The Truth About Natural Cold Remedies says that chicken soup is probably ineffective for actually preventing a cold, but once you have a cold, it's pretty effective for treating the symptoms - which of course we already knew:
Science has begun to support what mothers have understood for centuries. The heat, salt, and hydration provided by chicken soup may actually fight the cold virus (NIH, 2012). Laboratory studies have shown that ingredients of a chicken soup with vegetables could kill viral cells and prevent the growth of new ones (Rennard, et al., 2000). The soup may also provide an anti-inflammatory effect in the upper respiratory tract that helps soothe symptoms (Rennard, et al., 2000). Unfortunately, the benefits of chicken soup appear to be limited by how quickly the soup leaves the body (Babizhayev, et al., 2012). There is no indication that chicken soup would help in the prevention of colds, but its use as a remedy for symptoms is common. It is largely a safe remedy with no ill side effects, and scientific evidence points more in favor of its cold-fighting properties than against it.
Read More: The Truth About Natural Cold Remedies at HealthlineRelated: What Was Your Mom's Home Remedy for a Cold? (Images: 1. Emma Christensen; 2. Healthline)