Sample size: how many participants are needed in a cohort study?

Researchers estimated the prevalence of pertussis infection in school age children after implementation of the preschool pertussis booster vaccination in the UK in October 2001. A prospective cohort study design was used. Children aged 5-15 years who presented with persistent cough in primary care were recruited. Recruitment took place between November 2010 and December 2012. The primary outcome was pertussis infection, as diagnosed by an oral fluid anti-pertussis toxin IgG titre.1

The sample size was based on an anticipated 20% prevalence of pertussis in the study population. The precision of the estimate needed to be within five percentage points as assessed by the 95% confidence interval for the population prevalence—that is, a 95% confidence interval of 15% to 25%. The required sample size was 246. To allow for a potential 20% failure rate in obtaining oral fluid samples with sufficient total IgG for analysis, the required sample size was increased to 300.

In total, 294 children were recruited of whom 279 (94.9%) had sufficient total IgG for analysis. It was reported that 56 (20.1%, 95% confidence interval 15.4% to 24.8%) children had evidence of recent pertussis infection, including 39 (18.1%, 13.0% to 23.3%) of 215 children who had been fully vaccinated. It was concluded that a fifth of school age children who present in primary care …