Figure 1. Timeline of seasonal periods of analysis, by study year.

Note. Definition of periods as follows: Fall = five week period starting with first week of September; Pre-Influenza = interval from date by which 90% of elderly immunizations had been administered until onset of influenza period; Influenza = the interval between and including the first and last occurrences of at least two consecutive weeks with two or more influenza isolates reported; Peak = five week period including week with peak proportion of respiratory specimens positive for influenza +/− two weeks; Spring = interval from/including week after the influenza period to May 31; Summer = interval from/including first week of June to last week of August. * Pre-influenza period not defined in 2003–04 due to early season.