ca Ortqvist 2011

Methods	Prospective cohort study carried out on Stockholm Country inhabitants aged at least 6 months between week 44 and 52 in order to assess effectiveness of pandemic monovalent flu vaccine H1N1 (Pandremix by GSK) in preventing laboratory‐confirmed H1N1 flu cases. Estimates were calculated by linking data from different database: Sminet (for laboratory‐confirmed H1N1 flu cases), Vaccinera (on which data of vaccinated participants has been reported), Common Health‐Care Registers for Stockholm Country Council (GVR, for detect hospital admission cases due to Influenza H1N1), Statistic Sweden (for demographical data)
Participants	Inhabitants of Stockholm country (2,019,183 out of which 449,971 were aged under 19 years)
Interventions	Pandemrix (GSK), 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, DL‐a‐tocopherol and polysorbate 80 2 doses of 0.5 ml (participants aged at least 13 years) or of 0.25 ml (between 3 and 12 years) were administered. Only participants who belong 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 1st vaccine dose. Twenty‐ five cases of confirmed H1N1 flu cases had been observed between weeks 44 and 52 among participants who 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 persons who developed influenza > 14 days after being vaccinated out of the cumulated number of persons who had been vaccinated up until 2 weeks before with the rate of persons with an influenza diagnosis out of all non‐vaccinated persons, excluding persons 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 how exposed and not exposed was selected ‐ possible bias by indication