Not all children are required to get their immunizations. Some do not get them for health reasons or for religious reasons. But these children may be more likely to get sick.
A recent study found that religious exemptions for vaccines have been increasing in New York counties.
Religious exemptions let children skip their shots if their parents have an objection.
The counties with increasing religious exemptions have also seen increases in whooping cough for both vaccinated and unvaccinated children.
The study, led by Aamer Imdad, MD, of the State University of New York Upstate Medical University in Syracuse, looked at religious exemptions for immunizations and illness rates.
The researchers looked at the rates of religious exemptions in New York counties from 2000 through 2011.
Then they looked at the rates of whooping cough, also called pertussis, in these counties.
Whooping cough is an infectious disease caused by a bacteria. It causes severe coughing which can last for months.
Whooping cough can kill individuals who get it, but these deaths are almost always among babies under 3 months old. Others who die from it tend to be elderly or to be weak from other health conditions.
Throughout all of New York, an average of 0.4 percent of children got religious exemptions for at least one vaccine from 2000 to 2011.
However, the rate doubled over those 11 years. The rate was 0.23 percent in 2000 and increased to 0.45 percent in 2011.
When the researchers looked at the rate divided up by counties, they found large differences across the state in the religious exemptions rates.
In 34 counties, the rate of religious exemption use more than doubled over the time studied.
In the counties where at least 1 percent of children received religious exemptions for a vaccine, the rates of pertussis were higher than in the counties with lower religious exemption rates.
Counties with at least 1 percent religious exemption use had an average of 33 cases of whooping cough per 100,000 individuals.
In the counties with lower than 1 percent using religious exemptions, the average whooping cough rate was 20 cases per 100,000 individuals.
However, it was not only the unvaccinated children who had higher rates of whooping cough in the counties where rates went up.
"The risk of pertussis among vaccinated children living in counties with high exemption rate increased with increase of exemption rate among exempted children," the authors wrote.
Even children who had received their vaccinations had higher rates of whooping cough if they lived in an area where more children were skipping shots because of religious exemptions.
This finding is probably related to the idea of herd immunity. In herd immunity, a high rate of overall vaccination in a community protects all of the individuals in that community, whether they have had their shots or not.
However, if too many individuals in a community do not have their shots, then everyone in the community is at higher risk for diseases. This occurs because no vaccines are 100 percent effective.
The more individuals who are vaccinated in a community, the higher the protection is for everyone in that community.
"Religious reasons behind parental refusal deserve additional study because they drive the exemption rates in New York state," the authors wrote.
The study was published June 3 in the journal Pediatrics. The research was funded by a Resident Research Grant from the American Academy of Pediatrics.
One author, Dr. Halsey, receives payment from Merck and Novartis to participate in Safety Monitoring Committees for vaccines and their safety, but not for the vaccines studied in this research.
Dr. Halsey is also participating in GlaxoSmithKline's defense in a lawsuit related to a patent on immunization schedules.
Another author, Dr. Shaw, was paid for speaking in 2011 and 2012 for Merck. The other authors had no disclosures.