# Spatial and social determinants of the 1857 yellow fever epidemic in Lisbon

## TL;DR

This study analyzes the 1857 yellow fever epidemic in Lisbon to understand how social and environmental factors influenced its spread and impact.

## Contribution

The paper provides a historical analysis of yellow fever transmission dynamics in a pre-vaccination urban setting using spatial and social data.

## Key findings

- The 1857 Lisbon yellow fever epidemic had a basic reproduction number (R0) of approximately 5.
- High-risk areas were identified near the coastline in low-elevation, densely built-up neighborhoods.
- Women working at home had the highest probability of death during the epidemic.

## Abstract

Despite the availability of a highly effective vaccine, yellow fever virus (YFV) is still endemic in 47 countries globally. Although disease due to YFV was first recorded in 1635, factors contributing to its spread remain poorly understood today. Using archival data from the nineteenth century, we digitalised and mapped the 1857 yellow fever (YF) epidemic in Lisbon, Portugal, to understand how transmission dynamics and spatial and environmental characteristics led to disparities in health outcomes between sociodemographic groups. We modelled the basic and effective reproduction number (R0 and Rt) and found that transmission dynamics throughout this pre-vaccination era epidemic are consistent with prevailing estimates (R0 ≃ 5). Transmission peaked at the end of October 1857 when YF was declared an epidemic, then declined until January 1858. YFV killed 4.2% of the population with infection attack rates ranging between 10.3-13.5%. Out of the 34 parishes in urban Lisbon, our hotspot analysis identified 15 statistically significant high-risk parishes near the coastline. Our maps, combined with a digital terrain model, show that the highest number of deaths occurred within connected streets confined in low-elevation built-up areas with homes. We discuss the potential role of wind and temperature in aiding mosquito dispersal across Lisbon, which were believed as the main historical environmental drivers of YF. More people died at home than in hospitals, and although working-aged men accounted for most fatalities, the highest probability of death was found among women working at home. Our study highlights the role of human-environment interactions in shaping a historical YF epidemic in a pre-vaccination urban setting and enhances our understanding of modern-day transmission dynamics.

Yellow fever is a mosquito-borne disease transmitted to humans primarily through Aedes aegypti in urban areas. Currently, yellow fever is endemic in the tropical regions of South America and Africa. Historically yellow fever caused outbreaks and epidemics in North America and Europe too. Climate change and global connectivity are expanding the potential range of Aedes aegypti, increasing the risk of yellow fever re-emergence in Europe. To support future preparedness efforts, we examined a historical yellow fever epidemic that occurred in Lisbon, Portugal, that caused 18,000 cases and 5,652 deaths, and assessed its transmission characteristics using the basic and effective reproduction number. We digitalised and mapped archival data at the neighbourhood level to assess the contributions of both social (population, occupations, and gender dynamics) and environmental (temperature, precipitation, wind, and elevation) factors of the yellow fever epidemic in Lisbon. Research into a European epidemic that occurred 80 years before an effective yellow fever vaccine is useful for understanding population-environment dynamics in a pre-vaccination setting where yellow fever is not established. Our findings could strengthen current arbovirus control efforts and support epidemic preparedness in urban areas.