title: Late influenza vaccination is associated with reduced effectiveness.
abstract: 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.
authors.affiliation: Maccabi Healthcare Services, Ha'Mered 27, Tel Aviv 68125, Israel. Hodik_g@mac.org.il
authors.forename: Gabriel
authors.initials: G
authors.lastname: Chodick
authors.forename: Anthony D
authors.initials: AD
authors.lastname: Heymann
authors.forename: Manfred S
authors.initials: MS
authors.lastname: Green
authors.forename: Ehud
authors.initials: E
authors.lastname: Kokia
authors.forename: Varda
authors.initials: V
authors.lastname: Shalev
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006760:Hospitalization / Q000639:trends; D006801:Humans; D007115:Immunization Schedule; D007252:Influenza Vaccines; D007251:Influenza, Human / Q000276:immunology; D007557:Israel; D008297:Male; D008875:Middle Aged; D009026:Mortality / Q000639:trends; D011446:Prospective Studies; D016896:Treatment Outcome
chemical_list: D007252:Influenza Vaccines