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.
- Peak mortality occurs after 3 months (early summer):
- 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.
- Do nothing (unmitigated epidemic):
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.
- Might reduce peak healthcare demand by 2/3.
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.
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- 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.
- Policy measures:
-
- 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.
- Fatality rate of 0.9%.
- Fatalities: