Rather than focus on particular events or meetings, state health officials are looking at the big picture to develop comprehensive policies to control the outbreak, said Osterholm, who has been in frequent contact with Washington state health leaders.

Half of the states plus the District of Columbia require screenings or exams for preschoolers, according to the National Center for Children’s Vision & Eye Health. But California has no such requirement, said Xuejuan Jiang, an assistant professor of research ophthalmology at Keck School of Medicine of USC. California does require them for older children.