Refusal to vaccinate children against whooping cough might have played a role in the 2010 pertussis outbreak in California, researchers reported.
Analysis of the outbreak — the largest in the state since 1947 — showed that pertussis cases fell into two large temporal and spatial clusters, according to Saad Omer, MBBS, PhD, of Emory University in Atlanta, and colleagues.
And a 5-year analysis of so-called nonmedical exemptions from vaccination for children entering kindergarten found 39 spatial clusters, Omer and colleagues reported in the October issue of Pediatrics.
The two types of clusters tended to overlap, Omer and colleagues found, suggesting nonmedical exemptions might have been “one of several factors in the 2010 California pertussis resurgence.”
Several other possible causes of the outbreak have been put forward, including the notion that the acellular vaccine now used is less effective than the older whole-cell medicine.
Researchers have also suggested that even if it has the same initial efficacy as the whole-cell vaccine, its effectiveness may wane over time, leaving children vulnerable.
But Omer and colleagues have been concerned for some time about the risks of clusters of children who don’t get vaccinated against Bordetella pertussis.
Indeed, in 2009 they reported that across the U.S., vaccine exemptions had risen sharply and in some places were highly clustered.
“Herd immunity is important,” commented Robert Finberg, MD, of the University of Massachusetts Medical School in Worcester.
“If most of the population is not immunized, any given individual will have a much greater chance of being exposed to pertussis and therefore of coming down with the disease,” he toldMedPage Today.
In California, the rate of nonmedical exemptions — sometimes also called personal belief exemptions — has been rising, Omer and colleagues reported.
From 2000 to 2010, the overall rate in the state rose from 0.77% of children entering kindergarten to 2.33%. While those rates are still low, they noted, some schools reported as many as 84% of new students were exempted in 2010.
Data on how many children started kindergarten in each of 5 years — 2005 through 2009 — and how many in each school had exemptions, as well as exact locations, were available for 7,091 schools.
The researchers also had data, including location, on 8,521 of the 9,143 reported cases of pertussis (location was missing for 622 cases) with onset in 2010.
Analysis showed census tracts within a cluster of exemptions were more likely to be in a pertussis cluster (odds ratio 2.47, 95% CI 2.22-2.75).
Also, more pertussis cases occurred within exemption clusters than outside (incident rate ratio 1.20, 95% CI 1.10-1.30).
The associations remained significant after adjustment for demographic factors.
A bivariate analysis indicated that both types of cluster were linked to factors “characteristic of high socioeconomic status,” the researchers reported, including such things as lower population density, lower average family size, and higher median household income.
That’s surprising, commented Finberg, because it’s usually in lower socioeconomic areas where public health authorities have trouble getting kids vaccinated.
And that fact points to a gap in the study: “It is not clear what the nature of the non-medical exemptions were,” he said.
Without understanding the reasons parents are seeking exemptions, he said, it’s not easy to decide how to approach the problem.
Omer and colleagues pointed out some other gaps in the data. California records the fact of an exemption, but not the specific vaccines, so that it’s possible some children with an exemption might actually have been fully vaccinated against pertussis.
And the kindergarten data are snapshots that only serve as a “proxy for community-level vaccination coverage and immunity,” they said.