We knew of some mutations from previous pandemics, but whether that would apply to other flu viruses nobody knows".
Previous pandemics have been caused only by H1- (the 1918 pandemic), H2- (the 1957 Asian flu) or H3-type viruses (the Hong Kong flu of 1968).
Improved access to anti-viral drugs and the roll out of the vaccination programme made any repeat of deaths and illness on the scale of previous pandemics unlikely.
But until today, it was unclear whether another fearsome feature of previous pandemics would also come in to play.
"Bacterial pneumonia may be contributing to influenza-associated mortality in a manner similar to that in previous pandemics," said epidemiologist Matthew Moore of the Centers for Disease Control and Prevention.
Recent work by Cauchemez found lower transmissibility of the 2009 pH1N1 virus in US households than in previous pandemics.
Since the R0 value of a new pandemic strain of influenza is unknown, we covered a range of R0 estimates for previous pandemics [7], [8], [13].
Experience with previous pandemics suggests that the younger age bias observed for 2009 H1N1 may shift in coming months to older persons.
Although disease severity in previous pandemics has been attributed to bacterial superinfection [3], early reports among H1N1pdm-infected individuals in developed countries failed to correlate severe disease with coinfection [2], [4], [5].
Whereas the use of RT-PCR in the current pandemic may yield a smaller denominator (and thus an overestimate of CFR compared to previous pandemics), other pandemics could have involved substantial numbers of false-positive cases in the denominator.
So, although we can learn from past experience, current response plans need to consider the possibility that the eventual pandemic diffusion profile may differ substantially geographically and temporally from previous pandemics.
