ca Ortqvist 2011.

Methods	Prospective cohort study carried out on Stockholm County inhabitants aged at least 6 months between week 44 and 52 in order to assess effectiveness of pandemic monovalent flu vaccine H1N1 (Pandemrix, GSK) in preventing laboratory‐confirmed H1N1 flu cases. Estimates were calculated by linking data from different databases: Sminet (for laboratory‐confirmed H1N1 flu cases), Vaccinera (on which data of vaccinated participants has been reported), Common Health‐Care Registers for Stockholm County Council (GVR, for detection of hospital admission cases due to influenza H1N1), Statistic Sweden (for demographical data).
Participants	Inhabitants of Stockholm County (2,019,183, out of which 449,971 were aged under 19 years)
Interventions	Pandemrix, split‐virion, inactivated, monovalent AS03‐adjuvanted vaccine A dose (0.5 mL) contained 3.75 lg of an influenza A/California/7/2009 (H1N1)v–like strain and an adjuvant composed of squalene, D‐alpha‐tocopherol, and polysorbate 80. 2 doses of 0.5 mL (participants aged at least 13 years) or 0.25 mL (participants between 3 and 12 years) were administered. Only participants who belonged to at‐risk categories (i.e. diabetes mellitus or pulmonary, heart, liver, renal, and immunocompromising disease) were initially recommended for vaccination in the 6 months to 3 years age class; from week 46 onwards this was extended to the whole age class. Immunisation campaign started in week 42. Data about vaccination are recorded in the Vaccinera database, where date of vaccination, batch number of the vaccine, the person’s unique identification number, medical risk group of vaccinated are reported. A flu case was considered vaccinated if diagnosis/hospital admittance occurred more than 14 days after administration of the first vaccine dose. 25 cases of confirmed H1N1 flu cases were observed between weeks 44 and 52 among participants who had received 1 or 2 vaccine doses at least 14 days before diagnosis or hospitalisation. Out of them, 11 (10) were aged between 6 months and 12 years.
Outcomes	Laboratory Not assessed Effectiveness Cases of laboratory‐confirmed H1N1 flu cases notified to the Institute for Infectious Diseases Control and available in the Sminet database occurred between week 42 and 52 of 2009. Incidence rate ratios for a given week were calculated comparing the rate of people who developed influenza > 14 days after being vaccinated out of the cumulated number of people who had been vaccinated up until 2 weeks before with the rate of people with an influenza diagnosis out of all non‐vaccinated people, excluding people who had had a previous influenza diagnosis. Safety Not assessed
Funding Source	Government: funding was provided by the County Council of Stockholm and by the Department of Communicable Diseases Control and Prevention, Stockholm County. Authors declare that they have no conflicts of interest.
Notes	Authors attempted to identify possible risk factors associated with vaccine failure in the study population, analysing incidence of several chronic conditions in cases (total cases of vaccine failures, n = 25) and in vaccinated controls (matched for age and vaccination date) using a case‐control design. For both chronic renal or hepatic disease and immunocompromised condition, a significant association was found (whole populations). As authors self note in the Discussion, “the sampling for the sampling for an influenza diagnosis was not made systematically but in routine medical care”. The authors conclude that “monovalent AS03‐adjuvanted influenza A(H1N1)v vaccine was very effective in preventing the pandemic influenza in Stockholm County”.
Risk of bias
Bias	Authors' judgement	Support for judgement
PCS/RCS‐Selection Exposed cohort	Unclear risk	Voluntary vaccinee
PCS/RCS‐Selection Non Exposed cohort	Unclear risk	Refuse the vaccination
PCS/RCS‐Comparability	High risk	Insufficient description
PCS/RCS‐Assessment of Oucome	Low risk	Secure record
Summary assessments	High risk	Insufficient description as to how exposed and not exposed were selected ‐ possible bias by indication