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 Analysis, J-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.

• • 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.

• • 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.