Whooping cough is getting a foothold once again in the U.S., and it seems to be getting stronger. More than 20,000 cases have been reported so far this year, compared with only about 8,500 last year, and Washington State has already declared a whooping cough epidemic.

Now there's fresh evidence that the current vaccine against whooping cough may be less reliable than an older version. Scientists report this week that children in Australia were about four times more likely to catch whooping in 2011 when given only the vaccine, called DTap, instead of an older vaccine, known as DTwP.

The same trend is emerging with the same vaccine on the West Coast, Wired's Maryn McKenna explained on her blog this week. This could be contributing to the epidemic in the U.S., especially when vaccinations are spotty.

To pinpoint the source of Australia's whooping cough epidemic, doctors in Queensland examined the medical records for about 40,500 children who received the new DTap vaccine, the older version, or a combination of the two during the three-shot series.

The results were clear cut: The older vaccine was more effective at preventing whooping cough 10 to 12 years after vaccination. The findings are published in the current issue of the Journal of the American Medical Association.

Even though the study took place in Australia, it "absolutely corroborates very well with what we're seeing and learning in Washington state," says Maxine Hayes, a pediatrician at the University of Washington's School of Public Health, tells Shots. "There is waning protection from the DTap vaccine."

Whooping cough, also known as pertussis, is caused by the bacterium Bordetella pertussis. It hospitalizes about 50 percent of infants who get it and can lead to pneumonia in adults.

The first vaccine for whooping cough was introduced in the 1940s, and it cut infections from about 200,000 cases each year to only a few thousand cases annually in the 1970s.

But the vaccine wasn't perfect. Mild side effects were common, such as pain and swelling, and in rare cases, serious neurological problems.

So scientists developed safer vaccines, such as DTap, which is currently the main line of defense in the U.S.

The first children to receive DTap are now teenagers, and, they are catching whooping cough more often than they should, Hayes says.

Some 77 percent of children and adolescents who got whooping cough in Washington State last year were up to date on their required shots, she notes.

This is consistent with another study in California last year, which foundnew booster shots lost their potency after three years.

So what's next? Hayes says that the Centers of Disease Control and Prevention is going to examine more immunization records and figure out exactly how long the DTap vaccine works. Then they can decide if more frequent booster shots are required.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.