Experts say a 70% to 90% vaccination rate is needed for herd immunity, especially as more infectious COVID-19 variants take hold.

Yet shots-in-arms are stalling or falling off in many states, including Arizona.

Now, a new CDC survey looks at the groups least likely to move the needle.

Only 34% of adults ages 18–39 have received a COVID-19 vaccine. CDC surveys show that's unlikely to change — especially among people ages 18–24, with one-quarter saying they were unsure and another quarter saying they would not get vaccinated.

Other notably reluctant groups include Black adults, people living outside of metro areas, and those with less education, no insurance or lower household incomes.

Respondents commonly cited concerns about vaccine safety and effectiveness.