Zika virus is known to increase the risk of birth defects, including microcephaly, if an expectant mother is infected. With more than 5,600 cases reported in the United States as of February this year, concerns are heightened, even if the great majority are from travelers returning from affected areas. In U.S. territories alone, 37,175 additional cases have been recorded, mostly from Puerto Rico.

Based on those statistics, one might assume that people in the U.S. would applaud the development of a new vaccine and be ready to line up to get it.

That does not appear to be the case, however, and a new study explains why.

The report in the Journal of Public Health points out that the willingness to use a Zika vaccine once it is available is likely to hinge on how people weigh the risks associated with the disease and the vaccine but also on their misconceptions about other vaccines.

A study team led by researchers from the Annenberg Public Policy Center (APPC) of the University of Pennsylvania drilled down into factors that will affect the eventual acceptance or rejection of the Zika vaccine.

Among the strong determinants were erroneous beliefs about an association between the measles, mumps, and rubella (MMR) vaccine and autism—an allegation that, although it has repeatedly been disproven, frequently pops up in social media. Those who held that belief said they were less likely to get the Zika vaccine.

Other influences were their perceptions of the severity of the Zika virus and general belief in the power of science to solve problems, the study authors write.

“When a new disease arises, people who lack understanding of the new threat may extrapolate from their knowledge of other diseases,” explained Yotam Ophir, a PhD candidate at Penn’s Annenberg School for Communication who coauthored the study with APPC Director Kathleen Hall Jamieson, PhD.  “We found that the misbelief about the MMR vaccine’s association with autism was more influential on the decision of whether to get vaccinated for Zika than even perceptions of Zika itself, which is worrisome, especially in light of the persistence of that misinformation.”

To reach that conclusion, the researchers analyzed 2016 data from APPC’s Annenberg Science Knowledge survey conducted during the outbreak of the Zika virus and including responses from 3,337 individuals between August 25, 2016 and September 26, 2016. The results they focused on were only part of the more extensive, 34-week survey of U.S. adults on attitudes, behavior, and understanding of the Zika virus.

Getting the Zika vaccine was less likely the more respondents bought into the false association between autism and the MMR but more likely for those with faith in science to overcome problems, according to the responses.

Inaccurate information cut both ways, however. Those who believe that Zika causes the birth defect microcephaly—which is accurate—were more likely to intend to get the vaccination, but so were those who believed Zika is likely to cause death, which is inaccurate.

A surprising result was that respondents who took more actions to protect against Zika also said they were less likely to get the vaccination when it becomes available. Researchers suggest that might “be the result of their confidence that their actions pre-empt the need to be vaccinated.”

“Scientists often look at the effect of misinformed beliefs about the MMR vaccine on people’s intention to vaccinate children with the triple vaccine, but they don’t as often look at the dangerous spillover effects that these misbeliefs can have,” Ophir said.

A year ago, the National Institutes of Health announced the launch of the second-phase clinical trial of a DNA-based vaccine for Zika.