Top of Page How fast can a pandemic influenza strain spread?

• In a typical influenza season, we have as many as six months to prepare, and we have some knowledge of the circulating viruses. This allows for the production and delivery of vaccines against the circulating strains. • In a pandemic influenza outbreak, a new virus may reach the United States in hours or days because of international travel. Because few people have immunity to a new pandemic strain, it is likely to become widespread over a period of a few weeks. • In past pandemics, for example the 1918 Spanish flu, the outbreak occurred in several waves of infection. There might be a period of several weeks in which many people are ill, followed by a few weeks of fewer cases of illness, then another surge of illness. Planning must take into account the likelihood of more than one wave.  more