The UK government's influenza pandemic response plan can be found here [in PDF format].
Among the points made are that historically pandemics have come in waves:
Pandemic waves
In 1918/19 the A/H1N1 pandemic occurred in three distinct epidemic waves: early spring 1918, autumn 1918 and late winter 1919. The second wave was by far the largest and case-fatality rates were also higher than in the first wave12. The A/H3N2 pandemic caused an epidemic wave in the winter of 1968/69 but a more severe one in 1969/704. In contrast, the second wave of the 1957/58 pandemic in the UK was very small in comparison to the first3. Thus all planning should assume that more than one wave is possible (but not inevitable) and that a second wave could be worse than the first.
In the UK report, the epidemiology of the next pandemic is thought to be:
Summary of Epidemiology
1. The scale and severity of illness (and hence consequences) caused by
pandemic influenza generally exceed those of even the most severe winter
epidemics.2. Mortality in the UK is likely to exceed 50,000 deaths, possibly appreciably
higher.3. Besides the elderly, excess mortality is also likely in younger adults and
children.4. Modelling studies suggest that after a case occurs in Hong Kong, because of international travel, it may take 2-4 weeks for the virus to reach the UK.
5. Once cases begin to occur in the UK it will take only a few weeks before
activity is widespread6. It is possible that there will be more than one epidemic wave (with an interval of several months) and, if a second wave occurs, it may be more severe than the first.
7. Cumulative clinical and serological attack rates across all waves together may be in the order of 25% and 50% respectively.
