Longman says yearly means ‘happening or appearing every year or once a year’ Oxford says yearly means ‘Happening or produced once a year or every year’ Your Dengue outbreaks seem very confusing. In my country your ‘rainy season’ does happen once a year; that's why it's called 'the rainy season'. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.