COVID Shot Shows Less Efficacy in Few Blood Cancer Patients by Angela Mohan on April 19, 2021 at 12:10 PM Blood. The findings suggest that the mRNA COVID-19 vaccine may have reduced efficacy in people with chronic lymphocytic leukemia (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 trea...