(USA TODAY) -- Health officials said Thursday that the number of cases of whooping cough could reach the highest level in more than 50 years.
As of July, nearly 18,000 cases have been reported, more than twice as many as at this time last year, the Centers for Disease Control and Prevention reported. At this pace, the number of whooping cough cases will surpass every year since 1959.
Public health officials are concerned the uptick might be due in part to a switch from one vaccine type to another 15 years ago. The change was based in part on now-discredited concerns about the dangers of the older vaccine.
"We may need to go back to 1959 to find as many cases reported" halfway through the year, said Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases.
Whooping cough, called pertussis by doctors, is a highly contagious bacterial disease and very dangerous to infants and young children. Half of babies who get it are hospitalized, Schuchat said.
The disease leads to severe coughing that causes children to make a distinctive whooping sound as they gasp for breath. In rare cases, it can be fatal. Nine children have died this year.
Though 95% of toddlers are vaccinated against the disease, only 8.2% of adults are, and they are the ones most likely to infect babies, Schuchat said.
The highest rates of infection are in babies less than 12 months, and half of the cases are in those under 3 months. Babies are too young to be protected by the first vaccination, typically given at 2 months, so vaccinating their mothers and the people around them is key to protecting them.
Children get vaccinated in five doses, with the final shot given at 4 to 6 years. A booster shot is recommended around age 11. The vaccine's protection wanes, and health officials have debated moving up the booster shot.
Health officials don't know why pertussis, which tends to occur in waves every three to five years, is rising. They're investigating whether one reason might be a switch made in the type of vaccine given in the early 1990s.
Unproved and unscientific claims that there was a connection between the pertussis vaccine and brain injury pushed manufacturers to switch to another safer version, acellular pertussis vaccine. It has been used in the USA since 1997.
Health officials see some evidence that its effectiveness may wane more quickly than the previous form, contributing to a rise in whooping cough cases among children ages 10 to 14.
The uptick in infections in that age group is "different than what we've seen in previous waves," Schuchat said. "That's why we're recommending a booster at 11 or 12."
The vaccine is not 100% effective, but unvaccinated children are eight times more likely to be infected, Schuchat said.
"Without the vaccine, we know that we would have hundreds of thousands of pertussis cases each year," she said, adding that even if vaccinated children do get whooping cough, they don't get as sick and they're less infectious to others.
Public health officials push hard for pregnant women and all adults to get the pertussis booster vaccine, which is called Tdap and also protects against tetanus and diphtheria.
Washington state is in the midst of a major whooping cough outbreak.
"As of the end of last week, we've had more than 3,000 cases," said Mary Selecky, secretary of the state Department of Health. "My biggest concern is for the babies."
She said there have been no child deaths this year. Last year, two children died from the disease.
Chelsey Charles' daughter, Kaliah Jeffery, was one of them.
Charles of Lake Stevens, Wash., got whooping cough while she was pregnant. When Kaliah was 2 weeks old, "she started sneezing" and was hospitalized.
After six days, she was put on a ventilator and began having seizures. When Kaliah was 27 days old, Charles said, doctors said, "We're going to have to let her go."
On Aug. 15, 2011, Charles and Kaliah's father, Tanner Jeffrey, held her as she died. "They took her off life support, and she tried to take one breath and she couldn't," Charles said.Since her daughter's death, Charles has worked to educate pregnant moms about the risks of the disease.
"A lot of people didn't even know about it before," she said. "I tell people, 'Would you rather have a baby die because they're not vaccinated?' "