ba Eisenberg 2008a.

Methods	A prospective, population‐based case‐control study of hospitalisations attributable to laboratory‐confirmed influenza was performed in counties that encompass Nashville, Tennessee; Rochester, New York; and Cincinnati, Ohio, during the 2003–04 and 2004–05 influenza seasons. Each site conducted surveillance at sufficient hospitals to capture 95% of hospitalisations attributable to ARI or fever among children residing in the respective county. Study nurses enrolled children within 48 hours after admission to surveillance hospitals Sunday through Thursday in the 2003–04 influenza season and 7 days per week during the 2004–05 season.
Participants	Description of cases: Eligible children were county residents, 5 years of age or younger, with an admission diagnosis of ARI or fever with laboratory‐confirmed influenza. Description of controls: Children resident in the same county of cases who tested negative for influenza were control participants.
Interventions	Unclear. Matching is described as suboptimal.
Outcomes
Funding Source	Government
Notes	The authors conclude that “even in an influenza season (2004–2005) with a suboptimal vaccine match, more than 1 half of these visits could be prevented with recommended influenza vaccination Partial vaccination did not seem to be effective These results offer additional evidence in support of recommendations for vaccinating children against influenza and they highlight the importance of children receiving the recommended number of influenza vaccinations” No mention of blind exposure assessment is made and effects of vaccine on laboratory outcomes supposedly assesses effectiveness. Broken‐down data not provided.
Risk of bias
Bias	Authors' judgement	Support for judgement
CC‐Case Selection	Low risk	Independent validation
CC‐Control Selection	Low risk	Same population
CC‐Comparability	Low risk	Possibly adequate
CC‐Exposure	Low risk	Secure record
Summary assessments	Low risk	Plausible bias unlikely to seriously alter the results.