Link-Gelles R, et al. - This trial dealt with the evaluation of the generalisability, in terms of socioeconomic status, of estimates from a matched case–control study of 13-valent pneumococcal conjugate vaccine effectiveness for the prevention of invasive pneumococcal disease, in children in the USA. Despite the affluency of the enrolled cases than unenrolled cases, the estimates were disclosed to be externally valid with respect to socioeconomic status. This study design could provide valid estimates. The generalisability of observational data was found to be feasible, even when unenrolled individuals could not be contacted.