About a fifth of children seen by their doctors for persistent coughs may actually have whooping cough, a new study suggests.

The new results come from England, where adolescents don't receive a booster vaccination for the highly contagious disease; that booster shot is recommended in the United States.

The findings suggest that a shot may be warranted for adolescents in England, though further research is necessary, said Dr. Cameron Grant, a pediatrician at the Starship Children's Hospital and the University of Auckland in New Zealand, who was not involved in the study. [5 Dangerous Vaccine Myths]

Deadly but deceiving

Whooping cough, or pertussis, is preventable with the vaccine, but it is still kills nearly 300,000 people yearly worldwide, mostly in places where the vaccine isn't widely used.

In older children and adults, its early symptoms mimic the common cold. But after those symptoms resolve, pertussis causes a cough that can linger for weeks (the cough makes a characteristic whooping sound, hence the condition's name).

It's also incredibly infectious: Every person with whooping cough will infect, on average, 12 other people, in part because individuals are at their most infectious when it seems they just have a cold, Grant said.

"It's a sneaky bug," Grant told Live Science.

But although pertussis is mainly a nuisance for healthy adults, among infants it can be deadly. In babies, whooping cough may not even cause much coughing before the little one becomes purple or pale, and stops breathing.

And because the symptoms of whooping cough are worse at night, it can be hard to diagnose.

"Parents can have a terrible time in the middle of the night and take their baby to the doctor the next morning and the baby looks fine," Grant said. But later that night, their baby may still be in grave danger, he said.

Currently, there's a whooping cough epidemic in California, with more than 800 cases reported in the first two weeks of June.

Immunization schedule

In the United States and the United Kingdom, most children get three shots of the pertussis vaccine during the first several months of infancy, and then a booster shot in preschool. Children in the United States also receive an additional booster shot, which U.K. kids do not get.

"In the U.K., the main aim of the vaccination at the moment is to try and prevent transmission of whooping cough to vulnerable individuals," such as young infants, Wang told Live Science.

In the United States, adolescents started receiving booster shots at ages 11 or 12 in 2006, said study co-author Dr. Kay Wang, a general practitioner and researcher at the University of Oxford in England. That's because the vaccine given early in life only confers protection for four to 12 years, the authors wrote in the paper.

To estimate the prevalence of whooping cough, the researchers tested oral fluids from 279 children in England, between ages 5 and 15, who had gone to their doctors complaining of coughs that lingered for two to eight weeks.

About 20 percent of the children tested positive for pertussis, and the majority of those children were up to date on their vaccines, according to the study.

Adolescent shot?

Since the adolescent booster shot was introduced in the United States, pertussis has declined among teens, some research suggests.

In theory, lowering the rates of pertussis in teens and adults could also reduce whooping cough in infants, who can contract the disease from older siblings and caregivers. But vaccinating teens in the United States hasn't dramatically reduced the number of small babies admitted to the hospital with whooping cough,, other research suggests.

Either way, it's important to make sure little ones stay up to date on their vaccines, Grant said.

"We must immunize all of our young infants and immunize them on time," Grant said. Otherwise, whooping cough epidemics could become more common, he said.

The new findings were published today (June 24) in BMJ.

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