The PRIMAKid trial is a general practice based double-blind randomized placebo-controlled trial on the effectiveness and costs of combined influenza and pneumococcal vaccination in pre-school children with recurrent respiratory tract infections. A target number of 660 children aged 18-72 months with a history of two or more general practitioner attended episodes of RTI, are included. Exclusion criteria are diseases accompanied by a high risk of recurrent RTI and conditions chronically treated with corticosteroids. Over a period of 7 to 22 months follow-up, the number of febrile RTI-episodes as primary outcome is assessed, and as secondary outcomes the severity and length of febrile RTI-episodes, medical visits / interventions, health-related quality of life and productivity loss of parents.