Effective immunity and second waves: a dynamic causal modelling study

TL;DR

This study uses dynamic causal modeling to estimate that effective immunity lasts about three months, influencing the timing of second waves and informing public health strategies for COVID-19.

Contribution

It introduces a model-based approach to estimate immunity duration and its impact on second wave timing across multiple countries.

Findings

Effective immunity lasts approximately three months.

Second waves are deferred by about six months under current immunity assumptions.

Implications for vaccination timing and public health interventions.

Abstract

This technical report addresses a pressing issue in the trajectory of the coronavirus outbreak; namely, the rate at which effective immunity is lost following the first wave of the pandemic. This is a crucial epidemiological parameter that speaks to both the consequences of relaxing lockdown and the propensity for a second wave of infections. Using a dynamic causal model of reported cases and deaths from multiple countries, we evaluated the evidence models of progressively longer periods of immunity. The results speak to an effective population immunity of about three months that, under the model, defers any second wave for approximately six months in most countries. This may have implications for the window of opportunity for tracking and tracing, as well as for developing vaccination programmes, and other therapeutic interventions.