# Multi-lineage evolution in viral populations driven by host immune systems

## TL;DR

This paper models how viruses evolve under host immune pressure in a two-dimensional antigenic space, revealing conditions for single or multiple lineage coexistence and providing insights into influenza evolution.

## Contribution

It introduces a novel evolutionary model incorporating finite immune memory to explain diverse viral evolutionary patterns in antigenic space.

## Key findings

- Small mutation rates favor single lineage stability
- Large mutation rates enable multiple co-existing lineages
- Model constrains parameters for influenza virus adaptation

## Abstract

Viruses evolve in the background of host immune systems that exert selective pressure and drive viral evolutionary trajectories. This interaction leads to different evolutionary patterns in antigenic space. Examples observed in nature include the effectively one-dimensional escape characteristic of influenza A and the prolonged coexistence of lineages in influenza B. Here we use an evolutionary model for viruses in the presence of immune host systems with finite memory to delineate parameter regimes of these patterns in a in two-dimensional antigenic space. We find that for small effective mutation rates and mutation jump ranges, a single lineage is the only stable solution. Large effective mutation rates combined with large mutational jumps in antigenic space lead to multiple stably co-existing lineages over prolonged evolutionary periods. These results combined with observations from data constrain the parameter regimes for the adaptation of viruses, including influenza.