Design, sample size and recruitment criteria for studies included in this overview
| Reference | Year | Country | Age | Design | Outcome | Sample size | Case definition | Source of cohort, controls or series |
|---|---|---|---|---|---|---|---|---|
| Incidence studies: | ||||||||
| 4, 5 | 85 | N Zealand | 0–6 | Cohort | Incidence (asthma) | 1032 | Reported asthma | Population-based birth cohort |
| 6 | 89 | UK | 1–10 | Cohort | Incidence (wheeze) | 9670 | Reported wheeze | National birth cohort |
| 7 | 89 | USA (AZ) | 0–17 | Cohort | Incidence (asthma) | 739 | PD asthma | Random household sample |
| 8 | 90 | USA (MA) | 5–18 | Cohort | Incidence (asthma) | 722 | PD asthma | Schools-based cohort |
| 9,14 | 95 | USA (AZ) | 0–6 | Cohort | Incidence (wheeze) | 762 | Reported wheeze | HMO-based birth cohort |
| 10,11 | 95 | UK | 0–5 | Cohort | Incidence (wheeze) | 12530 | Reported wheeze | National birth cohort |
| 12,13 | 96 | UK | 0–16 | Cohort | Incidence (as/WB) | 4583 | Reported asthma/WB | National birth cohort |
| Natural history studies: | ||||||||
| 15 | 84 | USA (NY) | 0–9 | Cohort | Prognosis (early) | 236 | Wheeze 8y later | Bronchiolitis before age 2 |
| 16 | 86 | USA (NY) | 0–2 | Cohort | Prognosis (early) | 27 | Recurrent wheeze | Parainfluenza bronchiolitis |
| 17 | 87 | Israel | 0–5 | Cohort | Prognosis (early) | 80 | Persistent wheeze at age 5 | Atopic wheezy infants |
| 18 | 87 | Japan | 1–4 | Cohort | Prognosis (early) | 48 | Wheeze 22–44 months later | Wheezy infants |
| 19 | 88 | Sweden | 0–7 | Cohort | Prognosis (early) | 67 | Wheeze 4 years later | RSV+ illness before age 3 |
| 9 | 95 | USA (AZ) | 0–6 | Cohort | Prognosis (early) | 247 | Wheeze at age 6 | Wheeze before age 3 |
| 10 | 95 | UK | 5–16 | Cohort | Prognosis (later) | 1477 | Wheeze at age 16 | Wheeze before age 5 |
| 20 | 95 | UK | 7–23 | Cohort | Prognosis (later) | 1090 | Asthma/WB at 11, 23 | Asthma/WB before age 7 |
| Case-control studies: | ||||||||
| 21 | 74 | USA (MN) | 2–16 | C–C | Asthma (OP) | 628 | Asthmatic OP | Other OP (no atopic disease) |
| 22 | 90 | Italy | 1–12 | C–C | Asthma (OP) | 735 | Asthmatic OP | Routine health check |
| 23 | 91 | USA (NY) | 0–17 | C–C | Asthma (IP/OP) | 383 | Admitted or 2 OP visits | Private paediatric practice |
| 24 | 91 | Sweden | 3–15 | C–C | Asthma (OP) | 126 | New OP referrals | Two local schools |
| 25 | 92 | USA (NY) | 3–14 | C–C | Asthma (ER) | 114 | ER visit for asthma | Other ER patients |
| 26 | 92 | USA (MD) | c. 9 | C–C | Asthma (OP) | 346 | Asthmatic OP | Private paediatric practice |
| 27 | 93 | Canada | 3–4 | C–C | Asthma (IP) | 914 | First ER asthma visit | Population sample |
| 28 | 94 | Nigeria | c. 5.5 | C–C | Wheeze (OP) | 280 | Wheezy OP (no FH) | Neighbours |
| 29 | 94 | Ireland | 5–11 | C–C | Asthma (survey) | 211 | Asthma reported | Population survey |
| 30 | 94 | UK | 5–7 | C–C | Asthma (OP) | 62 | Asthmatic OP | Surgical OP |
| 31 | 94 | Israel | 3–15 | C–C | Wheeze (OP) | 400 | Asthma treatment | Neighbours |
| 32 | 95 | Malaysia | 0–5 | C–C | Asthma (IP) | 359 | First asthma admission | Non-respiratory admissions |
| 33,40 | 95 | Sweden | 1.5–4 | C–C | WB (IP) | 212 | WB treated in hospital | Random population sample |
| 34 | 95 | Sweden | 1–4 | C–C | Asthma (OP) | 511 | Asthma OP referral | Random population sample |
| 35 | 95 | UK | 12–18 | C–C | Wheeze (survey) | 961 | Frequent/severe wheeze | Population survey (no wheeze) |
| 36 | 95 | USA (NC) | 7–12 | C–C | Wheeze (OP) | 342 | ⩾2 Wheeze attacks | Paediatric clinic sample |
| 37 | 96 | S Africa | 7–9 | C–C | Asthma/wheeze (survey) | 620 | Asthma symptoms | Population survey (no wheeze) |
| 38 | 96 | UAE | 6–18 | C–C | Asthma (survey) | 406 | PD asthma on therapy | School classmates |
| 39 | 96 | USA (OH) | 4–9 | C–C | Asthma (OP) | 262 | PD asthma on therapy | Adjacent birth records |
| 41 * | 93 | USA (VA) | 2–16 | C–C | Wheeze (ER) | 114 | ER visit for asthma/BL | Other ER patients |
| 42 * | 87 | UK | 0–5 | C–C | Wheeze (OP) | 1285 | ⩾3 Wheeze attacks | Neonates in locality |
| 44 * | 90 | Canada | 1–17 | C–C | Asthma (OP) | 620 | Asthma diagnosed | Allergy clinic patients |
| Case series: | ||||||||
| 45 | 82 | Nigeria | 1–13 | Cases | Relative severity | 380 | Severity score | Asthmatic outpatients |
| 46 | 82 | USA (MI/MA) | 0–17 | Survey | Relative severity | 272 | Functional impairment | Current asthma (reported) |
| 47 | 87 | USA (NY) | 4–17 | Cases | Relative severity | 276 | ER visits per year | Asthmatic outpatients |
| 48 | 89 | Canada | 1–17 | Cases | Relative severity | 415 | Severity score | Asthmatic outpatients |
| 49 | 90 | USA (all) | 2–5 | Survey | Relative severity | 99 | Asthma medication | Current asthma (reported) |
| 50 | 90 | USA (all) | 0–5 | Survey | Relative severity | 117 | Hospitalisations | Current asthma (reported) |
| 51 | 92 | Canada | 1–17 | Cases | Relative severity | 807 | Severity score | Asthmatic outpatients |
| 52 | 93 | USA (ME) | 0–13 | Cases | Relative severity | 199 | Attack frequency | Asthmatic outpatients |
| 53 | 95 | USA (CA) | 5–12 | Cases | Relative severity | 300 | Intubation | Asthmatic inpatients |
| 54 | 96 | Canada | 1–10 | Cases | Relative severity | 68 | Readmission within 1 year | Asthmatic inpatients |
| Other studies: | ||||||||
| 43 * | 96 | Canada | 6–17 | Survey | Recent asthma | 892 | PD asthma and symptoms | Survey of complete town |
PD = physician diagnosed; WB = wheezy bronchitis; BL = bronchiolitis; RSV = respiratory syncytial virus; ER = emergency room; OP = outpatients; IP = inpatients; FH = family history. *Not included in the meta-analysis of case-control studies (table 3) but discussed in the text.