Incidence
Anaphylaxis following immunisation
is a rare event. Even the
largest pre‐licensure vaccine trials are unlikely to detect a single case, let alone provide an estimate of incidence. The onus for detection of anaphylaxis falls to national post‐marketing surveillance systems. The “yellow card” reporting system of the
Medicines and Healthcare products Regulatory Agency in the UK (
www.mhra.gov.uk) received 130 reports of anaphylaxis associated with immunisation in the six years from 1997 to 2003,
suggesting a rate of 1 per million doses.5 Likewise, the US Vaccine Adverse Event Reporting System (
http://vaers.hhs.gov) recorded 452 reports of “anaphylactoid reactions” in over
1.9 billion doses of vaccine administered countrywide over a 10‐year period.6 This yields an estimated incidence of
0.2 cases per million doses. All post‐marketing surveillance systems rely on passive reporting of cases and are prone to under‐reporting. Also these incidences are of overall rates of reaction and do not reflect incidences following individual vaccines.
There are a limited number of studies specifically addressing the incidence of anaphylaxis as an AEFI. Patja et al describe 30 cases of anaphylaxis occurring after MMR vaccination over a 14‐year period, deriving an incidence estimate of 1 per 100 000.7 In a retrospective
analysis of hospital discharge records, Bohlke et al identified five cases of anaphylaxis in 7.5 million doses of vaccine, giving an incidence rate of 0.65 cases per million doses.8 Yet in two of these five “cases”, uncertainties remained about the true nature of these events. The retrospective design of this study made it impossible to clarify these further. As with most advanced immunisation programmes, children received combination vaccines with multiple immunisations at a single clinic visit, making it impossible to attribute risk to a single vaccine or component. These studies exemplify the difficulty of describing anaphylaxis as an AEFI in any detail using retrospective analyses.