"During the early stages of a new influenza pandemic, travel restriction is an immediate and non-pharmaceutical means of retarding incidence growth. It extends the timeframe of effective mitigation, especially when the characteristics of the emerging virus are unknown. Chong and Chung (2012) describe a study in which they used the 2009 influenza A pandemic as a case study to evaluate the impact of regulating air, sea, and land transport. Other government strategies, namely, antivirals and hospitalizations, were also evaluated."
Having laboriously constructed graphics from published material on the spread of pandemic influenza viruses in 1918, 1957 and 1968, I can understand why this should be so: the 1968 flu spread like lightning compared to the 1957 and 1918 pandemics, which were still mainly carried by ship and by road - whereas far more frequent air travel in 1968 took the virus around the world in a few weeks.
Pandemic influenza virus graphic by Russell Kightley Media