The number of whooping cough cases in Indiana has doubled in the past year, state health officials warned on Thursday, urging parents to make sure their children are vaccinated.
There were 136 confirmed cases of whooping cough in the state, including one that was fatal, in the first half of 2017 — compared with 66 cases, none of them fatal, in the first half of 2016, according to the Indiana State Department of Health.
While the spike was startling, Dr. James D. Cherry, a whooping cough expert at the University of California, Los Angeles’s David Geffen School of Medicine, said it was not particularly worrying. Whooping cough rates tend to be cyclical, he noted, peaking every three years on average. And although a study in 2016 found a correlation between vaccination rates and the rates of both whooping cough and measles, confirmed cases have also risen because of increased awareness and better diagnostic tests.
In its mildest form, whooping cough, a bacterial illness formally called pertussis, causes coldlike symptoms — or, in many adults, no symptoms at all. But more severe cases, especially in children, cause violent coughing fits that can end with a characteristic whooping sound as the person struggles to inhale. While the infection is treatable with antibiotics, the symptoms can last for 10 weeks or more, and some people actually fracture ribs from coughing. In babies, it can be deadly.
The last major domestic outbreak of whooping cough happened in Washington State in 2012, when more than 1,200 cases were reported in fewer than five months — the most in Washington in at least three decades.
Nationally, reported cases have risen in the last 15 years. In 2003, there were 11,647 cases in the United States, according to the Centers for Disease Control and Prevention. Before that, the annual tally had not broken 10,000 since 1964. But since 2003, there have been over 10,000 cases each year.
In 2009, there were 16,858 reported cases. In 2012, the year of the Washington outbreak, there were 48,277 cases nationwide. In 2015, the most recent year for which the C.D.C. provided statistics, there were 20,762.
Much of this trend is attributable to the introduction of a more sensitive diagnostic test in 2005, Dr. Cherry said, not necessarily to an actual increase in infections. But researchers have also found that the current vaccine is less effective than the one that came before it and wears off within about three years.
Outbreaks of several other preventable childhood diseases, including measles and mumps, have become more common as more parents have sought exemptions to vaccination requirements.
An outbreak of measles that began at Disneyland in December 2014 was attributed to a decline in vaccination rates. This spring, Minnesota recorded its largest outbreak of measles in nearly 30 years, primarily among unvaccinated children of Somali immigrants. And this week, a study by researchers at Stanford University and Baylor College of Medicine found that measles could soon become endemic in the United States again, after being virtually eliminated from the country in 2000.
A simulation of vaccination rates and measles rates in children ages 2 to 11 showed that “a 5 percent reduction in measles, mumps and rubella vaccination coverage resulted in a threefold increase in annual measles cases, with an additional $2.1 million in public-sector costs,” the researchers reported on Monday in JAMA Pediatrics.
But Dr. Cherry said whooping cough outbreaks were far less dependent on vaccination rates than measles outbreaks, because the immunity provided by the whooping cough vaccine wears off faster.
“When there’s been large areas where people haven’t been vaccinated, it clearly shows up,” he said, but “the unvaccinated people aren’t as important in the epidemiology as they are, say, with measles.”