In 2000, an influenza vaccine was associated with unusual ocular and respiratory symptoms (known as “oculorespiratory syndrome” [ORS]) that possibly were due to numerous microaggregates of unsplit viruses present in the product. We assessed the potential for an improved vaccine formulation (for use in 2001–2002) to cause ORS and other symptoms in adults, using a double-blind, randomized, crossover study design. Symptoms were ascertained 24 h after 622 doses of vaccine and 626 doses of saline placebo were injected. The risk of ORS was 6.3% after vaccine injection and 3.4% after placebo...

In 2000, an influenza vaccine was associated with unusual ocular and respiratory symptoms (known as “oculorespiratory syndrome” [ORS]) that possibly were due to numerous microaggregates of unsplit viruses present in the product. We assessed the potential for an improved vaccine formulation (for use in 2001–2002) to cause ORS and other symptoms in adults, using a double-blind, randomized, crossover study design. Symptoms were ascertained 24 h after 622 doses of vaccine and 626 doses of saline placebo were injected. The risk of ORS was 6.3% after vaccine injection and 3.4% after placebo injection, which yielded a significant vaccine-attributable risk of 2.9% (95% confidence interval, 0.6–5.2). ORS symptoms were mild. Significant differences in risk after injection of vaccine versus placebo existed for ocular soreness and/or itching (2.4%), coughing (1.6%), and hoarseness (1.2%). Vaccine-attributable general symptoms were infrequent. We conclude that certain mild oculorespiratory symptoms were triggered by an influenza vaccine that was otherwise minimally reactogenic and, hence, that such symptoms might be associated with influenza vaccines in general.