This morning, Governor Jerry Brown signed a bill into law that will ban the use of religious and personal belief exemptions---ways that parents can opt out of certain vaccines for their kids before attending public or private school. California now joins only two other states, Mississippi and West Virginia, that have no exemptions in place (though students can still opt out if a doctor says they shouldn't get vaccinated for medical reasons). The hotly-contested law was passed in the hopes that increased vaccination rates will help stave off the spread of vaccine-preventable diseases like the measles, which broke out earlier this year in a cluster that started in Anaheim's Disneyland.
Some people will argue that the exemption ban won't have a noticeable effect on vaccine coverage and disease outbreaks in California. After all, the state already has decent vaccination rates in its kindergarteners---they hover around 92 percent, above the threshold that the CDC says is necessary to support herd immunity. And only 2.5 percent of kids in the state have personal belief exemptions on file, so eliminating the exemption won't increase vaccination rates that much.
Here's the thing, though: Whether or not the law has a significant effect on the health of California's kids, this is a prime opportunity to carefully study the effects of legislation like this on both vaccination and disease rates. Health officials would love to know for sure that SB277 will have a meaningful impact on public health. But they can't. It's notoriously hard to draw connections between statewide vaccine laws and disease numbers---we tried. It's really hard to prove a causal connection between historical lawmaking and vaccination numbers, because all the analysis has to be done retroactively. And certain diseases, like pertussis, can crop up in vaccinated populations as well, making it more difficult to draw a straight line between regulation and infection, too.
Just take a look at the vaccination rates in other states, compared to where they stand on the two types of exemptions: religious and personal belief. The top-vaccinated state in the nation, Mississippi, is one of the only two states that doesn't allow either exemption. Hey! Look at that! A correlation! But traveling down the list, the connection between exemption status and vaccination rate gets a little murkier. West Virginia, the other no-exemption state, doesn't appear in the list of the top-ten vaccination rates (it's number 18), and three states that allow both types of exemptions appear in the top-ten list.
Now let's look at the bottom of the pile, at states that are below or dangerously close to falling below the vaccination threshold necessary for herd immunity. Seven of those states have both exemptions in place, with Colorado falling at the bottom of the list. But again, correlation doesn't imply causation. It's impossible to say, based simply on a state's exemption status and its vaccination rates, whether or not those exemptions made a difference.
If you look broadly at these figures, though, it does seem like states with more exemptions are more likely to have low vaccination rates. Now is the time to test that hypothesis. California is at a historical inflection point. If public health researchers and politicians can look carefully at the state of the state's vaccination rates and disease numbers before and after SB277 is enacted, they'll get a powerful tool to either support more bans of these exemptions---several of which are on the table in other states right now---or drive the United States toward different, perhaps more effective strategies to reduce vaccine-preventable disease. Let's see what comes out of the lab.
