Non-Pharmaceutical Interventions Reshape Network Immunization Outcomes

TL;DR

This paper investigates how non-pharmaceutical interventions influence herd immunity and immunization strategies, revealing counterintuitive effects and the importance of contact structure in network-based epidemic models.

Contribution

It uncovers the complex interactions between non-pharmaceutical interventions and immunization strategies on spatial contact networks, highlighting the role of contact numbers in herd immunity outcomes.

Findings

Uniform vaccination is most effective in sparse networks.

Naturally acquired immunity surpasses vaccination as contact numbers grow.

Real-world data confirms phenomena observed in synthetic network models.

Abstract

Herd immunity is shaped not only by the infection capacity of a spreading epidemic or the contact structure of the hosting population, but also by how and under what circumstances individuals acquire immunity. Immunization strategies may interact with ongoing non-pharmaceutical interventions, which commonly aim to reduce social contact numbers. We demonstrate that these interactions can induce unexpectedly strong and counterintuitive effects on herd immunity. We explore these phenomena on spatially embedded contact networks and uncover a reversal in the relative effectiveness of disease- versus vaccine-induced immunization schemes, highlighting the average number of contacts as a critical determinant of emerging herd immunity. In sparse geometric networks with limited degree heterogeneity, uniform vaccination proves most effective; however, as average contact numbers increase, naturally acquired immunity ultimately becomes the better strategy. We show that this phenomenon may emerge not only in synthetic networks but also in real-world mixing networks, observed during non-pharmaceutical intervention periods across multiple states of the United States.