Table 1

Design, sample size and recruitment criteria for studies included in this overview

ReferenceYearCountryAgeDesignOutcomeSample sizeCase definitionSource of cohort, controls or series
Incidence studies:
4, 5 85N Zealand0–6CohortIncidence (asthma)1032Reported asthmaPopulation-based birth cohort
6 89UK1–10CohortIncidence (wheeze)9670Reported wheezeNational birth cohort
7 89USA (AZ)0–17CohortIncidence (asthma)739PD asthmaRandom household sample
8 90USA (MA)5–18CohortIncidence (asthma)722PD asthmaSchools-based cohort
9,14 95USA (AZ)0–6CohortIncidence (wheeze)762Reported wheezeHMO-based birth cohort
10,11 95UK0–5CohortIncidence (wheeze)12530Reported wheezeNational birth cohort
12,13 96UK0–16CohortIncidence (as/WB)4583Reported asthma/WBNational birth cohort
Natural history studies:
15 84USA (NY)0–9CohortPrognosis (early)236Wheeze 8y laterBronchiolitis before age 2
16 86USA (NY)0–2CohortPrognosis (early)27Recurrent wheezeParainfluenza bronchiolitis
17 87Israel0–5CohortPrognosis (early)80Persistent wheeze at age 5Atopic wheezy infants
18 87Japan1–4CohortPrognosis (early)48Wheeze 22–44 months laterWheezy infants
19 88Sweden0–7CohortPrognosis (early)67Wheeze 4 years laterRSV+ illness before age 3
9 95USA (AZ)0–6CohortPrognosis (early)247Wheeze at age 6Wheeze before age 3
10 95UK5–16CohortPrognosis (later)1477Wheeze at age 16Wheeze before age 5
20 95UK7–23CohortPrognosis (later)1090Asthma/WB at 11, 23Asthma/WB before age 7
Case-control studies:
21 74USA (MN)2–16C–CAsthma (OP)628Asthmatic OPOther OP (no atopic disease)
22 90Italy1–12C–CAsthma (OP)735Asthmatic OPRoutine health check
23 91USA (NY)0–17C–CAsthma (IP/OP)383Admitted or 2 OP visitsPrivate paediatric practice
24 91Sweden3–15C–CAsthma (OP)126New OP referralsTwo local schools
25 92USA (NY)3–14C–CAsthma (ER)114ER visit for asthmaOther ER patients
26 92USA (MD)c. 9C–CAsthma (OP)346Asthmatic OPPrivate paediatric practice
27 93Canada3–4C–CAsthma (IP)914First ER asthma visitPopulation sample
28 94Nigeriac. 5.5C–CWheeze (OP)280Wheezy OP (no FH)Neighbours
29 94Ireland5–11C–CAsthma (survey)211Asthma reportedPopulation survey
30 94UK5–7C–CAsthma (OP)62Asthmatic OPSurgical OP
31 94Israel3–15C–CWheeze (OP)400Asthma treatmentNeighbours
32 95Malaysia0–5C–CAsthma (IP)359First asthma admissionNon-respiratory admissions
33,40 95Sweden1.5–4C–CWB (IP)212WB treated in hospitalRandom population sample
34 95Sweden1–4C–CAsthma (OP)511Asthma OP referralRandom population sample
35 95UK12–18C–CWheeze (survey)961Frequent/severe wheezePopulation survey (no wheeze)
36 95USA (NC)7–12C–CWheeze (OP)342⩾2 Wheeze attacksPaediatric clinic sample
37 96S Africa7–9C–CAsthma/wheeze (survey)620Asthma symptomsPopulation survey (no wheeze)
38 96UAE6–18C–CAsthma (survey)406PD asthma on therapySchool classmates
39 96USA (OH)4–9C–CAsthma (OP)262PD asthma on therapyAdjacent birth records
41 * 93USA (VA)2–16C–CWheeze (ER)114ER visit for asthma/BLOther ER patients
42 * 87UK0–5C–CWheeze (OP)1285⩾3 Wheeze attacksNeonates in locality
44 * 90Canada1–17C–CAsthma (OP)620Asthma diagnosedAllergy clinic patients
Case series:
45 82Nigeria1–13CasesRelative severity380Severity scoreAsthmatic outpatients
46 82USA (MI/MA)0–17SurveyRelative severity272Functional impairmentCurrent asthma (reported)
47 87USA (NY)4–17CasesRelative severity276ER visits per yearAsthmatic outpatients
48 89Canada1–17CasesRelative severity415Severity scoreAsthmatic outpatients
49 90USA (all)2–5SurveyRelative severity99Asthma medicationCurrent asthma (reported)
50 90USA (all)0–5SurveyRelative severity117HospitalisationsCurrent asthma (reported)
51 92Canada1–17CasesRelative severity807Severity scoreAsthmatic outpatients
52 93USA (ME)0–13CasesRelative severity199Attack frequencyAsthmatic outpatients
53 95USA (CA)5–12CasesRelative severity300IntubationAsthmatic inpatients
54 96Canada1–10CasesRelative severity68Readmission within 1 yearAsthmatic inpatients
Other studies:
43 * 96Canada6–17SurveyRecent asthma892PD asthma and symptomsSurvey 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.