Starting today, students without up-to-date immunization records aren’t allowed to attend school in some Colorado districts.

Weld County District 6 officials expected the deadline, which is always the first Monday in November, to affect less than 50 of the district’s 21,000 students. In the Poudre School District, where the deadline is usually Nov. 1, about 400 students lacked up-to-date records as of Friday, but a spokeswoman said many likely remedied that over the weekend. Most of the affected students were sixth-graders lacking their Tdap booster shot, which protects again tetanus, diphtheria and pertussis.

Such “exclusion dates” are not only perfectly legal, immunization advocates say they can be an effective way to get parents to comply with state law and prevent outbreaks of diseases like whooping cough and chicken pox.

In Colorado, which has among the lowest immunization rates in the country, students must have proof they’ve received certain vaccinations to attend school. In lieu of that, their parents may sign a form exempting their children from shots for medical or religious reasons, or because of “personal belief.”

The practice of “exclusion” comes into play when immunization paperwork, whether it’s a child’s up-to-date shot record or a signed exemption form, is missing. While districts like Weld 6 and Poudre typically use exclusion dates as a last-line-of-defense measure, the strategy raises questions about the impact of lost instructional time, often for the students who can least afford it.

“We always have concerns about instructional time,” said Theresa Myers, the district’s director of communications.

Still, she said the district makes a concerted effort to give parents advance notice about the exclusion date, through letters and follow-up phone calls. It also hosts immunization clinics and offers information about community resources for immunization.

FAQ on HB 14-1288 A new set of frequently asked questions, posted today by the Colorado Department of Public Health and Environment, provides information about a new state immunization law.

“We give them ample chance and opportunities to get the vaccination records in,” she said.

Danielle Clark, Poudre’s director of communications, said it’s unclear how many students are absent today because of the exclusion deadline since they are lumped in with all absent students. Also unknown is the length of time excluded students stay out of school because of missing immunization paperwork.

The state, which now requires schools to disclose immunization and exemption rates upon request, doesn’t mandate exclusion dates, or track the number of schools or districts that use them. But immunization advocates say exclusion letters—often a version of a state form letter—are fairly common and often get quick results from families.

“Sometimes that’s the best way to get parents’ attention,” said Kathleen Patrick, assistant director of health and wellness at the Colorado Department of Education.

Who gets excluded?

In the K-12 system, kindergarteners and sixth-graders are most likely to miss school because of exclusion dates since certain vaccinations are required by age five and around age 11. In districts offering preschool, the deadlines may affect younger children, who not only need a raft of required shots but typically have greater vulnerability to vaccine-preventable disease.

Schools can actually bar kids who don’t have their shots-or a medical, religious or personal belief exemption-much earlier than November. That’s because State Board of Health rules allow exclusion once parents are given 14 days to get their children vaccinated or prove that the process is underway.

Theoretically, if parents are notified on the first day of school, unvaccinated students could be barred starting the third week—say late August or early September. In practice, that doesn’t happen.

“At the beginning of school it’s a little bit busy,” said Patrick. ”So sometimes immunizations might take a little bit of a back burner.”

Even after things settle down, there’s the October 1 count day to think about. With school funding dependent on how many students show up that day, implementing exclusion dates then doesn’t make sense.

But after count day—and usually a series of reminders from school nurses and even principals—exclusion dates are fair game. Some administrators don’t use them because they want kids in seats, but others believe the threat of communicable disease is a greater concern, said Patrick.

Some districts use a form letter from the Colorado Department of Public Health and Environment to let parents know their children are missing required shots. The letter warns that children can be excluded if the records aren’t submitted and allows school staff to fill in an exclusion date.

“Sending home an exclusion letter is definitely a tool in the tool kit,” said Stephanie Wasserman, executive director of the Colorado Children’s Immunization Coalition. “That child that’s out of compliance could bring disease that impacts the whole classroom, the whole school.”

The Boulder Valley School District uses the state letter, said Susan Rowley, the district’s director of health services.

“Non-compliance is truly very low,” she said. “I don’t know that there have been specific kids excluded except in pre-K.”

New legislation

A new Colorado law intended to make schools’ immunization rates more transparent may bolster efforts to ensure students are up to date on their shots. House Bill 14-1288, which took effect July 1, requires schools to disclose immunization rates and exemptions upon request.

Patrick said the state is in the process of offering guidance to school districts about how to calculate immunization and exemption rates. In general, that information should be ready for release by second semester, she said.

“It’s a soft implementation, but definitely in the coming months and for next school year….there’s going to be a lot more support and messaging around telling parents, ‘Did you know you can ask your school what their rates are?’” said Wasserman. “It’s a great tool for parents and parents are thrilled about it.”

Some advocates believe the new law could serve as a consumer-driven lever to push up immunization rates. While many states have rates in the 90-95 percent range for three common kindergarten vaccinations, Colorado and Arkansas bring up the rear with rates in the low to mid-80s. These rates—below the threshold for herd immunity–are a concern for public health experts, particularly because of outbreaks of whooping cough in recent years. The highly contagious disease, also known as pertussis, can be deadly for babies and young children.

Along with disclosure of immunization rates, the new law directs the state to create an online education module on immunization. State officials say that module, which must be approved by the State Board of Health next spring, should be ready in mid-2015.