AZ-SWED is a study for preschool children (18 months to 5 years old) who have severe wheezing. Wheezing is a whistling sound when breathing. In this study, children will either get a medicine called Azithromycin (AZ) or a placebo (a fake pill with no medicine) for 5 days. The study is double-blind, meaning neither the doctors nor the families know who gets the real medicine. Researchers want to see if AZ helps more than the placebo by using a tool called the Asthma Flare-up Diary for Young Children (ADYC). Parents will fill out this diary each day for 5 days. Some children will have 2 extra visits to check if the medicine causes problems or changes in bacteria in their airways. The study involves up to 2,000 children from six emergency departments (EDs). Children must not have used antibiotics or steroids recently, and they should not have other serious health problems. The study is short-term with 3 main visits.

• Study involves up to 3 visits over a few weeks.
• Parents fill out a daily diary for 5 days.
• Some kids will have 2 extra check-up visits.