Q: Is there a connection between shellfish allergies and iodine?
Equating an allergy to shellfish with an allergy to iodine is a fairly common misconception. Fish and radiographic contrast allergies are also erroneously equated with iodine allergies.
Iodine is essential for proper thyroid function. Without it, people become ill with thyroid problems. As it turns out, seafood and crops fertilized with seaweed are a good source of iodine. As people moved from coastal areas inland, the incidence of thyroid deficiency increased. Beginning in the 20th century, it became a common additive in many varieties of table salt. As a result, now you have to work hard to completely avoid it.
The allergen in shellfish is a protein, not iodine. Some people with iodine allergies really have a topical sensitivity to iodine (e.g., povidone iodine; Betadine), usually a much different kind of reaction than the immediate reaction found with anaphylaxis.
Bottom line: A shellfish allergy should almost never preclude the use of iodine for water disinfection. If you are concerned, get more information about the true nature of the allergy. If the person has not had problems with other seafood (saltwater) or table salt, iodine is not the culprit. There are, of course, other reasons for not using iodine as a water disinfectant.