Eyeballing the census numbers on page 19: https://www.census.gov/content/dam/Census/library/working-papers/2002/demo/POP-twps0056.pdf
It looks like all population growth took a hit starting in the 1960s. This was the "woman's liberation" movement and widespread adoption of hormonal birth control. This is also around when the released the live-attenuated polio vaccine, the first MMR vaccines, and started pushing the "preventative" vaccines for things like pneumonia and influenza.
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