Background: Extensive research has been carried out to quantify the efficacy and immunologic responses to influenza immunization, however, only little was reported on whether the timing of vaccine administration affects its effectiveness. The study aims were to compare post-immunization hospitalization and mortality rates among high-risk patients who were vaccinated against influenza in different periods of autumn.
Methods: Data for the current population-based historical prospective study were obtained from a large health organization in Israel insuring 1,600,000 members. Study population (n = 68,166) included high-risk patients aged 55 and above, who were immunized against influenza between October and November 2003.
Results: Although at baseline, individuals vaccinated in late November (12-25/11/2003) were significantly younger and had lower burden of illness compared to those vaccinated in early October (1-13/10/2003), they had higher hospitalization rates and longer hospital stay during the following winter (December to March). They also had an increased risk for all-cause mortality (RR = 1.78; 95%CI: 1.13-2.80). No such differences were observed during the following summer (April-August).
Conclusions: In countries where influenza generally peaks between December and early March, vaccines administered after mid-November might be less beneficial.