# Deceleration and rebound of hemagglutinin divergence in influenza B/Victoria across COVID-19 NPI phases (2016–2024)

## TL;DR

The study shows how influenza B/Victoria evolved in response to pandemic restrictions and their relaxation in three countries.

## Contribution

It reveals a transnational slowdown in hemagglutinin evolution during NPIs and a rebound afterward.

## Key findings

- Influenza B/Victoria hemagglutinin evolution slowed during pandemic NPIs across China, the U.S., and Australia.
- After NPI relaxation, HA genetic distance increased significantly compared to pre-pandemic levels.
- The magnitude of HA rebound varied by country, aligning with policy timing and intensity.

## Abstract

The COVID-19 pandemic reshaped global viral transmission and created a natural experiment to test how non-pharmaceutical interventions (NPIs) influence the evolution of co-circulating pathogens. We investigated the evolutionary dynamics of the influenza B/Victoria lineage across China, the United States, and Australia—three countries that implemented distinct COVID-19 policy strategies. Using hemagglutinin (HA), neuraminidase (NA), and nucleoprotein (NP) sequences from GISAID (2016–2024), we performed temporal phylogenetic analyses and estimated rate of genetic distance change. Although the duration and strictness of NPIs varied among the three countries, our analysis revealed a transnational common pattern: the evolution of HA showed a marked slowdown during the NPI period. During the post-relaxation period, the population genetic distance of HA in the three countries showed a highly significant increase compared to the pre-pandemic period (comparison between post-relaxation vs. pre-pandemic period, p < 0.001). The magnitude and trajectory of these shifts differed by country, broadly aligning with policy timing and intensity. These findings highlight the sensitivity of influenza evolution to population-level interventions and underscore the value of integrating policy metrics into influenza surveillance and pandemic preparedness.