Fundamental principles of epidemic spread highlight the immediate need for large-scale serological surveys to assess the stage of the SARS-CoV-2 epidemic

Jose Lorenco, Robert Paton, Mahan Ghafari, Moritz Kraemer, Craig Thompson, Peter Simmonds, Paul Klenerman, Sunetra Gupta

In: not yet published

Date: 25 March, 2020


  • Three phases of the spread of a novel pathogenic infectious agent:
    • Slow accumulation of new cases (often undetectable).
    • Rapid growth in cases of infection, disease and death.
    • Slowdown of transmission due to depletion of susceptible individuals (termination of first wave).
  • This typical progression assumes:
    • No control measures (social distancing, travel bans, etc.).
    • The infection elicits a protective immunity.
  • Susceptible-Infected-Recovered model (SIR).
    • Explore sensitivity of the system to fraction of population vulnerable to disease and death.
  • Means: explore what level of the population has already been infected and is now immune:
    • Higher number of people currently immune -> lower number of people vulnerable to future infection (herd immunity).
    • Lower the number of vulnerable people -> less people may require hospitalization.
  • Conclusions:
    • Current wave of the COVID-19 epidemic should last 2-3 months.
    • Infections may have started up to 1 month before first death occurred (UK, Italy).
    • If that is true, the current number of infections may be far higher than estimated.
      • Perhaps as high as half of the population.
    • Implications:
      • The vast majority of infections develop very mild or no symptoms at all.
      • Fewer than 1 in 1,000 infected require hospitalization.
      • May already have a substantial level of herd immunity in both countries.
    • Urgent need to for testing to understand actual level of immunity.
      • Understand what stage of the epidemic we are in.


  • Confirms the importance of understanding who has already been infected (and how long immunity lasts, etc.).
    • Many people that contract the disease are without symptoms.
    • So without proper testing, we don’t know.
  • The study has not yet been formally published or peer-reviewed.
  • Criticism of the study argues that its assumptions may not be in line with some real world data:
    • For instance, the Lombardy region in Italy is well beyond 1 in 1,000 people infected requiring hospitalization.
    • In fact, 1 in 1,000 of its entire population required hospitalization (with a death rate is 0.42 per 1,000 people)

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