METHODS
We conducted a systematic literature search in PubMed for studies published between 1 January 1990 and 9 December 2015. We restricted studies to those conducted in the USA to minimize potential geographic variation of associations. The following search string was used: ‘(bacteria[All Fields] OR bacterial[All Fields]) AND (virus[All Fields] OR viral[All Fields]) AND (lower respiratory tract infection[All Fields] OR LRTI[All Fields] OR lower respiratory tract[All Fields] OR LRT[All Fields] OR lower respiratory infection[All Fields] OR LRI[All Fields] OR pneumonia[All Fields] OR bronchitis[All Fields]) AND (“1990/01/1”[PDAT]: “2015/12/09”[PDAT]) AND United States[All Fields] AND (time[All Fields] OR temporal[All Fields] OR season*[All Fields])’. Among 464 articles written in English, exclusions were made based on titles, abstracts and full articles. We excluded reviews, in vivo and in vitro experiments, and studies of immunocompromised populations. Nine articles were retrieved from the literature search and three additional studies were selected from the reference list of retrieved articles.