One disease’s encore
An old childhood disease reared its head in Minnesota last year, infecting five young children and killing one of them, according to a recent report by the Centers for Disease Control and Prevention. The outbreak, of a disease known as “Hib” for short, is the latest of several contagious diseases making encore appearances after having been all but vanquished through immunization. Although the Hib outbreak was small, public health officials worry that parents opting not to vaccinate their children -- or delaying until kids enter school -- may put more communities at risk for similar outbreaks in the future.
Haemophilus influenzae type b (Hib) is a bacterial disease that can cause serious and sometimes fatal illness in children younger than 5. Hib was once the leading cause of bacterial meningitis, an infection that can lead to brain damage and deafness. The bacterium can infect other tissues as well, causing complications such as pneumonia, serious blood infections or severe swelling in the throat. In its less invasive form, Hib also used to be the most common cause of ear infections in children.
Before vaccines against Hib became widely available in the early 1990s, an estimated 20,000 children in the U.S. were seriously sickened by Hib each year and nearly 1,000 died. Since then, it has virtually disappeared. “Haemophilus influenzae type b was the most important bacterial disease that we encountered in pediatrics for half of my career as a pediatrician,” says Dr. Wilbert Mason, who heads the division of infectious diseases at the USC Keck School of Medicine.
No cases of Hib have been observed in Los Angeles in 2008 or so far this year, says Dr. Jonathan Fielding, director of public health for Los Angeles County. But, he adds, “I am concerned that over time this will be an increasing problem.” That is because an increasing number of parents are obtaining exemptions from school vaccination requirements.
The disease may be practically gone, Mason says, but “the organism is still around.”
The CDC report, published Jan. 30, noted that in three of the five Minnesota cases, children had not been vaccinated against Hib. (The other two cases were of a child who hadn’t completed the full series of shots and a child with immunodeficiency.) The outbreak happened to occur when vaccine supplies were low, but the shortage did not prevent any of the five from being immunized, the report said
But the vaccine shortage may have been a factor in the Minnesota outbreak by increasing the number of carriers of the bacterium, says Dr. Nancy Messonnier, who heads the Meningitis and Vaccine Preventable Diseases branch at the CDC and co-wrote the agency’s report. Faced with rationing doses, physicians in Minnesota put off the final booster, considered the lowest priority of the series of four shots. Kids who weren’t boosted likely had sub-optimal protection, increasing the chance that they harbored the bacterium even if it didn’t make them sick. More carriers means more chances for the bacterium to be transmitted.
Outbreaks of other vaccine-preventable diseases have occurred recently. In 2008, 131 measles cases were reported nationwide from January through July, more than twice the typical yearly average, according to an August CDC report. Fifteen people were hospitalized, four of them infants.
Measles, like Hib, has been virtually eradicated from the U.S. thanks to vaccination. As well as the familiar rash and fever, the viral infection can cause serious illnesses such as encephalitis and pneumonia. Imported cases -- which can occur when people travel to a country where the disease is still prevalent -- cause 50 or so measles cases in the U.S. on average each year. But in the 2008 outbreaks, a surprising number of people -- nearly 100 -- contracted the disease domestically, presumably through contact with recently infected travelers, says Dr. Jane Seward, acting deputy director of the CDC’s National Center for Immunization and Respiratory Diseases. The first outbreak that year was initiated by a 7-year-old unvaccinated child who contracted measles in Switzerland. Once the child arrived home in San Diego, the disease spread to others in the family and in the community. Twelve cases were reported and some 70 people were quarantined in their homes to prevent further spread.
By the end of the year, seven outbreaks had occurred across the country. “Many of them were in groups of people whose parents had refused vaccinations for their children,” Seward says.
It’s one thing to suspect a link between unvaccinated populations and increased risk for outbreaks, and another thing to prove it. The case appears strongest for another once-common childhood disease. A 2008 study published in the American Journal of Epidemiology showed that communities in Michigan with high rates of parents refusing vaccines were associated with outbreaks of whooping cough.
Saad Omer, an epidemiologist at Emory University in Atlanta and first author of the study, estimates that the odds of whooping cough outbreaks were 2.7 times higher in communities with greater numbers of vaccine-refusing parents. “Even if you vaccinate, your child may be at risk,” he says. That’s because the immunization does not guarantee 100% protection.
Michigan is one of 21 states, including Minnesota and California, that allow “personal belief” exemptions from school vaccination policies in addition to exemption for religious or medical reasons. “The practical implication of our findings is that everyone who is living in a community with a high proportion of unvaccinated individuals has an elevated risk,” Omer says.
Some parents don’t refuse vaccinations for their children, but may delay them until they run up against school requirements. But that leaves children unprotected at the most vulnerable ages, says Kristine Sheedy, a behavioral health communications specialist at the CDC. “Delaying is not a risk-free decision,” she says.
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