“It’s a really savvy co-option of reproductive rights and the movement’s framing of the issue,” said Lisa Ikemoto, a law professor at the University of California-Davis Feminist Research Institute. “It strengthens the meaning of choice in the anti-vaccine space and detracts from the meaning of that word in the reproductive rights space.”

Framing the decision to vaccinate as a singularly personal one also obscures its public health consequences, Ikemoto said, because vaccines are used to protect not just one person but a community of people by stopping the spread of a disease to those who can’t protect themselves.

Celinda Lake, a Democratic strategist and pollster based in Washington, D.C., said “My Body, My Choice” is no longer polling well with Democrats because they associate it with anti-vaccination sentiment.

“What’s really unique about this is that you don’t usually see one side’s base adopting the message of the other side’s base — and succeeding,” she said. “That’s what makes this so fascinating.”