The differences between elimination, eradication and herd immunity — and what it means for Covid-19

After more than a year of politicized science and amid a wave of vaccine hesitancy, the long war against Covid-19 stands at a critical stage.

Hope for an eradication akin to smallpox — or even a polio- or measles-style elimination — is a towering aspiration. Herd immunity, meanwhile, is a moving target that requires a lot of things to go right — and stay right, experts say. People will need to trust science, put their communities over personal comfort and realize that pathogens have no respect for state or national borders.

How previous disease fights have played out — from measles in the United States to anthrax in Kenya to the global defeat of smallpox — offers lessons for how humanity might overcome the latest scourge. Some variables — variants, for instance — are largely out of people’s control, but so many other proven measures are fully within their power.

“If we had done a better job of social distancing and continued it rigorously through the time that the vaccine became available, I think — not think, I know — we would’ve seen fewer cases and death, but that takes an enormous amount of discipline,” said Dr. Howard Markel, director of the University of Michigan’s Center for the History of Medicine.

Eradication, elimination or herd immunity?

With the coronavirus endgame in mind, let’s first take a look at the words infectious disease warriors use to describe their successes.

Herd immunity requires a certain percentage of people to be infected or vaccinated to stop the spread, but experts say it depends on the herd, or community, as well as its density, the number of susceptible people and other factors. No one knows the percentage until a community reaches it. It differs among diseases. With Covid-19, it will likely hinge on continued vaccinations.

“I think we are going to be seeing (Covid-19) or its cousins or variants for years to come,” Markel told CNN, predicting it might require annual vaccinations, like with influenza, where vaccinations are reengineered to adapt to changes in the virus.

Eradication is the unicorn of infectious disease. Markel calls it “exquisitely rare.” It’s been achieved only twice: with rinderpest, which sickens cloven-hoofed animals like cattle and buffalo, and with smallpox.

Elimination is more common. It’s when cases are reduced to zero or near zero in a specific area, owing to continual efforts to prevent transmission. In the United States, examples include measles, rubella and diphtheria — all of which were largely stamped out by vaccination.

The key word is largely. Measles demonstrates the tentative nature of elimination if control measures aren’t maintained.

The United States declared measles eliminated in 2000, but cases continue to pop up, ranging from 55 in 2012 to 1,282 in 2019, according to the US Centers for Disease Control and Prevention. The latter tally included the largest US outbreaks since 1992, all of them linked to travel-related cases that reached at-risk populations and spread within “underimmunized close-knit communities.”

Thus, Markel and other experts frown on words like elimination and eradication, even if they’re the industry standard.

“Elimination, for me, is not precise enough a word,” he said, adding he prefers “‘eliminate by vaccine’ or ‘suppression by vaccine’ because we know the measles virus does circulate. It’s out there somewhere.”

The battle to vanquish smallpox

“I always have second thoughts about those words also,” said Dr. Bill Foege, the epidemiologist credited with instituting the tactics integral to ending smallpox worldwide (it is the disease that was eradicated, he emphasizes; the virus still lives in American and Russian labs).

Comparing diseases, responses and outcomes across locales isn’t always helpful, but strategies used in the smallpox fight, which came to a successful end in 1980, can be applied to Covid-19, he told CNN.

“It’s different, but from the beginning my suggestion (for Covid-19) has been that if you combine vaccination with contact tracing you could do it in such a way that you might well achieve success,” said Foege, who led the CDC from 1977 to 1983. “One thing we have not done very well is contact tracing and the use of vaccine as a tool.”

In 1966, health authorities believed 80% of a population needed to be inoculated to wipe out smallpox in an area — similar to numbers tossed around with Covid-19 — but in Nigeria, doctors had nowhere near that supply of vaccine, nor was it expected to arrive with any haste, Foege told CNN.

When cases were confirmed in a village in eastern Nigeria, Foege and his cohorts went on the attack. They examined maps and coordinated with missionaries via ham radio to identify cases, which they then isolated. They tapped their limited vaccine stock to inoculate those who might have been exposed, then denizens of villages where their contacts and relatives lived, as well as the markets villagers frequented — a process known as ring vaccination, where doctors cut off spread by monitoring and vaccinating a “ring” around infected patients.

Within weeks, they’d snuffed out the disease with what Foege estimates was a 7% vaccination rate. Meanwhile, a city in eastern Nigeria boasting a 96% vaccination rate was still experiencing outbreaks, he said.

“We showed you didn’t need Step One in the (World Health Organization) strategy, which was mass vaccination,” Foege said. “We showed you can go right for the outbreaks. … This idea of herd immunity — you hear it used all the time now in print, on TV — people don’t understand what they’re talking about.”

Ring vaccination and the surveillance/containment strategy Foege and his team employed became the standard for fighting smallpox, which killed hundreds of millions of people in the 20th century alone. To those who say contact tracing in the United States is too arduous, Foege isn’t hearing it.

When Foege and his team arrived in India in 1973, the nation had the bulk of the world’s smallpox cases. The following year was even deadlier. It took nine months to assimilate surveillance/containment techniques to Indian conditions, and by the time they were ready to launch their assault, there were 48,000 cases.

A year later, there were zero, with no smartphones or computers in the field, so Foege doesn’t believe it when he hears some political and public health leaders in the technology-drenched United States say it can’t be done now.

