Pertussis infections have reached epidemic proportions in the state of Washington, officials there said, with more than 10 times as many cases this year as were counted by early May in 2011.
The state's health department has recorded 1,284 cases of pertussis, familiarly known as whooping cough, through May 5. Last year, 128 cases were reported through the first 18 weeks.
In six of the past eight weeks, more than 100 cases were reported. Although there were only 54 in the week ending May 5, the health department noted that additional cases may have occurred that had not been reported to the department.
By far, the highest infection rates have been in children and teens. Infants younger than 12 months and children 10 to 13 years old have had rates approaching 1 per 1,000.
Among the 86 infants infected, 23 were hospitalized. Most of those admissions were in infants younger than 3 months.
A CDC report last June indicated that only 9 states had lower rates of pertussis vaccination among children entering kindergarten in 2009 than Washington's 91.6%. Moreover, the state had the nation's highest rate of parent-signed exemptions from vaccination for kindergartners, at 6.2%.
A new law that went into effect last summer required parents seeking exemptions for their children to receive information from a licensed healthcare professional on the risks and benefits of vaccination. It's unclear whether the law reduced the number of exemptions filed.
But whether the low vaccination rates in Washington are a factor in the current outbreak also was unclear. A CDC official told the New York Times that most children diagnosed with the disease in Washington had been vaccinated.
Also, changes in the vaccine in the past 20 years, intended to increase its safety, may have lessened the durability of its protection against infection, the official said.
State officials projected that, at the current rate, more than 3,000 children would come down with pertussis this year. That would be the highest level since vaccines were introduced in the 1950s.