Infectious disease modelling currently informing US and UK governments

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   Don Wposted 10 months ago

   The epidemiological modelling currently informing US and UK government policy can be found here:
   
   https://www.imperial.ac.uk/media/imperi … 3-2020.pdf
   
   Some key points:
   
   1. The Infection Fatality Ratio, aka Case Fatality Rate, is around 0.9% (for the UK) when corrected for demographics such as age. This may be different for the US depending on demographics and other variables.
   
   "The age-stratified proportion of infections that require hospitalisation and the infection fatality ratio (IFR) were obtained from an analysis of a subset of cases from China. These estimates were corrected for non-uniform attack rates by age and when applied to the GB population result in an IFR of 0.9% with 4.4% of infections hospitalised (Table 1)..."(1)
   
   Note, the fatality rate can be affected by the age of the population, availability of hospital care, standard of care etc, so will vary by country and regions within countries.
   
   Therefore some countries/regions will have far greater fatality rates than others, reflecting the different variables in each country/region. Based on standard of care available in the US, the fatality rate should be relatively low. The fatality ratio will be most affected by how overwhelmed the healthcare system becomes.
   
   2. Mitigation alone will not be enough to prevent emergency surge capacities (the maximum care capacity for the healthcare system) being exceeded. The modelling shows that suppressing infections is the only way to flatten the peak of the epidemic sufficiently, and suppression measures need to start relatively quickly to be most effective.
   
   "Perhaps our most significant conclusion is that mitigation is unlikely to be feasible without emergency surge capacity limits of the UK and US healthcare systems being exceeded many times over. In the most effective mitigation strategy examined, which leads to a single, relatively short epidemic (case isolation, household quarantine and social distancing of the elderly), the surge limits for both general ward and ICU beds would be exceeded by at least 8-fold under the more optimistic scenario for critical care requirements that we examined. In addition, even if all patients were able to be treated, we predict there would still be in the order of 250,000 deaths in GB, and 1.1-1.2 million in the US . . .
   
   We therefore conclude that epidemic suppression is the only viable strategy at the current time. The social and economic effects of the measures which are needed to achieve this policy goal will be profound. Many countries have adopted such measures already, but even those countries at an earlier stage of their epidemic (such as the UK) will need to do so imminently"(2).
   
   3. Infection suppression measures will need to be kept in place for a significant period of time, to prevent a "second wave" of infection. The following graph shows the projected result of increasingly intrusive suppression strategies relative to Intensive Care Unit (ICU) bed capacity. The last two strategies keep the peak of the epidemic below the surge capacity, assuming they are kept in place for five months. With all interventions, a "second wave" of infection exceeding the surge capacity is projected after the suppression measures are lifted (in 5 months). So suppression measures would need to be kept in place (perhaps on and off) until a vaccine is developed, which could be more than 18 months.
   
   (3)
   
   "The more successful a strategy is at temporary suppression, the larger the later epidemic is predicted to be in the absence of vaccination, due to lesser build-up of herd immunity..."(4).
   
   "To avoid a rebound in transmission, these policies will need to be maintained until large stocks of vaccine are available to immunise the population – which could be 18 months or more"(5)
   
   4. It's uncertain whether suppression measures will be effective in the long term due to the difficulty in maintaining them. The current situation is unprecedented and the effectiveness of them relies on the willing cooperation of a significant proportion of the population.
   
   "However, we emphasise that is not at all certain that suppression will succeed long term; no public health intervention with such disruptive effects on society has been previously attempted for such a long duration of time. How populations and societies will respond remains unclear"(6)
   
   I'd suggest reading the report to get a fuller picture of the advice received by the US and UK governments, which may give some insight into how closely decisions made by government reflect scientific advice, and how that advice is being used to inform political decisions (or not).
   
   (1) https://www.imperial.ac.uk/media/imperi … 3-2020.pdf (p.5)
   (2) ibid (p.16)
   (3) ibid (p.19)
   (4) ibid (p.11)
   (5) ibid (p.15)
   (6) ibid (p.16)

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