Strengths and limitations
This study utilized a well-defined population with detailed observations and personal histories covering a wide range of variables at clearly defined time points. However, by being restricted to a single center the findings are not as applicable in a wider context. Moreover, as we did not have information on the residential addresses of the participants, we used data from one air quality monitoring station as a proxy for personal exposure to PM2.5. Furthermore, confirmed viral test was not available for all participants. Also, diagnosing asthma in young children is challening as it is difficult to instruct this patient group to do a spirometry test. Finally, given the retrospective nature of this study, we had limited information regarding some details of specific variables. For example, we were not able to obtain detailed information on smoking status, e.g. whether the parents of the children smoke on a daily basis or occasionally.