They do this because the immunity from pertussis (whooping cough) vaccines wanes over time. So, giving you a combo vaccine is just a time and cost saving measure to make sure you get your immunity boosted against both diseases at the same time.
Right now, pertussis is a serious problem. Adults or teens can harbor the bacteria without showing symptoms. But if they encounter a baby, they can pass those bacteria to the baby and cause a life-threatening infection. Since babies cannot be immunized against pertussis until the age of two months, the public health community is trying to protect these babies by making sure that everyone around the baby is protected. Also, pregnant women are now being vaccinated in the hope that their antibodies will give the baby some protection until it can be immunized.
Finally, comments from another scientist. Thank you mom.
Researchers have identified a number of genes associated with autism. Studies of people with autism have found irregularities in several regions of the brain. Other studies suggest that people with autism have abnormal levels of serotonin or other neurotransmitters in the brain. These abnormalities indicate that autism usually results from the disruption of normal brain development early in fetal development caused by defects in genes that control brain growth and that regulate how neurons communicate with each other. Were these children vaccinated during fetal development. I don’t think so.