The two-dose mRNA COVID-19 vaccine may be less effective in people undergoing treatment for certain types of blood cancers, researchers have claimed in two new studies.

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The studies, published in the journal Blood, suggest that the mRNA COVID-19 vaccine may have reduced efficacy in people with chronic lymphocytic leukaemia (CLL) and multiple myeloma, two types of blood cancer compared to healthy people. However, vaccination against COVID-19 is strongly recommended in both groups, the researchers said.

"Even though response rates were not optimal, patients with CLL should still get the vaccine and, if appropriate, it may be better to do so before CLL treatment starts although the disease itself may affect the response", said lead author Yair Herishanu, Associate Professor in haematology and head of the CLL service at the Tel Aviv Sourasky Medical Centre in Israel. In the first study of 167 patients with CLL and 53 healthy adults, all participants received two doses of BNT162b2 messenger RNA (Pfizer) COVID vaccine. The immune response among patients with CLL was found dependent on their cancer treatment process: those undergoing active cancer treatment had significantly lower response rates (16 per cent) to the vaccine.

Treatment naive patients (those whose disease is being watched but not yet treated) had a 55.5 per cent response rate. Response to the vaccine was markedly higher in people who completed CLL treatment at least a year before vaccination (94 per cent). "Overall, the response rate to the vaccine was significantly less than what we see in the general population, which is most likely attributed to the presence of cancer itself and certain CLL treatments", said Herishanu. In addition, patients with CLL also had lower antibody titers, which means the intensity of the response was also lower, explained Herishanu. Similar results were reported in the case of patients with multiple myeloma. After the first dose of the same vaccine in elderly patients with multiple myeloma, only 20.6 per cent showed neutralising antibodies.