ba Anonymous 2005.

Methods	Case‐control study based on the 45 British Columbia surveillance system sites in which during 2004 to 2005 sentinel physicians were encouraged to take more swabs. Cases were people who reported to sentinel physicians with acute onset respiratory illness with fever and cough and 1 or more of sore throat, arthralgia, myalgia, or prostration and had a positive specimen for influenza A. Controls were all other symptomatic reportees who tested negative. Once the specimens were taken, a questionnaire with details of the case was attached. The authors report that "there were 219 separate submissions of respiratory specimens by a known sentinel physician during the 2004 to 2005 surveillance period. Of these, only 32 (15%) had all questionnaire information completed on the original laboratory requisition; 187 required follow‐up interview with the submitting physician to complete missing information and 133 were completed. From the 165 patients with complete records, specimens were collected between 4 October, 2004 and 31 March, 2005 with the distribution of submissions mirroring the distribution of sentinel visits for ILI overall"
Participants	165 out of 219 participants had sufficient information as required by the study protocol. Of these, 134 were from the period of greatest circulation. 40 and 7 cases, respectively, had specimens positive for influenza A and B, and only 7 overall were aged 19 or below. The text appears to suggest that matching was partial.
Interventions	Trivalent influenza vaccine (various suppliers) formulations were standardised to contain 15 µg each of A/H1N1/New Caledonia/ 20/99, A/H3N2/Wyoming/3/2003 (antigenically equivalent to A/H3N2/Fujian/411/2002), and B/Jiangsu/10/2003 strains.
Outcomes	Laboratory Specimens were swabs or nasal washouts on which PCR was used.
Funding Source	Government
Notes	The authors conclude that "We found age‐adjusted point estimates for VE against medical consultation for laboratory‐confirmed influenza A during the mismatched 2004 to 2005 season to range as low as 40% and as high as 75%. VE varied with age, definition of immunisation status and whether analysis was restricted to presentation within 48 hours of ILI onset. Overall, our estimates suggest cross‐protection for the 2004 to 2005 season despite vaccine mismatch. Our VE estimates mostly reflect the protection conferred to young healthy adults; the sample included few elderly persons or those with underlying conditions. The higher than expected reports of facility outbreaks in 2004 to 2005 in BC may have reflected an even lower VE amongst the frail elderly. Because of small sample size, estimates are unstable with wide confidence intervals. The possibility of no protection cannot be ruled out". Attrition, small sample size, recall and performance bias. High risk of bias
Risk of bias
Bias	Authors' judgement	Support for judgement
CC‐Case Selection	Low risk	Independent validation and representativeness series of cases
CC‐Control Selection	Low risk	Selected from the same population
CC‐Comparability	Unclear risk	Only sex and age adjustment
CC‐Exposure	High risk	No descriptions
Summary assessments	Unclear risk	Attrition, small sample size, recall and performance bias