Trachoma vaccines are usually assayed by testing their ability to protect monkeys or baboons against subsequent challenge of the conjunctiva with a pathogenic strain of trachoma/inclusion conjunctivitis (TRIC) agent. In such experiments the course of infection in vaccinated baboons was compared in terms of arbitrary scores assigned to a range of clinical signs, and of counts of TRIC inclusions in conjunctival scrapings. Analysis of many such scores indicated that after a large challenge dose of strain MRC-4s, the scores for signs of inflammation reached their maximum earlier than the follicle score; the inflammation score was closely related to the number of inclusions, whereas the follicle score was not. With this system, the optimum periods for eliciting differences between vaccinated and control measures varied according to the sign used; it was later for follicles than for inflammation or inclusions. For assessing the influence of vaccination, the mean of the inflammation scores read weekly for the first 3 weeks after challenge and the mean inclusion score over the same period were equally satisfactory, and either was rather better than the mean of three follicle scores taken over the period 3–6 weeks.

For assessing the influence of vaccines or therapeutic agents on experimental trachoma it is important to determine which signs discriminate best between treated and control animals, and the optimum times for measuring them.