Why match in case-control studies?

Researchers investigated the effectiveness of a monovalent rotavirus vaccine against severe rotavirus diarrhoea in children aged under 2 years. A case-control study design was used. In total, 323 children were recruited from seven hospitals in El Salvador after admission with laboratory confirmed rotavirus diarrhoea. For each case three controls, matched for age (within 30 days of date of birth) and neighbourhood, were recruited.1

Vaccination history was confirmed after inspection of vaccination cards held by the parents. Potential risk factors—including demographics, socioeconomic factors, birth weight, premature birth, current body mass index, history of breast feeding, day care attendance and medical history—were collected from hospital records or in interviews with parents. The researchers concluded that the monovalent rotavirus vaccine was highly effective against admissions for rotavirus diarrhoea in children aged under 2 years in El Salvador. No differences were reported between cases and controls in breastfeeding patterns, premature birth, maternal education, or …