Impact of non-pharmaceutical interventions (NPIs) to reduce COVID19 mortality and healthcare demand

By: Neil M Ferguson, Daniel Laydon, Gemma Nedjati-Gilani, Natsuko Imai, Kylie Ainslie, Marc Baguelin, Sangeeta Bhatia, Adhiratha Boonyasiri, Zulma Cucunubá, Gina Cuomo-Dannenburg, Amy Dighe, Ilaria Dorigatti, Han Fu, Katy Gaythorpe, Will Green, Arran Hamlet, Wes Hinsley, Lucy C Okell, Sabine van Elsland, Hayley Thompson, Robert Verity, Erik Volz, Haowei Wang, Yuanrong Wang, Patrick GT Walker, Caroline Walters, Peter Winskill, Charles Whittaker, Christl A Donnelly, Steven Riley, Azra C Ghani.

In: Report 9, the WHO Collaborating Centre for Infectious Disease Modelling within the MRC Centre for Global Infectious Disease AnalysisJ-IDEA, Imperial College London.

Date: 16 March, 2020

Summary

  • Analyzes the likely impact of public health measures on the spread and impact of COVID-19.
    • Simulates impact of a number of different public health measures in UK and US.
    • Measures analyzed, alone or in combination, include home isolation, home quarantine, social distancing and school/university closure.
    • Measures are generally aimed at reducing contact rate in the population and transmission of the virus.
    • Assumes there will not be a vaccine in the foreseeable future.
  • Focus of the simulation is to measure the impact on health / healthcare.
    • Not on broader economic and social impact or costs.
  • Key observations from the simulation scenarios:
    • Do nothing (unmitigated epidemic):
      • Peak mortality occurs after 3 months (early summer):
        • 510 thousand fatalities in UK.
        • 2 million fatalities in the US.
      • Peak critical care / ICU demand more than 30 times of current capacity.
      • Does not take into consideration impact of overwhelmed healthcare systems.
    • Mitigation strategy: not sufficient.
      • Flatten the curve to slow down the spread of the virus.
      • Involves home isolation of suspect cases, quarantining of case households, social distancing high risk individuals.
      • Reduces peak healthcare demand (2/3 reduction) and fatalities (50% reduction) substantially.
      • Does not avoid overloading the healthcare system.
    • Suppression is the only viable strategy.
      • Interrupt transmission of the virus, get to low case levels.
    • Minimum suppression policies involve a combination of:
      • Population-wide social distancing (largest single impact).
      • Home isolation of cases.
      • School/university closures (to break contacts between households).
    • Suppression may keep peak demand on healthcare facilities within capacity.
      • Reduction of peak demand by up to 95%.
      • Reduction of fatalities by about 80-90%.
    • Suppression policies likely needed in place for 18 months.
      • Until a vaccine is available.
      • Perhaps not all the time (switch certain policies on/off based on triggers).
      • Social distancing likely to be in place for 2/3 of the time.
    • Appropriate policies may evolve and change.
      • Intensive testing, contact tracing.
    • Multiple, layered interventions are needed.
      • Impact of one intervention in isolation is likely limited.
      • Combination of multiple interventions has substantial impact.
    • Choice of measure depends on:
      • Feasibility of implementation.
      • Effectiveness in different social contexts.

Mitigation: not the preferred strategy

  • Not sufficient to avoid overwhelming healthcare system.
    • Flatten the curve: reduce, but not necessarily stop, reproduction number (not necessarily below 1).
    • Flattening the curve.
    • Reduce peak healthcare demand and overall deaths.
    • Protect most at risk of severe disease from infection.
    • Allow population immunity to build up.
  • Most effective combination of policies:
    • Home isolation of suspect cases.
    • Quarantining those living in the same household as suspect cases.
    • Social distancing of elderly and at risk.
  • Possible outcome:
    • Might reduce peak healthcare demand by 2/3.
      • But healthcare systems still overwhelmed (8x, instead of 30x).
    • Might reduce deaths by 50%.
      • But still 100 thousands of deaths.

Suppression: may be the only viable strategy

  • May keep peak demand within healthcare capacity.
    • Reduce reproduction number (to below 1).
    • Reverse epidemic growth.
    • Reduce case numbers to low levels (or eliminate transmission).
    • Maintain that situation indefinitely.
  • Policies required:
    • Social distancing of the entire population.
    • Home isolation of cases.
    • Household quarantine of family members of cases.
    • (School and university closures).
  • Possible outcome:
    • Reduction of peak demand by up to 95%.
    • May be able to keep peak demand on healthcare facilities within capacity.
    • Reduction of fatalities by 80-90%.
  • Challenges:
    • Need to be triggered early in the epidemic to keep peak demand under control.
    • Lesser build-up of herd immunity (if interventions are relaxed, infections will rise again).
    • High social and economic costs.
    • Needs to be maintained until vaccine is available (perhaps 18 months).
    • Intermittent relaxation of social distancing may be possible.

General simulation observations

  • Model restrictions.
    • COVID-19 is a new, emergent virus: much remains unknown about transmission.
    • Population response to measures vary between countries and communities.
    • Unforeseen, spontaneous population behavior changes.
  • Selected model assumptions and outcomes:
      • 1/3 in households.
      • 1/3 in schools / workplaces.
      • 1/3 in the community.
    • Incubation period: 5 days.
    • Ro = 2.4 (2-2.6)
    • Symptomatic individuals:
      • 50% more infectious than asymptomatic individuals.
      • 2/3 of cases are sufficiently symptomatic to self-isolate within 1 day.
    • Recovery:
      • Individuals are immune in the short term.
      • Infection in the same or following season unlikely.
    • Infection:
      • Doubling rate of 5 days.
      • 81% of population ultimately infected.
      • Mean hospitalization of 10.4 days.

Table 1 -

    • Policy measures:
      • Mitigation: in force for 3 months.
      • Suppresion: in force for 5 months or longer.
      • 2-3 week lag in impact on hospitalization numbers.

Table 2-

    • Fatalities:
      • Fatality rate of 0.9%.
        • 4% of population requires hospitalization (8 days).
        • 30% of hospitalized require critical care (16 days).
        • 50% of critical care die.

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