Experts have been warning for years about the small but growing number of people who decline or delay immunizing their children because they fear side effects or believe the diseases are gone.
Haemophilus influenzae type b, which is unrelated to the influenza known as the flu, used to infect 20,000 children under age 5 each year in the United States before vaccines were licensed for children 1987 and infants in 1990. About 1,000 died, and many of those who survived were left mentally retarded.
"It was just a horrible disease," said Caroline Johnson, director of the Philadelphia Health Department's Division of Disease Control.
Johnson's last personal encounter with Hib came during her medical training two decades ago. The city's last previous case was in 1997.
Recent epidemics of whooping cough and measles, two other vaccine-preventable diseases, also have been linked to lack of vaccination. Just yesterday, three measles cases were reported near Pittsburgh, bringing the state's total to four - already more than in any year since 2003. The fourth, a University of Pennsylvania student whose case was reported in February, was Philadelphia's first case since 2001.
The Hib situation is complicated by a vaccine shortage.
Merck & Co. Inc. announced in December 2007 that it was withdrawing 1.2 million doses of its vaccine because of possible contamination of its equipment in West Point, Pa.
Sanofi Pasteur, which makes another Hib vaccine at its plant in Swiftwater, Pa., increased production but could not completely make up for the shortfall.
Until vaccine is again fully available - Merck now expects manufacturing to be ramped up by mid- to late 2009, a spokeswoman said - the government has issued guidelines to conserve supplies.
The recommendations direct providers to delay giving the "booster" shot, which is normally administered at age 12 months to 15 months, except for children with specific known risk factors. That delay would ensure sufficient stocks so all children could receive the "primary" series of shots in infancy.
The strategy appears to have worked. There have been no reports of children missing the critical primary series because of the Hib vaccine shortage.
But the primary series consists of three doses for some versions of Hib vaccines and two doses for others.
As a result of confusion among some doctors, officials said a small number of children may not have gotten the full primary series.
"Physicians need to make every possible effort to give the initial three doses to every kid," said Veronica Urdaneta, Pennsylvania's state epidemiologist.
Most vaccines confer immunity in two ways: individual and communal. A vaccinated individual is largely, though not completely, protected.
Nearly complete protection, known as the "herd" effect, occurs when nearly everyone in a community is vaccinated. This reduces the amount of infectious organisms in circulation, raising the level of protection for both people who are vaccinated and those who are not.
Vaccine experts believe that Hib has reappeared now because unvaccinated children currently have less protection from a "herd" that is largely missing the booster.
The U.S. Centers for Disease Control and Prevention advised public-health workers in January about five independent Hib cases in Minnesota - all involving children who had received none or just some of the primary vaccine series - that it believed were due to higher levels of the bacteria being carried by vaccinated children.
Oklahoma had two cases in February. One child was too young to get the vaccine; the other had just gotten a dose, and full immunity probably had not yet developed.
A child in New Jersey who had received the full primary series died of Hib disease last summer, a spokeswoman for the state Health Department said yesterday. She was unable to provide more details.
Johnson, the Philadelphia disease control director, got her first hint of a local problem in January, when a child from Chester County was being treated at a city hospital.
"I remember hearing about the case and shrugging my shoulders and thinking, what a shame," she said.
Then, on March 11, she got a report of a 4-year-old city child who had died of meningitis due to invasive Hib disease, which also can cause death through pneumonia and bloodstream infections known as sepsis.
The child's family belongs to a church that eschews modern medicine. The city threatened court action to force vaccination of the child's siblings, she said.
The Medical Examiner's Office, in a review of recent deaths, then determined that a 2-year-old from a community with religious objections to vaccines had died of a related disease Jan. 24.
This is the one suspected Pennsylvania case that has yet to be proven.
Since August, the state Department of Health has confirmed three cases in Lancaster County and one in Perry County. The Chester County case brought the state's total of confirmed cases to six, which a CDC spokesman said yesterday was the most from any state.
To Paul A. Offit, chief of infectious diseases at Children's Hospital of Philadelphia, the Hib outbreak is a sign of what can happen when some parents decide not to vaccinate their children.
Although the risk to each individual in the broader community isn't high, he said, "the question is, when do you reach a tipping point?"
Contact staff writer Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.