the 1‐2% of children who have egg allergy [4] and a significant number of adults who have either persisting or adult-onset egg allergy. Each year the European Pharmacopoeia publishes guidance on the maximum allowable ovalbumin content of influenza vaccines. Up to the end of 2005, conventional split and subunit influenza vaccines were allowed to contain a maximum of 1mg ovalbumin per dose, whereas virosomal influenza vaccines were permittedo50ng/dose. In an apparent retrograde step, however, from 2006 all influenza vaccines were permitted a maximum ovalbumin content of up to 1mg/dose [5, 6]. Therefore, virosomal vaccines can no longer be relied upon to contain safe quantities of egg protein for the egg-allergic population. Furthermore, there is considerable variation in egg protein content between different brands of influenza vaccine and even between batches of the same vaccine [7]. Therefore, recommendations based on vaccine type that are applicable to the total egg-allergic population are not possible.