Dáil Éireann Debate, Tuesday - 27 July 2021
2247. Deputy Róisín Shortall asked the Minister for Health if he will outline the underlying assumptions in the NPHET models in respect of the hospitalisation projections for the delta variant under each of the four scenarios and the reason these are so different to the experience with delta in the UK; and if he will make a statement on the matter. [37154/21]
View answerThe modelling scenarios begin with some basic assumptions on the effective level of close social contact and the increased transmissibility of the delta variant. The level of close social contact is assumed to increase either to the level seen in early summer 2020 (moderate social contact) or late summer 2020 (higher social contact). The transmissibility of the delta variant is uncertain, so the scenarios use conservative estimates (where the delta variant is 1.97 times more transmissible than the variants circulating in 2020, based on estimates from ECDC) or higher estimates (where delta is 2.4 times more transmissible, based on published UK estimates of the relative transmission advantages of the alpha and delta variants.
This gives four scenarios for transmission, levels of infection and case numbers: optimistic (moderate social contact, conservative transmission advantage for delta), central 1 (higher social contact, conservative transmission advantage for delta), central 2 (moderate social contact, higher transmission advantage for delta), and pessimistic (higher social contact, higher transmission advantage for delta).
We know that the widespread administration of vaccines means that there will be fewer infections, and that infections are less likely to lead to severe disease, hospitalisation and mortality. The models assume that the first dose of a vaccine is approximately 60% effective in preventing symptomatic infection with the alpha variant, and the second dose increases effectiveness in preventing symptomatic infection to 80-90%. Furthermore, one dose of vaccine is assumed to offer 70-85% protection against severe disease, hospitalisation and death due to infection with the alpha variant, with a second dose increasing vaccine effectiveness against severe disease to 90-95%.
Vaccines are known to be less effective in preventing symptomatic infection with delta, and this reduction in vaccine efficacy is taken into account in projecting the number of infections and cases in each scenario. However, the available evidence suggests that vaccines are equally effective in preventing severe disease with delta compared to alpha, and this is assumed to be the case in the models.
When all these factors are taken into account, on average per 1000 cases, the models project 15-25 hospitalisations, 2-3 admissions to ICU, and 2-4 deaths. These ratios are comparable to what is being seen in the UK and other jurisdictions.
