Could the dtap vaccine (whooping cough shot) interfere with the results of a CBC test?

Stress response. Any stress will potentially effect a cbc, usually producing a mild rise in the total WBC count, dominated by mature forms. It is less likely if the CBC is drawn at the same time or soon after the shot, & more an hour or so later as the stress response kicks in. The dr ordering the test will take a mild stress effect into account when interpreting the study.