def vida_pop(cluster, start_age, end_age, input_db):
    """Returns the denominator as per Vida's population numbers
    for a given health cluster and age band"""
    pop = input_db.query(
        "vida_pop",
        columns=["popn"],
        conditions={            
            "cluster_id": cluster,
            "start_age>": start_age,
            "end_age<": end_age,
        },
    )
    pop = pop.popn.sum()
    return pop