Now that we're in the thick of flu season, the topic of vaccines is everywhere — as is the (somehow still) continuing fervor against getting them. For those in that camp, a new study reveals the type of people who are most likely to refuse mandatory vaccines, reports The New York Times. The study, which will be published in an upcoming issue of the American Journal of Public Health, led researchers to look at vaccine data for about 17,000 California kids between 2007 and 2013. Specifically, they wanted to know how many parents opted their kindergarteners out of receiving mandatory vaccines with personal belief (nonmedical) exemptions — and how that number has changed in recent years. Results showed that the number of kids vaccinated varied pretty widely throughout the state. But those areas with the highest percentage of kids exempt from vaccinations — some up to 50% — tended to be predominantly white and higher-income. And, overall, the number of those personal belief exemptions doubled to about 3% between 2007 and 2013. This is of public health concern, since it's recommended that the vast majority of kids get vaccinated in order for herd immunity to take effect, which can protect those who aren't able to be vaccinated. In terms of ideology, another recent study found that people who don't vaccinate tend to fall into four categories: Those who are complacent don't necessarily feel like there's enough of a problem to warrant getting their shots. The unconfident don't feel like vaccines are truly safe. Those who are calculating do plenty of research, but don't feel like their personal risks warrant vaccination (and don't consider herd immunity to be their responsibility). And, finally, there are those who simply don't have access to vaccines. So it wouldn't be a huge surprise if, as the authors suggest, the best way to convince people to get their shots may depend on why they're refusing in the first place.