(RxWiki News) More than 9,000 people contracted whooping cough in California in 2010. It was the worst outbreak of the disease, also called pertussis, since 1947 in that state.
A recent study found that one factor contributing to the epidemic was families choosing not to vaccinate their children.
The study found that people were more likely to catch pertussis if they lived in an area where many people sought legal exemptions not to vaccinate their children.
When too many people choose not to vaccinate, it decreases the protection against that disease for the entire community.
"Talk to your doctor about the CDC immunizations schedule."
The study was led by Jessica E. Atwell, MPH, a PhD candidate in the Department of International Health at John Hopkins Bloomberg School of Public Health.
She and her colleagues used a computer program to identify clusters of non-medical vaccine exemptions among California kindergarteners from 2005 through 2010.
Non-medical exemptions let parents choose not to vaccinate their children for public school requirements for reasons related to their religious, philosophical or personal beliefs.
Religious exemptions exist in all but two states, and 19 states have some form of a non-religious belief-related exemption option.
In California, the number of non-medical exemptions tripled from 0.77 percent in 2000 to 2.33 percent in 2010. Some schools had non-medical exemption rates as high as 84 percent.
Then the researchers used the same program to identify clusters of particularly high cases of pertussis during the 2010 epidemic.
Then they looked for overlaps between those clusters that were statistically not a result a chance.
The researchers identified 39 clusters of non-medical exemptions.
These were areas where high numbers of families used non-medical vaccine exemptions compared to surrounding, outside areas.
The researchers found two clusters where the pertussis cases were more highly concentrated than in other places in the state.
One pertussis cluster included most of central California between May 2010 and October 2010. The other included San Diego County between July 2010 and November 2010.
When the researchers looked for overlaps, they found that residents living in a non-medical exemption cluster were 2.5 times more likely to be living in a pertussis cluster as well.
This finding means that the grouping of families with non-medical exemptions was contributing to the greater number of pertussis cases in the epidemic.
This finding remained after the researchers controlled other factors that could influence the results — such as family income, family size, race/ethnicity percentage, and the percentage of people with college degrees.
Those living inside a geographical area with a higher number of non-medical exemptions were 20 percent more likely to contract pertussis than those outside the clusters.
Because the researchers did not look at the individual vaccination status of each person in the geographical areas, their 20 percent increase number applied, on average, to everyone in the area.
The ability to contain a vaccine-preventable disease depends on several factors, including the effectiveness of the vaccine, the contagiousness of the disease, and the level of herd immunity in an area.
Herd immunity occurs when enough people in a population are vaccinated so that the disease cannot be transmitted through the population. It protects those who cannot be vaccinated or have weak immune systems.
"Pertussis is in the most contagious disease category — more contagious than the flu or smallpox," said Paul Offit, MD, chief of the Division of Infectious Diseases and director of the Vaccine Education Center at Children's Hospital of Philadelphia.
Approximately 95 percent of a community needs to be vaccinated against pertussis to keep cases and outbreaks lower, according to background research information in the study.
"Parents think they're erring on the side of caution by not vaccinating their kids, but their whole risk-benefit calculation is off," said Atwell.
"They don't trust in decades of scientific research," she said, referring to parents who don't vaccinate because of fears about vaccine safety.
A large part of the return of pertussis in recent years is that the current vaccine's protection does not last as long as an older vaccine discontinued in the 1990s, but Atwell said this study shows that parents' not vaccinating also plays a part.
"Our parents grew up seeing and fearing these diseases, but many of the parents today have not seen them and take it for granted that they're not around," she said.
Tracie Newman, MD, a pediatrician with Sanford Health in Fargo, North Dakota, said that tetanus, diphtheria and pertussis can all be very serious diseases, especially in young children.
"Infants are most at risk for severe, life-threatening complications from pertussis, which is why vaccinating this population and pregnant women is so vital," Dr. Newman said.
"Pertussis or whooping cough causes severe coughing spells, respiratory distress and vomiting," she said. "For higher risk patients, hospitalization and the potential for complications, which could include pneumonia or death, are greater."
The study was published September 30 in the journal Pediatrics.
The research was funded internally. The funding for collection of data and disease cases at the California Department of Public Health was funded by the Centers for Disease Control and Prevention.
Atwell is a former employee of the California Department of Public Health and was paid for contract work related to introducing a vaccine from the GAVI Alliance and PATH.
Another author has received vaccine-related research grants from Crucell and has served on the safety monitoring review boards at Merck and Novartis. He also consulted for GlaxoSmithKline during a patent lawsuit.
The authors declared no potential conflicts of interest.