Monkeys vaccinated with the attenuated A2497P⁻ strain are protected against a challenge infection with virulent A2497 organisms. Vaccinated animals and naive unvaccinated controls were challenged 4 wk after the last vaccination. (A) Six unvaccinated control monkeys (individual data points are plotted). (B) Six A2497P⁻ vaccinated monkeys (individual data points are plotted). Solidly protected monkeys were (RML 641, 642, and 647) culture negative and developed no ocular pathology after challenge. The three partially protected monkeys (RML 145, 147, and 643) had reduced chlamydial infection burdens; however, this reduction did not significantly alter the intensity of ocular inflammation. Chlamydial burden is the geometric mean of recoverable IFU for both eyes of individual monkeys monitored by culture from conjunctival swabs. Clinical disease score is the aggregate clinical score (hyperemia and follicle formation) for both eyes of individual monkeys. The experiment was done once.