I'm not sure that that is what the article is saying. It wrote "the vaccine is at least 85% effective in preventing most strains of whooping cough in Australia." I read that to mean that the vaccine is 85% effective against the strains that it protects against, and that most of the strains in Australia are covered against that are present in Australia. But, it didn't mention whether those strains are the most common in Australia. For example, it may be that we have 10 strains in Australia (I'm making up these numbers), and 8 of them are covered in the vaccine (which protects people 85% of the time for those 8 strains), while the 2 that are not covered may be the most common strains infecting people at the moment.
On the other hand though, it may be exactly how you interpreted the information, and that of all the strains (including the ones not covered in the vaccine), only 15 out of 100 people that are vaccinated will catch whooping cough when exposed.
I guess that's the problem with media... they aren't 100% clear in what they write. When I get a chance I'm going to try to look up the actual journal article and have a read.

