A pandemic response to a disease of predominantly seasonal intensity

To the Editor: Kelly showed that the pandemic (H1N1) 2009 influenza epidemic experience in Victoria was similar, overall, to a moderately severe influenza season.1 As with seasonal influenza, 0–4 year olds had the highest hospital admission rates,2,3 with those in the first year of life most likely to be admitted to intensive care units (ICUs).

“Swine flu” did have differences to seasonal flu, with “younger” people having disproportionally more serious illness.2,3 However, these younger groups were not necessarily what most would regard as young. Eighty per cent or more of deaths associated with pandemic (H1N1) 2009 influenza were among those aged over 35 years, and most had identifiable risk factors.2,3 The peak 5-year age group for ICU admissions was 50–54 years.2

Those aged over 65 years seemed relatively protected — presumably because most had pre-existing immunity resulting from infection with H1N1-type viruses circulating since 1918. Pre-existing immunity was also not uncommon in others. In 18–65-year-olds, 30% had protective antibody levels to pandemic (H1N1) 2009 influenza before vaccination.4

Using data from New South Wales, we can ascertain the severity of pandemic (H1N1) 2009 influenza in different age groups.2 In NSW last winter, 1214 people were hospitalised, 225 were admitted to ICUs and 48 died with pandemic (H1N1) 2009 influenza (17.2, 3.2 and 0.7 per 100 000 population, respectively). Pregnant women had a 10 times higher risk of death (1.4 per 100 000) than other women of their age. For the overall population aged under 40, the death rate was 0.4 per 100 000, with most having identifiable risk factors. Thus, the death rate for those aged under 40 years, but with no known risk factors, was about one per million people.

Most of the mortality predictions for this epidemic have been consistently wrong and exaggerated. Some were suggesting 10 000 deaths in NSW alone.5 These, and other predictions of second and third killer waves, have generated needless fear and inappropriate responses.6 Many experts and even health departments were postulating that more than 20% of the population would become infected, with an associated case fatality rate of 1% or more. That translates to a population mortality rate of 200 per 100 000 people, which is 300 times higher than what actually occurred.

We need to learn from our recent experience so we can better plan and act in the future. Analysis such as that performed by Kelly is essential if we do not want to repeat the mistakes we made during this pandemic, caused by a virus of relatively low virulence.