This will only hurt you a bit. Not getting the shot may hurt someone else. Photographer: Spencer Platt/Getty Images
This will only hurt you a bit. Not getting the shot may hurt someone else. Photographer: Spencer Platt/Getty Images

One of America's essential national defense shields is under attack from within, and the aggressors are people you wouldn't normally consider dangerous: parents of young children. As more and more resist having their school-age kids vaccinated, they are destroying the herd immunity that everyone relies on for protection against whooping cough, measles and other dangerous, sometimes fatal, infectious diseases.

The dangers of vaccines have been thoroughly debunked. A report published this week in the journal Pediatrics, however, shows how hard it is to convince anti-vaccination parents of this. In fact, the harder researchers pushed the benefits of vaccines, the more the parents resisted. Political scientist Brendan Nyhan of Dartmouth College, one of the study's authors, notes that policy measures might be more effective than public communication at improving vaccine rates.

Consider then what Vermont is doing. Its new disclosure rule requires all licensed schools, public and private, and day-care centers to publish the percentage of children who have received each vaccine required for attendance, making clear what percentage have received exemptions. Lawmakers in Colorado, which is in the midst of a whooping cough epidemic, will consider a similar bill this month. All states need such provisions.

For each 1 percent rise in exempt children in a school, the risk of a pertussis outbreak increases by 12 percent, according to a Colorado study. The percentage of parents claiming exemptions continues to grow.

A small percentage of kids (less than 1 percent) can't be vaccinated because they are allergic to vaccines or have compromised immune systems. This is why medical exemptions are necessary. In the past, this group was protected by herd immunity: If about 95 percent of a community is immunized against a pathogen, it can't find sufficient hosts and dies off.

Now, many geographic clusters of vaccine refusers have poked holes in the protective blanket, and are contributing to outbreaks of measles and pertussis. Parents of kids who can't be immunized need to know if they are living in such a hole.

Just being vaccinated doesn't protect people completely, and certainly not if they haven't had the full course of doses. Only herd immunity can eliminate the risk of exposure to a disease. Transmission between schoolchildren isn't the only concern. An exempted child can sicken those who temporarily can't be immunized -- newborns, for example, or pregnant women or anyone who is too ill to get a scheduled vaccine.

Vermont's disclosure law hasn't worked perfectly. School vaccination rates, published for the first time in January, are hard to find on the state Department of Health website. Ideally, schools should post and publicize this information. Such problems are easily fixed.

The newly revealed data should galvanize support for state measures making it harder for parents to get exemptions from vaccines on philosophical grounds and be used to block the far greater number of proposals (31 versus five, in a study of bills introduced from 2009 to 2012) to make it easier.

In the 20 states that allow philosophical exemptions, nine merely require a parent or guardian to sign a form. Washington was one of those states, but since 2011 it has forced parents to obtain the signature of a licensed health-care provider who has given them information about immunization risks and benefits. Since then, the percentage of exempted kindergartners has fallen by 24 percent.

For too long a tiny minority of people -- often in well-off, educated communities -- have risked the lives and health of everyone else. The national conversation needs to change from what the refusers are doing to their own children to what they are doing to everyone else's.

To contact the senior editor responsible for Bloomberg View's editorials: David Shipley at davidshipley@bloomberg.net.