In cluster trials of interventions which target a subset of the at-risk population, contemporaneous event counts in a non-target group can be used in estimating the population direct effect if indirect effects are unlikely and not of interest, but the application of this approach has been limited despite efficiencies over conventional methods. We describe the rationale for using the contemporaneous counts approach in the WHO’s Malaria Vaccine Programme Evaluation and how it was applied, and in a re-analysis of a human rotavirus vaccine trial, we demonstrate its application and compare results to the conventional method of using event rates per person-time in the target group. The intervention effect is estimated by comparing between arms the ratio of event counts in the target group to the non-target group. Expanding surveillance into a non-target group may often be possible without significant additional costs. Inclusion of person-time denominators may lead to a relatively small further gain in efficiency, which may not justify their measurement. Confounding due to chance imbalance  is reduced implicitly insofar that there is better balance in the relative difference between the target and non-target groups than in the variable itself within the target group alone. The approach can improve efficiency when target-group event counts are more strongly correlated with non-target-group counts than they are with target-group person-time, which is more likely as heterogeneity increases. The contemporaneous counts approach can increase the efficiency and internal validity of cluster trials and reduce their operational complexity since person-time denominators are not used.