IMMEDIATE RECTIONS
Immediate hypersensitivity reactions to vaccines are rare, with a frequency that vary between 1 per 50,000 –1,000,000 doses7,8. They typically occur between a few minutes and up to 4 hours after vaccination, and urticaria is the most frequent manifestation occurring four times more frequently than anaphylaxis9. Other skin reactions include erythema, isolated pruritus and angioedema especially involving the face and lips. Respiratory symptoms, such as rhinoconjunctivitis, sensation of throat closure, dyspnoea and wheezing are less commonly reported10.
Anaphylaxis is defined, according to EAACI11 as a life-threatening reaction characterized by acute onset of symptoms involving different organ systems and requiring immediate medical intervention and, when suspected to be vaccine-related, has to be evaluated according to the Brighton Collaboration Working Group Criteria recently reviewed with emphasis on objective symptoms and signs12. They define anaphylaxis as the involvement of at least two organs and provide a combination of major and minor criteria for classifying increasing levels of diagnostic certainty differing from Sampson et al. anaphylaxis clinical criteria commonly used in clinical settings.
Overall, being characterised by a broad range of possible symptoms, a number of immediate adverse events following immunization could be misdiagnosed as anaphylaxis and differential diagnosis and alternative potential triggers has always to be considered whenever an episode appears to coincide with vaccine administration13, see Tab 1.
Since post-vaccination anaphylaxis is very rare, usually it starts to be reported to passive pharmacovigilance during post-marketing surveillance and data are often influenced by under- and over-reporting, incomplete information and lack of denominators13. Recently, Miller et al. assessed current VAERS sensitivity for anaphylaxis ranging from 13% to 76%14, that highlights the need of a correct diagnostic framework performed by allergists or immunologists expert in vaccine allergy for a correct vaccination management. Being rare, the incidence varies among different studies: in a study population consisted of children and adolescents Bohlke et al.15 reported 5 cases of anaphylaxis after administration of 7,644,049 vaccine doses, for a risk of 0.65 cases/million doses; while, McNeil et al.5 identified 18 cases of anaphylaxis after administration of 12,403,201 vaccine doses to 0-17 age group, for an incidence rate of 1.45 cases per million vaccine doses.
Treatment of anaphylaxis in the setting of vaccine administration is reviewed in Castells et al.16.
Although rare, the precise diagnostic management of a suspected anaphylaxis post-vaccination is of paramount importance due to the risk of a potential serious reactions after re-exposure and, not secondly, because of overdiagnosis of severe allergic reactions to vaccines might lead to an increase in the number of children that interrupt vaccinations, resulting in an individual and collective risk of loss of protection against immune preventable diseases.