Estimating the number of infections and the impact of nonpharmaceutical interventions on COVID-19 in 11 European countries

By: Seth Flaxman, Swapnil Mishra, Axel Gandy, H Juliette T Unwin, Helen Coupland, Thomas A Mellan, Harrison Zhu, Tresnia Berah, Jeffrey W Eaton, Pablo N P Guzman, Nora Schmit, Lucia Callizo, Kylie E C Ainslie, Marc Baguelin, Isobel Blake, Adhiratha Boonyasiri, Olivia Boyd, Lorenzo Cattarino, Constanze Ciavarella, Laura Cooper, Zulma Cucunubá, Gina Cuomo-Dannenburg, Amy Dighe, Bimandra Djaafara, Ilaria Dorigatti, Sabine van Elsland, Rich FitzJohn, Han Fu, Katy Gaythorpe, Lily Geidelberg, Nicholas Grassly, Will Green, Timothy Hallett, Arran Hamlet, Wes Hinsley, Ben Jeffrey, David Jorgensen, Edward Knock, Daniel Laydon, Gemma Nedjati-Gilani, Pierre Nouvellet, Kris Parag, Igor Siveroni, Hayley Thompson, Robert Verity, Erik Volz, Patrick GT Walker, Caroline Walters, Haowei Wang, Yuanrong Wang, Oliver Watson, Charles Whittaker, Peter Winskill, Xiaoyue Xi, Azra Ghani, Christl A. Donnelly, Steven Riley, Lucy C Okell, Michaela A C Vollmer, Neil M. Ferguson and Samir Bhatt

In: Report 13, the WHO Collaborating Centre for Infectious Disease Modelling within the MRC Centre for Global Infectious Disease AnalysisJ-IDEA, Imperial College London.

Date: 30 March, 2020

Estimated actual infections substantially higher than detected infections

  • Infections estimated for 11 European countries by back-calculating from observed deaths.
    • 2-3-week period between infection, onset of symptoms and outcome.
  • Orders of magnitude fewer infections detected than estimated true infections.
    • Due to mild and asymptomatic cases and limited testing capacity.
  • Significant number already infected, average of about 5%.
    • 7 – 43 million individuals estimated infections up to 28th March.
      • 2-11% of the population.
      • Highest in Spain (15%) followed by Italy (10%).
      • Lowest in Germany and Norway.
    • Wide confidence intervals.
  • Not close to herd immunity.
    • Implies that the virus may spread rapidly should interventions be lifted.
    • Infection estimate needs to be validated by newly developed antibody tests.


Transmission substantially reduced by interventions

  • Initial Ro estimate of around 3.87.
    • Uncertain due to high number of imported cases and potential under-reporting of fatalities.
  • Assess the impact of interventions on R.
    • Assumes each intervention has same effect on transmission across countries and over time.
    • Results driven by countries with more advanced epidemic (and data).
  • Interventions seem to have reduced R.
    • All countries seem to have managed to reduce R.
  • Current estimates of R:
    • 1.43 average for the 11 countries.
      • 64% reduction compared to pre-intervention.
      • Estimates range from 0.97 (Norway) to 2.64 (Sweden).
    • High level of uncertainty in estimates.
      • Probability of R being < 1.0 is 44% on average across the countries.
  • Interventions may have averted about 59,000 deaths up to 31 March.

Most of this in Italy (38k) and Spain (16k).

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