Impact of mRNA COVID-19 vaccination on hematological parameters in patients maintained on clozapine: A retrospective study from Qatar

Abstract

Background

COVID-19 vaccines are known to cause transient changes in white blood cell counts as part of the immune activation process. Clozapine, an antipsychotic agent primarily prescribed for treatment-resistant schizophrenia, possesses both immunosuppressive and pro-inflammatory properties that may influence vaccine-related immune responses. The concurrent use of clozapine during COVID-19 vaccination has therefore raised concerns regarding potential hematological adverse effects. Despite increasing global research in this area, data from Arab populations remain scarce, highlighting the need for region-specific evidence. This study aimed to investigate the incidence of white blood cell and absolute neutrophil count abnormalities in patients receiving clozapine who were vaccinated with mRNA COVID-19 vaccines.

Method

A retrospective study was conducted within Qatar's public mental health services. The study included patients on clozapine who received at least one dose of a COVID-19 vaccine between February 2020 and November 2022. Hematological parameters were assessed at three time points: pre-vaccination, shortly post-vaccination, and three months post-vaccination. Demographic, clinical, and vaccine-related factors were also examined.

Results

Of 111 vaccinated patients, 74 had complete blood test data across the three time points. No cases of agranulocytosis or other serious hematological adverse effects were observed. Mild leukopenia occurred in 6.8–8.1 % of patients, and mild neutropenia in 5.4–6.8 %. These changes were transient and not associated with clozapine dosage, vaccine type, or other clinical variables.

Conclusion

COVID-19 mRNA vaccines appear safe for individuals maintained on clozapine, with only minor, temporary changes in white blood cell counts. These findings support continued vaccination efforts in this population.