One side effect of a yellow fever immunization is listed as "throbbing ears". Though that doensn't seem like a common immunization to China/Cuba travel recommendations.
[1] https://www.nytimes.com/2014/02/18/health/the-answer-to-the-...
>Results Of 24 individuals with suspected exposure identified by the US Department of State, 21 completed multidisciplinary evaluation an average of 203 days after exposure. Persistent symptoms (>3 months after exposure) were reported by these individuals including cognitive (n = 17, 81%), balance (n = 15, 71%), visual (n = 18, 86%), and auditory (n = 15, 68%) dysfunction, sleep impairment (n = 18, 86%), and headaches (n = 16, 76%). Objective findings included cognitive (n = 16, 76%), vestibular (n = 17, 81%), and oculomotor (n = 15, 71%) abnormalities. Moderate to severe sensorineural hearing loss was identified in 3 individuals. Pharmacologic intervention was required for persistent sleep dysfunction (n = 15, 71%) and headache (n = 12, 57%). Fourteen individuals (67%) were held from work at the time of multidisciplinary evaluation. Of those, 7 began graduated return to work with restrictions in place, home exercise programs, and higher-level work-focused cognitive rehabilitation.
>Conclusions and Relevance In this preliminary report of a retrospective case series, persistent cognitive, vestibular, and oculomotor dysfunction, as well as sleep impairment and headaches, were observed among US government personnel in Havana, Cuba, associated with reports of directional audible and/or sensory phenomena of unclear origin. These individuals appeared to have sustained injury to widespread brain networks without an associated history of head trauma.
[0] https://jamanetwork.com/journals/jama/article-abstract/26731...
> The mystery spread to China this spring, when the first employee fell ill, and fears escalated last month when the government warned other employees to seek medical attention if they experienced unusual ailments. So far this week, another employee, his wife and their two children were evacuated after the parents exhibited neurological symptoms. Officials said they expected that at least some others would be flown out of the country as well.
Not a huge sample size to guarantee something is happening... but I'd imagine the US gov has looked into it pretty closely and isn't overreacting without some good evidence of deteriorated health.