Modelling the dynamics of infection, waning of immunity and re-infection with the Omicron variant of SARS-CoV-2 in Aotearoa New Zealand

Executive summary

• Waning of infection-derived immunity means people become increasingly susceptible to being re-infected with SARS-CoV-2 over time. This waning creates the possibility of subsequent waves of infection, even after substantial population exposure, driven largely by re-infections.
• We investigated a model that included the effects of waning of vaccine-derived and infection-derived immunity under scenarios representing different levels of behavioural change relative to the first Omicron wave, and varying assumptions about the durability of infection-derived immunity, which is uncertain.
• In all scenarios investigated, a second wave of infections occurred in the second half of 2022. The timing of the peak of the second wave depended primarily on the speed at which immunity wanes following infection and was typically between August and November 2022 in the scenarios tested.
• The peak number of daily cases in the second wave was smaller than in the first wave.
• The peak hospital occupancy in the second wave was also generally smaller than in the first wave, but this was sensitive to the number of infections occurring in older age groups. A scenario with increased contact rates in older groups led to peak hospital occupancy higher than in the first wave.
• Scenarios with relatively high transmission rates (whether a result of relaxation of public health measures or voluntary behaviour change) did not necessarily lead to higher peaks. However, they generally resulted in more cumulative cases and sustained demand on healthcare systems (>250 hospital beds occupied throughout the winter period).
• The estimated health burden of Covid-19 in the medium term is sensitive to the strength and durability of infection-derived and hybrid immunity against severe illness, which are uncertain.

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