Odds Beaten by Ocean-Crossing Epidemics via Ships

On December 22, 1874, the H.M.S. Dido sailed into Fiji from Sydney, Australia, carrying approximately 200 passengers and an unseen threat. Among its occupants were a Fijian king and his son, who had contracted measles during their journey. Upon disembarking in Fiji, they unknowingly began an epidemic that would claim around 20,000 lives—up to one-fourth of the country's population at the time. 
 
Such devastating outbreaks resulting from overseas travel were rare in those days when voyages by sail or steam took weeks or even months to complete. As per a recent study published in the Proceedings of the National Academy of Sciences last week, which used mathematical models to illustrate how viruses faced long odds trying to cross oceans via seafaring vessels back then, More often than not, diseases would burn themselves out on board before these ships ever reached port. 
 
Today, however, we live in fear as new diseases like COVID-19 spread almost instantly across continents due to advances in air travel technology and increased connectivity between countries. But when did this shift occur? Yale doctoral student Elizabeth Blackmore, along with James O. Lloyd-Smith from the University of California, set out to find this inflection point. 
 
John McNeill, a historian at Georgetown University, commended Ms. Blackmore's use of advanced mathematical modeling as it helped quantify transmission likelihoods—something no historian has managed thus far. 
 
Ms. Blackmore said that she got interested in studying maritime disease transmission while pursuing her master’s degree after realizing that first smallpox reports only surfaced much later despite extensive shipping activities since the early centuries. She was further intrigued by reading "Pox Americana: The Great Smallpox Epidemic" by historian Elizabeth Fenn, where she discovered Boston experienced lengthy intervals between smallpox epidemics throughout the 18th century. 
 
The researchers utilized data related specifically to three infectious diseases—flu, measles, and smallpox—for their study. Their model showed that flu was the hardest to spread due to its short infection period of three days on average. Measles and smallpox, however, with their longer infectious periods, could potentially be transmitted more easily. 
 
They also studied 18 historical voyages, including that of Santa Maria, which carried Christopher Columbus to the Americas, and Mayflower's journey in 1620. Their calculations showed that even if one person had flu while sailing on these ships, the chances of disease transmission were less than 0.1 percent for flu, 24 percent for measles, and 33 percent for smallpox. 
 
Despite historically notorious conditions onboard these ships—overcrowded spaces, lack of sanitation., etc.—disease seemed unlikely to survive long enough journeys across the sea due to inherent limitations like the number of susceptible individuals present or the length of the voyage itself. 
 
In the mid-19th century, though, things started changing when steamships revolutionized travel by reducing travel times significantly and being able to carry far greater numbers. This increased speed, coupled with larger passenger loads, meant that diseases now had a much higher chance (70% for measles and 74% for smallpox) of surviving the voyage and infecting new populations upon docking. 
 
This risk escalated further during World War I when extremely fast troop carriers transported soldiers by thousands from home fronts to battlefields. These movements are often cited as one key reason why the Spanish Flu spread so rapidly worldwide in 1918. 
 
Ms. Blackmore plans on extending this research further by testing her models against actual outbreaks recorded in historical archives. Her goal is not just academic, but she believes understanding how slowly a disease can actually spread will help us appreciate effectiveness measures like social distancing better than simply assuming every contagious disease spreads 'like wildfire'.