So, which of these is true?
I'll start by pointing out that the subset of people who said they were "not sure" about the vaccine-autism issue could probably also fall into the subset of people who said they thought vaccines were safe in the other survey --- people's positions on issues can be pretty elastic, depending on how questions are asked and what kind of answers they think the pollsters want to hear. I think people who fall somewhere in the middle might have even more wiggle room than those who come down more strongly on one side or another.
The undecided group in the Harris/HealthDay poll, at 30% of respondents, was quite a bit bigger than the group professing a belief that vaccines cause autism (18%).
Thus, the proportion of all people who don't actively believe that vaccines cause autism in that survey is 82%, similar to the 79% of people who said they considered vaccines to be safe in the survey from two years ago.
Also, the two surveys sampled slightly different subsets of the U.S. population: the Harris/HealthDay poll was open to anyone over 18 (n = 2,026), while the two Pediatrics articles restricted their data set to respondents with minor children (n = 1,552). One of the articles went further, using only data from parents of children younger than 6 (n = 475).
Compared with the general population of the U.S. (reflected in 2010 census data), the group of people studied in the two Pediatrics articles is younger (the all-parents group was split about 60/40 between people under 40 and people 40+; two-thirds of the parents of young children were under 35; in the U.S. as a whole, a little less than half of all people, and about 60% of all adults, are older than 40), a lot richer and more likely to be female (split about 60/40 in favor of women).
What most interested me was the implication, in the article on parents of young children, that some of the people who think vaccines are generally safe, and believed vaccination was important for a child's health, are also worried that vaccines might cause autism:
Parents reported their attitudes regarding the safety and necessity of vaccines (Tables 2 and 3). The majority of the parents were either confident or very confident in vaccine safety (79.0%) and believed that vaccines are important to children's health (79.8%). Similarly, 73% of parents somewhat or strongly believed that the benefits of vaccines outweighed the risks. ...
Parents were asked to respond to a series of 11 potential vaccine-related concerns and were also given the opportunity of stating that they had no vaccine-related concerns (Table 4). Just more than one-fifth (20.8%) of the parents reported that none of the 11 issues listed were of concern to them. The most common concern reported by the parents was that it is painful for children to receive multiple shots during 1 doctor's visit (44.2%). Other concerns reported by >25% of the parents included their child receiving too many vaccines in 1 doctor's visit (34.2%), vaccines causing fevers in their child (28.3%), children getting too many vaccines in their first 2 years of life (27.8%), and vaccines causing learning disabilities such as autism (26.2%).
The article's authors suggest that these parents are probably weighing the potential risk of autism against the protection from disease, and choosing to vaccinate despite their misgivings because they think the benefits outweigh the risks. (Data from the Harris/HealthDay poll would also tend to support this idea of a cost-benefit analysis; even among people who believe that vaccines can cause autism, a majority thinks schools should require children to be vaccinated).
The other article, the one that studied all parents of minor children, also looked into what sources parents considered credible in the matter of vaccine safety.
Overwhelmingly, the person they trusted most was their children's doctor (76% said they trusted their children's doctor "a lot," 22% said "some" and only 2% said they did not trust their children's doctor); just as overwhelmingly, people put very little stock in what celebrities have to say about vaccine safety (in a clean reversal of the figures for doctors, 2% of parents said they trusted celebrities "a lot", 24% said they trusted them "some", and 74% did not trust them at all). For all the other types of people --- health-care providers apart from their children's doctor, government officials, friends and family, and parents who believe their children were harmed by a vaccine --- most people said they felt "some" trust. Similarly, of the people who look at media relating to vaccine safety, most placed "some" trust in all of the media categories listed: medical organizations' websites (the only category that also had a large number of people saying they trusted it "a lot"), government websites, newspaper or magazine articles, TV, vaccine manufacturers' websites and anti-vaccine websites.
These survey results suggest to me that people aren't the gullible sheep skeptics often accuse them of being. They are wary of anecdotal evidence, give more weight to experts' opinions than to laypeople's, and give the most weight to those authorities they consider neutral, or to have their child's best interests specifically in mind. Also, even without an extensive grounding in virology, immunology or epidemiology, they seem to grasp the importance of universal vaccination in maintaining herd immunity.
People are wiser than the conventional wisdom gives them credit for being.
Freed, G., Clark, S., Butchart, A., Singer, D., & Davis, M. (2011). Sources and Perceived Credibility of Vaccine-Safety Information for Parents PEDIATRICS DOI: 10.1542/peds.2010-1722P
Kennedy, A., Basket, M., & Sheedy, K. (2011). Vaccine Attitudes, Concerns, and Information Sources Reported by Parents of Young Children: Results From the 2009 HealthStyles Survey PEDIATRICS DOI: 10.1542/peds.2010-1722N
*Poll conducted by Harris Interactive and HealthDay, on January 11-13, 2011
**The annual HealthStyles survey, conducted by the CDC and the public-relations firm Porter Novelli, throughout the month of January in 2009