The dynamics of outbreaks and pandemics
Seasonal cycles, natural selection and the success of control efforts all play a role
It is notable that past flu pandemics, including Spanish flu, occurred in waves, with later waves sometimes being more severe than earlier ones. This was also a feature of the 2009 pandemic.
In part this reflects the fact that flu is traditionally spread during winter months (hence ‘seasonal flu’). The strongest factor underlying this seasonality appears to be absolute humidity, which is lower in the winter in temperate climates. Although seasonal effects are less pronounced for pandemic influenza, there is evidence that it is also affected by humidity.
Superimposed on such biological effects are behavioural and cultural factors that affect the spread of disease, such as the opening of schools after holidays.
Waves of infection
In the UK the H1N1 pandemic occurred in a series of waves, the first peaking in July 2009. A second wave began in mid-September 2009, as schools reopened. A third wave was seen during the winter of 2010–11. This third wave caused more severe disease and tended to affect adults rather than young people.
At the same time, the virus is not standing still but evolving. Sequencing of viral genomes has provided insight into different circulating strains, and has also suggested that the second wave was seeded by strains from the first wave that somehow survived, rather than being reintroduced.
Indeed, the second- and third-wave viruses showed increasing signs of adaptation to human hosts. The virus underwent multiple changes that enabled it to infect and multiply better within human cells – natural selection acting to increase its ‘fitness’. These adaptive changes may have enabled the 2009 H1N1 virus to persist beyond the pandemic and become established as a seasonal circulating strain in the UK.
- Absolute humidity and the seasonal onset of influenza in the continental United States (2010)
- Influenza A(H1N1)pdm09 in England, 2009 to 2011: a greater burden of severe illness in the year after the pandemic (2012)
- Evolutionary dynamics of local pandemic H1N1/2009 influenza virus lineages revealed by whole-genome analysis (2012)
- Accumulation of human-adapting mutations during circulation of A(H1N1)pdm09 influenza virus in humans in the UK (2014)
- Information on swine flu (H1N1) from the NHS