Read: California dems push vaccine mandate

Now, overall, most respondents still think vaccines are important. But a shift has certainly occurred. The combined categories of "extremely important" and "very important" totaled 84 percent this time around — compared to 94 percent in 2001.

Where did the 10 percent go?

• 11 percent called vaccines "somewhat important," compared to 4 percent in 2001;

• 2 percent said they were "very important," a category that had no responses in 2001;

• 2 percent said they were "not at all important," compared to 1 percent in 2001.

What happened?

Part of the answer may be a greater general discussion in the media and elsewhere about the efficacy of vaccinations. People who have heard about disadvantages of vaccines have nearly doubled, Gallup said.

Additionally, the increase in autism awareness is a likely factor, Gallup suggests. The autism "connection" is the part of the discussion that's heated up since the 2001 survey, and Gallup found that this belief that vaccinations can cause autism can be found across all demographic groups.

"Americans are much more likely now than 14 years ago to report having heard about the disadvantages of vaccines, while at the same time they have grown a bit more likely to have heard about the advantages. … The discussion of vaccine disadvantages may be having some effect, as the percentage who say vaccines are extremely important is down slightly," Gallup said. "Additionally, a small segment of the population remains skeptical about the benefits or safety of vaccines — including 9 percent who say vaccines are more harmful than the diseases they are designed to protect, and 6 percent who say certain vaccines can cause autism."