From plague to polio: how do pandemics end?

In the mid 17th century as Britain battled outbreaks of smallpox, plague and typhus, John Graunt, widely viewed as the founder of epidemiology, sat poring over four decades of mortality records collected by parish clerks.

These “Bills of Mortality”, he had realised by 1666, could be used to prove a simple idea: that epidemics ended not when the disease disappeared but when deaths returned to rates seen in normal times.

This measure of mortality would later become known as “excess deaths” and nearly 400 years later, Andrew Pollard, chief investigator for the Oxford vaccine, put forward a similar thesis for Covid-19. “The end of the pandemic is not the end of this virus — it’s the end of an unsustainable impact on health systems,” he said. “If we can convert it into something more innocuous, then we’ll have the end of the pandemic in sight.” 

Since the start of the year, as more transmissible coronavirus variants have driven up infections, and governments have extended restrictions, it has sometimes seemed that the pandemic will never end.

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History, however, teaches otherwise. Pandemics do finish but their endings are rarely neat. Diseases are seldom eradicated and outbreaks never end everywhere at the same time.

Influenza A, which has various strains in circulation throughout the world, is a classic example of a disease that has risen to epidemic proportions before retreating and then returning in unpredictable waves, ever since it first appeared among human populations in the late 1500s.

A particularly virulent version was the Spanish influenza pandemic of 1918 to 1920. Medical historians continue to dispute how it ended. Some argue that it infected enough of the population to generate a barrier of natural immunity. Others say it mutated to become less deadly over time. Either way, intense outbursts persisted around the world until 1922.

The first vaccine to protect against Influenza A was created in the 1940s, yet the smouldering embers of that pandemic remain. Genetic relatives of the Spanish flu were responsible for the Asian flu outbreak in 1957, the Hong Kong flu pandemic in 1968, and continue to reignite each year, a full century later.

“If we can get to a point like with flu where most people are OK, where we have a coronavirus season every year, then we can cope,” Pollard said.

Nursing volunteers learn how to treat patients during the 1918 Spanish influenza pandemic, which killed millions around the world over the next four years © Apic/Getty Images; Underwood Archive/Getty Images

A tipping of the scales

Like Influenza A, Covid-19 will probably never be eliminated. Instead, the perceived risk is likely to reduce over time.

Pandemics end when they “change from something that we as a society deem to be unacceptable, into things that can be fatal, but just in the background”, said Erica Charters, associate professor of the history of medicine at Oxford university.

Scientists refer to this as the moment when a disease that was once a pandemic or epidemic becomes endemic.

In a recent essay, Charters and independent scholar Kristin Heitman defined it as “the point when the urgency of the disease outbreak has sufficiently diminished so that public attention is redirected to the moral and social crises that the disease has engendered or exposed”.

Bar chart showing Estimated deaths as a % of the population for various epidemics

With Covid-19, the hidden toll could be the 168m children around the world who have forgone almost a full year of education, according to Unicef. Or the 114m jobs that were lost in 2020, according to the International Labour Organization.

In some countries, the scales are already perceptibly shifting towards these concerns. They may become the public’s greater preoccupation before a government or international body such as the World Health Organization officially proclaims the “end” of the pandemic.

The Smallpox exception

Smallpox is one of the only pandemics that has been completely vanquished. But the exception, frequently highlighted by epidemiologists, is also a cautionary tale.

The campaign to eradicate the disease, which had plagued humanity for millennia and was even found in the mummified corpses of Egyptian pharaohs, began in 1966 and ended in Somalia in 1979.

“What smallpox teaches us is that a vaccine alone does not eradicate a disease,” cautioned Alexandre White, assistant professor of the history of medicine at Johns Hopkins University. The smallpox vaccine was first developed in Britain in the 18th century but western doctors were largely uninterested in taking the shot to far-flung corners of the globe until increased travel threatened to bring it back again.

Smallpox killed 500m people in the 20th century alone. The vaccination programme ultimately succeeded but that success concealed an uglier side when the campaign became coercive and violent in its final days.

“Women and children were often pulled out from under beds, from behind doors, from within latrines,” wrote Stanley Music, a senior WHO epidemiologist sent in 1973 to Bangladesh, where some citizens were still refusing injections. “When they locked their doors, we broke down their doors and vaccinated them.”

Although vaccination for smallpox has been around since at least the 18th century, vaccinations in places like Bangladesh were not common until the 1970s. Pictured, a doctor from the CDC talking to villagers in Bangladesh in 1970 © Ann Ronan Pictures/Print Collector/Getty Images; Smith Collection/Gado/Getty Images

Much of the Covid-19 vaccine hesitancy found in black, Asian and minority ethnic communities today can be traced, some experts argued, back to the exploitative and often brutal medical campaigns of the past.

“Vaccines are certainly a major weapon against the spread of infectious disease but we also see the risks of failing to carry out vaccination campaigns that are compassionate in nature,” White said.

Ended for who?

History also shows that pandemics have never ended at the same time for everyone. Polio is a thing of the past for Europe, the Americas and Australia, but remains a stubborn threat in small parts of Africa and south Asia. 

“One way that a pandemic can end is that it can become someone else’s problem,” argued Dora Vargha, senior lecturer in medical humanities at Exeter university.

The first major polio epidemic was documented in 1894 in the US, and reached its peak in 1952, when nearly 60,000 children were infected, thousands were paralysed and 3,000 died.

The American virologists Dr Thomas Francis and Dr Jonas Salk, the creator of the flu shot, invented a vaccine for polio in 1955 and by 1979 it had been completely eradicated in the US.

Dr Jonas Salk gives a young girl the polio vaccine he invented in 1953. In Africa, the battle to banish polio continues, with ongoing vaccination programmes © Hulton Archive/Getty Images; AP

The rollout of polio vaccines in the 1960s and 1970s was similar in Europe and the Soviet Union but campaigns in the global south have been far less effective. Africa was finally declared polio free in August, only for South Sudan in November to report new cases of vaccine-derived polio — a rare form of the illness which occurs when the weakened virus in the vaccine mutates.

Polio showed, Vargha said, that “getting the vaccine out and distributing it evenly is very important”, but that adequate health infrastructure and investment was also needed to make a campaign successful.

With Covid-19, that responsibility sits in large part with the WHO-backed Covax scheme that aims to ensure that all countries can get access to vaccines. The facility has progressed slower than many had hoped but delivered its first doses of the AstraZeneca vaccine to a handful of African nations last month.

John Graunt, the father of modern epidemiology, analysed the Bills of Mortality and focused on excess deaths to determine when a plague outbreak could be over © Oxford Science Archive/Print Collector/Getty Images; Universal History Archive/ Getty Images

But just as vaccines are not the only solution, they should not be the only legacy, Thomas Bollyky, a senior fellow at Washington’s Council on Foreign Relations, argued in his book Plagues and the Paradox of Progress.

Pandemics such as cholera and yellow fever that would batter cities in the US every 10 or 15 years slowly disappeared, Bollyky said, not because of vaccination, but through investments in sanitation and hygiene.

When the Covid-19 pandemic struck, some countries suffered badly in the early months in part because their public health infrastructure was already overstretched. The WHO was also in a difficult position, heavily reliant on philanthropic organisations and the private sector to plug a shortfall in funding from member states.

Pandemics tend to expose failures to invest in health infrastructure and require governments to make sweeping changes to ensure they do not return, Bollyky said.

“The question is: will we get the same societal reforms that brought the end of previous pandemics this time around?”

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