Anecdotal, but this is something I’ve seen for years. As an added piece of totally non-rigorous evidence (especially because including Australia throws all of my northern hemisphere seasonal biases off), Google seems to agree:

http://www.google.com/tren​ds?q=%22runny+nose%22+%22s​ore+throat%22&ctab=0&geo=a​ll&date=2010&sort=0