Jordan 2007.

Study characteristics
Methods	Case‐control study nested within a cohort of older people registered with 79 participating general practices in central England. People were included in the identified cohort if aged 65 to 89 years and if they consulted their general practitioner (or other emergency medical services) for an acute episode of respiratory infection or acute exacerbation of pre‐existing respiratory disease, between 1 October 2003 and 31 March 2004. People with simple upper respiratory tract infections were excluded.
Participants	Cases were defined as all people admitted to hospital with acute respiratory disease. Only the first admission during the study period was included. Surviving cases were invited for interview. Controls were defined as people presenting with acute respiratory disease but who were managed in the community. 6 controls were invited per case to mitigate for a potential low uptake, in order to achieve 4 controls interviewed per case. Controls were matched to cases for age (within ± 5 years where possible), sex, and consultation date (within ± 7 days where possible). 3970 eligible participants were identified. 500 participants were admitted to hospital. Altogether 44.1% of invited cases and 54.5% of controls agreed to interview; 157 cases and 639 controls were finally interviewed. The proportion of cases vaccinated against influenza before entry to the study was 74.5% and in controls was 74.2%.
Interventions	Influenza vaccination and admissions to hospital for acute respiratory disease
Outcomes	—
Notes	The authors conclude that in a winter typical of the current levels of circulating influenza, they were unable to demonstrate that influenza vaccination had a specific effect on preventing hospitalisation among elderly people clinically ill with acute respiratory disease, although there was a possible effect during the peak weeks of influenza activity. Relying solely on the influenza vaccine to control the annual winter bed pressures in hospitals is unlikely to be a sufficiently effective yearly strategy, and continuing attention to other factors (e.g. the effective vaccination of healthcare workers, treatment of comorbidities, indoor housing conditions) is essential.