The United States is in the midst of the worst epidemic of pertussis, or whooping cough, in more than 50 years, and Allegheny County is mirroring the national trend.
Nationwide, the outbreak is startling: 32,000 cases reported as of Oct. 24, along with 16 deaths, most of them infants. The all-time recorded peak was in 1959, with 40,000 cases.
In Allegheny County, the incidence has almost quadrupled: 201 cases so far this year compared with 51 cases in all of 2011. Patients have ranged from babies to those in their 70s, but most have been 12 or 13.
Megan Casey, nurse epidemiologist at the Allegheny County Health Department, said the local incidence peaked in June; the number of cases showing up now are back to normal levels. No one has died, she said, although seven have been hospitalized.
"We don't really know why the numbers jumped or why they're fading out again," Ms. Casey said.
At least one study, in California, suggests that the vaccine's effectiveness is waning in three years time -- a shorter duration than many doctors thought. The same vaccine has been given since 1995; it is considered safer than the earlier version that had side effects.
Other possible causes for the increase include better detection and reporting or a mutation of the pertussis bacteria.
The disease is marked by violent, uncontrollable coughing that makes it hard to breath. When the sufferer takes a gulp of air it sounds like a "whoop."
The common treatment is antibiotics. Left untreated, it can last four to 12 weeks.
Pertussis is highly contagious, so the county requires all children to be vaccinated before they begin school. It is most dangerous to infants who have not yet been immunized. Last month, the U.S. Centers for Disease Control and Prevention in Atlanta recommended booster shots for all pregnant women, as well as all family members and caregivers who spend time with babies.
Whooping cough is also more dangerous for older adults, who tend to have other complications.
Even with immunization, whooping cough remains common in the U.S. The CDC says outbreaks tend to occur every three to six years, although the last peak year was only two years ago, with more than 27,000 cases reported nationally. Chances are the incidence was higher, however, because symptoms mimic those of other illnesses, such as colds and flu.
"This pattern is not completely understood," the CDC website says, "but that's why it's important that everyone get vaccinated. If it weren't for vaccines, we'd see many more cases of whooping cough."
The immunization schedule for children calls for the DTaP shot (diphtheria, tetanus and pertussis) at 2, 4 and 6 months, 15 to 18 months and 4 to 6 years, with a booster shot, or Tdap, at 11 or 12 years.
Allegheny County used to offer free vaccinations for all children, but last month the health department stopped including those with insurance. Ms. Casey said there was no connection to the epidemic, which began long before that policy change.
Sally Kalson: firstname.lastname@example.org or 412-263-1610.