Epidemiologists are steeling themselves for the next influenza pandemic, and looking to the past for lessons about control measures. Hence the paper by Christina E. Mills and colleagues elsewhere in this issue (Nature 432, 904–906; 2004), in which the authors have looked at the transmission dynamics of the 1918 pandemic in the United States.
Global outbreaks of influenza occurred three times in the last century. The 1918 pandemic probably killed at least 20 million people: the flu strain responsible spread rapidly, and individuals who became infected were ten times more likely to die than was the case in other pandemics. No wonder people so feared infection — and, for example, that ‘influenza camps’ were set up, at Lawrence, Maine.
The warning signs are there for the future. Highly pathogenic strains of avian influenza have in very recent times crossed from birds to humans. As virologists Richard J. Webby and Robert G. Webster have put it (Science 302, 1519–1522; 2003): “An old foe has again raised its head, reminding us that our worst nightmare may not be a new one.”
How easy would it be to control a new influenza pandemic? By analysing data on deaths from pneumonia and influenza in 45 US cities during 1918, Mills et al. estimate the reproductive number (R), that is, the number of secondary cases produced by each primary case. This is a measure of viral transmissibility, and given the rapid spread of the 1918 pandemic one might expect it to be rather high. But it turns out that its transmissibility was actually quite modest, with each primary case giving rise on average to just three or four secondary cases.
In the absence of global stocks of antiviral drugs and vaccines, warn the authors, control of a similar influenza strain would probably require aggressive measures early in an outbreak to reduce contacts between persons, whether or not they had already been diagnosed with flu. If a variant virus cropped up with a similar pathogenicity but a much higher transmission rate, we could be in for real trouble.
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Howlett, R. Past and future of an old foe. Nature 432, 817 (2004). https://doi.org/10.1038/432817b
Nature Reviews Microbiology (2005)