We are told by some that because Australia survived their flu season, the impact of pandemic H1N1 on the Northern Hemisphere is likely to be mild. Let’s consider what did and did not happen in Australia to see if that is a reasonable assumption.
First, the numbers.
There were 179 deaths associated with the pandemic (H1N1) 2009 virus in Australia between 19 June 2009 and 25 September 2009.
The median age of confirmed cases that died is 51 years (range 2-86 years of age), compared to the median age for deaths from seasonal flu from 2001 to 2006 which is 83 years.
Of particular note is the difference in the age distribution of this novel influenza virus to seasonal influenza and the increasing median age as the severity of the disease progresses: 21 years for all confirmed cases; 31 years for hospitalised cases; 43 years for ICU cases; and 53 years for deaths.
The median number of annual deaths in Australia for the years 2001 to 2006 from influenza and pneumonia is 3,089 and for laboratory diagnosed influenza is 40.
Although mortality data from all causes are generally not available for the current year, some information on influenza and pneumonia deaths are reported by individual jurisdictions from their Births, Deaths and Marriages Registers. In Western Australia, pneumonia and influenza deaths accounted for approximately 14.5% of all deaths in the final week of August, which is below levels seen at the same time in 2007 and 2008 (Figure 13).
Let’s consider what these numbers mean.
First, the age of people who died was much younger than during a normal flu year; the median age for deaths is 51 for pandemic flu but 83 for seasonal flu. Life expectancy at birth in Australia is currently 81. Thus, seasonal flu likely has little impact on the life expectancy of most Australians. In sharp contrast, pandemic flu caused severe illness and killed much younger Australians who likely would otherwise have lived much longer. Thus, simply comparing numbers of deaths caused by seasonal and pandemic H1N1 is misleading.
Second, in a normal flu-year, there are only 40 laboratory-confirmed influenza deaths. This year, there were 179, 4 times as many. Although there did not appear to be more deaths in the broader pneumonia and influenza category, we should remember that the people dying in this group are primarily very elderly. The numbers of young people who died of pandemic flu in Australia vastly exceeds the number who would die in a normal flu year.
Third, many interventions which are not usually undertaken were applied during this year’s flu season. Schools were closed for at least some period of time. This has previously been shown to limit the spread of flu viruses which would thus protect older Australians from getting infected. In addition, Tamiflu was aggressively prescribed for patients exhibiting flu this past season. This likely decreased the death rate from pandemic flu but also may have broken chains of infection, which would also likely decrease the number of older Australians infected with flu.
What can we expect based on the Australian results? A much higher rate of death in younger people than with seasonal flu, something we have already seen in the US and elsewhere in the Northern Hemisphere. Australia was much more aggressive in treating patients with Tamiflu than the United States is. As a result, we can likely expect higher death rates both directly from lack of treatment and indirectly because we are less likely to interrupt chains of infection. This will result in more Americans getting infected and, consequently, more deaths.
Although the lessons of the Australian experience would appear obvious, there is no indication that the CDC has learned anything from it.