A Case of Sudden Very Late Onset of Severe Clozapine-induced Agranulocytosis Following an Influenza Vaccine.

Agranulocytosis can be a life-threatening condition because of its high risk of serious infection. It is extremely rare, with an incidence of one to five cases per million in the population per year. About 70% of the cases are associated with medications. Clozapine-induced leukopenia is a well-known clinical entity, justifying regular hematologic surveillance. Most cases of clozapine-induced neutropenia and agranulocytosis occur during the first three months of treatment. It's extremely rare after the first year of treatment. However, we present the case of a late-onset of sudden severe agranulocytosis following an influenza vaccine, after more than 156 months of stable neutrophil counts on clozapine. Clinicians must keep in mind that this complication can occur at any time during the treatment course and may wish to increase the frequency of hematologic surveillance following an influenza vaccine or even consider a risk-benefit approach. Considering the importance of the influenza vaccination and the seriousness of agranulocytosis, further studies are needed to elucidate a potential increase in the risk of severe neutropenia in patients on clozapine receiving the influenza vaccine.