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Figure 1 | BMC Infectious Diseases

Figure 1

From: Prior immunity helps to explain wave-like behaviour of pandemic influenza in 1918-9

Figure 1

Model for unobserved processes. Before wave 1, people are either susceptible (S 1 ) or resistant (R 1 ) because of prior immunity. Persons exposed to the virus either express symptoms (E 1 ), or have an asymptomatic infection (A 1 ). Others are unexposed (U 1 ). After exposure, persons become immune (E 1 R or A 1 R), and a proportion become susceptible again prior to wave 2 (E 1 S 2 , A 1 S 2 and R 1 S 2 , plus the susceptible persons who escaped exposure in wave 1 (U 1 S 2 ). All susceptible persons are at risk of exposure in wave 2 (see figure), and either express symptoms, have an asymptomatic infection, or remain unexposed. The extension to wave 3 adds an additional layer of complexity, but there are no new principles invoked.

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