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Cycles in infections

  • 23 Dec 2022 5:41 PM
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    Reply # 13033923 on 13031784
    Chris Lloyd

    Terrific and clear answer Ken.

    I had this naïve idea that I took to the bank about 40 years ago that cycles are typically explained by (a) celestial rotations (b) some kind of negative feedback. The example I studied in detail was the prey-predator model.

    In the present case, the cycle is driven by a different dynamic but is still a kind of prey-predator model. The predator is the immune system and the prey is the virus. The prey changes occasionally, randomly and suddenly and the predator adapts but with a lag.

    I guess the other natural way to think of the cyclic graph is as a superposition of several single wave infections of a sequence of different but related diseases. 

    Thanks so much.
  • 22 Dec 2022 8:33 AM
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    Reply # 13032626 on 13031784
    Ken Beath

    Roughly the peaks represent the different variants. The NSW up to early November attached is clearer, maybe because the Australian has some spatial effects. First peak is original Omicron BA.1, then second strain BA.2 peaking in March, BA.5 in August. The missing variants are in the data but didn't have sufficiently greater infectiveness to produce a peak. There are also some artefacts due to start of school terms and changes in testing policies. The final peak started earlier than October but there was reduced testing due to changes in the use of RATs. The final peak is a mix of various descendants of BA.2 and some recombinant variants. 

    Generally each new variant evolves to escape the immune response to the previous variants. So each time we have a new peak as the population is closer to herd immunity for that variant.

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    Last modified: 22 Dec 2022 10:51 AM | Ken Beath
  • 21 Dec 2022 8:29 PM
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    Message # 13031784
    Chris Lloyd

    Here is a graphic of Covid infections in Australia over recent months. Does anyone know why there are cycles? There is no obvious prey-predator dynamic or negative feedback that I can think of.

    But I am sure that an epidemiologist reading this will have a ready answer. 

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