25.05.2020
Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold
Modeling LockdownImmunity
Gomes MGM et al
Pre-prints
Main result
While a second wave is predicted after lockdown end for a homogeneous population, a heterogeneity of susceptibility, or of infectivity and susceptibility (assumed to be perfectly correlated), limits the amplitude of the second wave and lowers the threshold of herd immunity. The population heterogeneity must therefore be taken into account.
Takeaways
When population heterogeneity in terms of susceptibility, or, infectivity and susceptibility, is taken into account, herd immunity is achieved after infection of a smaller proportion of the population than in a homogeneous model.
Strength of evidence Weak
The model is a bit simplistic and the presentation lacks rigor. There are typos (1/N in surplus) in the calculation of R0. Data sources are not specified. The code is not provided and it is not possible to reproduce the results.
One can criticize the shape of the infection strength chosen in the model version where infectivity and susceptibility vary.
Objectives
What is the effect of population heterogeneity on the threshold at which herd immunity is reached?
Method
Method: SEI(R) model, system of ODEs, parameters adjusted using data from confirmed cases in several countries. The authors estimate the R0, the time when the epidemic started locally (t0), and the effect of social distancing measures (d). The other parameters of the model are set (relative infectivity of exposed individuals, time to develop symptoms, time to cure).
Data: Confirmed case data for different countries, but the source is not specified.
Data: Confirmed case data for different countries, but the source is not specified.
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