Researchers Propose ‘Shield Immunity’ Model to Slow Virus Spread

Current Pandemic Context

The global coronavirus pandemic appears to be slowing in several countries, aided by measures such as travel restrictions and the closure of non-essential businesses. While these strategies have shown some success, they are not sustainable in the medium to long term. As nations begin to ease restrictions, concerns about a potential second wave of infections persist.

Introduction of Shield Immunity

A new study published in *Nature Medicine* explores a less restrictive approach to mitigate virus transmission. Researchers from the Georgia Institute of Technology introduce the concept of “shield immunity.” This model relies on the principle of interaction substitution, which posits that individuals who have recovered from COVID-19 can interact safely with both susceptible and infectious individuals without spreading the virus.

Mechanics of Shield Immunity

If this premise is proven accurate, recovered individuals could function as “interaction substitutes.” They would act as intermediaries to reduce direct contact between infectious and susceptible individuals. To assess the potential effects of this model, researchers developed a series of mathematical models. These models illustrate how shield immunity could lower the peak infection rates and reduce the duration of the outbreak.

The models focus on a key variable: the individual preference for engaging with a recovered substitute instead of interacting randomly, denoted as α. A higher preference for these shield interactions correlates with a more significant decrease in outbreak severity.

Model Descriptions and Predictions

The researchers categorized their models as either fixed or flexible, based on whether the increased interactions of shielding individuals were relative to others or to the baseline interaction rate. For instance, in a fixed shielding scenario with a preference value of 3, the model predicts that recovered individuals would have four times more contacts compared to their baseline. This intervention could lead to only 25% of the population being infected during the epidemic, in contrast to approximately 90% in the absence of shielding.

Limitations of the Study

Despite the mathematical evidence supporting the effectiveness of this approach in reducing the reproduction number (R0) of a viral epidemic, several limitations exist. Chief among these is the unverified assumption that recovered individuals can safely interact with both infectious and susceptible individuals without transmitting the virus. Although there is some evidence of immunity in recovered individuals, it remains unclear whether this immunity is complete or how long it lasts.

Additionally, while the study thoroughly examines the mathematical and theoretical aspects of shield immunity, it lacks consideration of the practical and sociological challenges that might arise. The importance of individual preferences is noted, as the success of this approach hinges on the willingness of recovered individuals to serve as middlemen. Given the uncertainties surrounding immunity to COVID-19, it cannot be assumed that these individuals would agree to act as “shields.”

Conclusion

In conclusion, the study presents a compelling alternative strategy for controlling the spread of viral epidemics. However, further research is necessary to explore both the scientific aspects of immunity and the practical implications of implementing such a model before it can be deemed feasible.

Written by Michael McCarthy
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Source: Weitz JS, Beckett SJ, Coenen AR, Demory D, Dominguez-Mirazo M, Dushoff J, et al. Modeling shield immunity to reduce COVID-19 epidemic spread. Nat Med. 2020.
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