You bet your sweet bupkiss we are. And I don't need a scientific study to convince me of that.
The fact is, the more comfortable we become about visiting a place, the easier it becomes to get to a place, the more likely it is we will eventually make a mistake and bring something with us we didn't intend to bring.
Granted, scientists take GREAT CARE to disinfect anything they are sending to Mars or any other celestial body, but the fact is, accidents do happen.
Fortunately, the Martian environment is very harsh (especially for microbes used to living on Earth) so anything which did travel there would face an especially hostile new home. This adds to the level of protection the planet has against any earthly incursions.
Still, O'Neill makes a good point in this article, "Could New Rover's Wheels Deliver Germs to Mars?":
Curiosity's landing on the Martian surface will contrast greatly with anything that's gone before. After entering the Martian atmosphere, parachute jettisoned and heat shield ditched, the large rover requires something more powerful to control its landing. Using the much-discussed "sky crane" -- a rocket-powered platform -- to lower Curiosity to the ground, the first part of the rover to make contact with the regolith will be its wheels. This is the first time a Mars rover will use its wheels as its landing gear.
And herein lies the problem, says Schuerger.
Soujourner, Spirit and Opportunity's wheels all had a period of time sitting under the sun, being baked by savage ultraviolet light, before coming into contact with the soil -- any surviving bacteria would have been fried. Curiosity's wheels, on the other hand, will make immediate contact with the soil. Should any hardy bacteria have survived the trip, nestled in the rover's tread, they could be buried quite nicely into the uppermost layer of Mars regolith.
In fact, according to the study, a contaminated wheel would be quite effective at harvesting bacteria in the Martian regolith. 31.7 percent of the samples delivered into the simulated Mars environment showed growth. Sadly (for the bacteria), their survival rates plummeted soon after -- the ultraviolet radiation and high carbon dioxide environment is a huge buzz-kill for microbe development.
So, there's a tiny chance that if Earth Brand™ microbes make it past the sterilizing process, if they survive the harshness of the interplanetary environment during transit to Mars, and if they are fortunate to be buried deep enough by the rover's wheels to be shaded slightly from the sun, then there might be a tiny glimmer of hope that our intrepid single-celled travelers live for more than a few minutes.
Click here to read the rest of O'Neill's article.