# Quantifying the spatiotemporal dynamics of the first two epidemic waves of SARS-CoV-2 infections in the United States

## TL;DR

This paper analyzes how SARS-CoV-2 spread across the U.S. during its first two waves, showing how quickly and where infections spread.

## Contribution

The study introduces new tools to quantify and visualize the speed and spatial spread of SARS-CoV-2 epidemic waves.

## Key findings

- The second wave of SARS-CoV-2 spread faster than the first, likely due to multiple introduction events.
- The origins of the waves were linked to large gatherings and relaxed masking policies.
- Regional differences in spread highlight the need for localized public health responses.

## Abstract

SARS-CoV-2 infection rates displayed strikingly organized patterns of temporal and spatial spread as new variants were introduced and subsequently transmitted within the United States. While these spatio-temporal “waves” of infection have been described previously, attempts to quantify the speed and extent of these waves have been limited. Here, we estimate and compare the wavefront speed and spatial expansion of the first two major infection waves in the United States, illustrating these dynamics through detailed visualizations. Our findings reveal that the origins of these waves coincide with large gatherings and the relaxation of masking mandates. Notably, we found that the second wave spread more rapidly than the first, possibly driven by multiple introduction events. These analyses highlight regional heterogeneity in epidemic dynamics and underscore the importance of localized public health measures in mitigating ongoing outbreaks.

In this work, we developed a set of tools and methods designed to produce high-resolution visualizations, as well as to quantify the complex and intricate patterns associated with the spatial and temporal spread of SARS-CoV-2 in the United States of America. Our focus was on examining and characterizing the dynamics of transmission as they unfolded during the first two major waves of infections that were observed across the contiguous United States territory. In developing such tools, we were able to generate detailed visual representations of how the virus had moved and spread geographically over time, while also providing a means to numerically capture and describe the speed of those spreading patterns. The approaches we developed were applied comprehensively across the lower 48 states of the USA, allowing us to track and document how SARS-CoV-2 propagated across different regions during each of the two significant waves of infection that defined the early period of the pandemic in those regions.