ca Slobodniuk 2002a.

Methods	"Cohort study of inactivated trivalent influenza vaccines compared with no treatment over 3 years. An additional aim of the study was to assess the impact on the immune system of vaccinating children for 3 years in a row. Children were immunised during three epidemics in 1998, 1999 and 2000 and controls were students from parallel classes, who received no intervention. The efficacy of the vaccines was determined from total morbidity rate for influenza and ARIs during outbreak periods 25/01/99 to 14/03/99; 10/01/00 to 21/02/00 and 21/01/01 to 23/02/01 in a boarding school in Yekaterinburg, Russia"
Participants	564 pupils of the boarding school aged 8 to 14 years
Interventions	In 1998‐99 and 1999‐2000 seasons Fluarix inactivated commercial vaccine (SmithKline Beecham) containing A/Singapore/6/86 (H1N1), A/Beijing/32/9 (H3N2), and B/Panama/45/90 was used. In 2000 to 2001 Grippol polymer subunit vaccine containing influenza virus strains A1, A3, and B was used.
Outcomes	Serological Immune response was evaluated before and 30 days after vaccine was administered. Tests were carried out by serological status (i.e. in seropositive and seronegative children) in 70 children in year 1, 109 in year 2, and 73 paired sera in year 3. Effectiveness Number of children with influenza or ARI during outbreak period each year Safety N/A
Funding Source	Unclear
Notes	The authors conclude that the vaccines offered increased protection with each new season, in effect having an additive effect. The first season the efficacy of Fluarix was low in the epidemic period (1.3?); the second inoculation achieved 2‐fold protection compared to the control group. The final year Grippol reduced morbidity by 2.8 times. According to the authors, a fourth injection could be unnecessary. The study is very difficult to interpret, there is no information on participants, community, matching, viral circulation disparity between paired sera and enrollees, etc.
Risk of bias
Bias	Authors' judgement	Support for judgement
PCS/RCS‐Selection Exposed cohort	Unclear risk	Not described
PCS/RCS‐Selection Non Exposed cohort	Unclear risk	Not described
PCS/RCS‐Comparability	Unclear risk	Not described
PCS/RCS‐Assessment of Oucome	Unclear risk	Not described
Summary assessments	High risk	Insufficient information to assess study design