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.