“I just don’t buy it,” he said. “They don’t have the courage to do it.”

‘There was not a combined national commitment’

Several hurdles prevented a smooth US Covid-19 response, including federalism, politics, scientific advice “colored with partisanship” and “toxic nonsense,” such as the notion that the country could achieve herd immunity by letting enough healthy people get sick, said William Hanage, an associate professor of epidemiology at Harvard. Many Covid-19 outbreaks were entirely avoidable, especially the ones that occurred after we began to understand the virus, he said.

“Unfortunately, as soon as a president — either the current one or the previous — says something, large portions of the country will sort themselves into camps and disagree,” he told CNN. “That sort of partisanship is a real struggle to overcome.”

Covid-19 preyed on shortcomings in the American health care system, including varying levels of quality and access. Hospitalized Americans had to navigate a variety of government and commercial insurance and prescription plans. Medical centers had to compete for resources, including personal protective equipment.

Incoherency reigned in a federal system that leaves health care to the states, which operate under different rules and reporting methods and contain numerous (sometimes at-odds) local and regional health agencies, Hanage said.

“There was not a combined national commitment to handling this,” he said.

Another factor seldom raised “is not part of health care, per se, but a huge part of public health”: the lack of paid sick leave across industries, especially in low-wage jobs, which forced people to choose between quarantining or paychecks, he said.

“We talk about the clever things that we can do,” he said, “but we haven’t done the really simple things.”

How other nations have reined in disease

Resolve to Save Lives, headed by former CDC Director Dr. Tom Frieden, is a public health initiative of the nonprofit, Vital Strategies. It recently detailed how Covid-19 success stories in countries with a fraction of the United States’ resources did not come without some discomfort.

Still vigilant from the 2002-2003 SARS outbreak, Vietnam tapped the military to help with contact tracing, quarantined those who’d come in contact with infected people, reinforced mask and distancing policies and delivered free Covid-19-related health care.

Mongolia and Senegal took similar measures, with Senegal adding a robust education campaign — but there was blowback. Protests erupted in both countries. Still, the results are hard to debate, judging from Johns Hopkins University’s numbers:

• Mongolia (population 3.2 million) has had about 46,000 cases and 184 deaths.

• Senegal (population 16 million) has had about 41,000 cases and 1,120 deaths.

• Vietnam (population 103 million) has had about 3,600 cases and 35 deaths.

These examples show how outbreaks can be reined in without reaching the incredible milestones of elimination, eradication or herd immunity. Frieden’s initiative also dives into past case studies to detail what it calls “epidemics that didn’t happen,” demonstrating how successful responses differ from one country and disease to the next:

• Brazil, which eliminated urban yellow fever in 1942, staved off an uptick of more than 2,000 cases between 2016 and 2018, despite a depleted vaccine stockpile. It ramped up vaccine production, administered partial doses to provide short-term immunity (and stretch supply) and prioritized surveillance of animal outbreaks. In 2019, it reported 85 cases.

• When the Democratic Republic of Congo declared an Ebola outbreak in 2018, Uganda enacted emergency protocols, testing everyone entering the country and opening treatment and rapid-testing facilities along the DRC border. While the DRC suffered the second-largest Ebola outbreak ever, almost 3,500 cases, only five cases were recorded in Uganda.

• In August 2019, a herder and two students in Narok, Kenya, fell ill from anthrax, which primarily affects animals but can infect humans who come in contact with infected animals or inhale spores. A Red Cross volunteer texted the country’s surveillance system. Within days, almost 25,000 cows and sheep were vaccinated. Health experts took to the radiowaves, met with farmers to build trust and instructed teachers how to screen children. Only one death was recorded.

• When monkeypox reappeared in Nigeria’s Akwa Ibom state in 2017, teams trained doctors in sample collection and provided education to reduce stigma. Patients were directed to an infectious disease hospital, while residents were warned to avoid contact with animals and self-quarantine while samples were collected. The outbreak was contained within a month.

Like diseases, solutions must be ‘global and local’

None of these four illnesses is prevalent in Western nations, of course, but it’s important to remember viruses don’t honor political borders, nor do they care if governments consider animal and human health separate disciplines.

They do, however, thrive on apathy and unpreparedness, and Foege believes thinking narrowly costs more lives, he said. New infections — be they monkeypox or hemorrhagic fevers like Ebola — pop up about once a year, and with each outbreak, leaders vow to strengthen investment and infrastructure, but as infections diminish, so does their enthusiasm.

Effective solutions require broad approaches, he said. Two-thirds of new infections are zoonotic, so scientists should be studying animal and human health hand in hand, Foege said. They must also think globally, which with a virus as transmissible as Covid-19, means wealthier nations sharing the vaccine.

“I’m concerned we are very late coming to that conclusion,” he said. “When people ask, ‘When will the US get back to normal?’ I just tell them, ‘When Mozambique gets back to normal.'”

There’s no place on Earth that isn’t local and global, said Foege, who has joined fights against polio, guinea worm disease and river blindness, and headed the CDC when it set its sights on eliminating measles.

“This is global and local, and that’s the way we have to be thinking. You can’t be a nationalist,” he said before aptly paraphrasing Albert Einstein: “Nationalism is an infantile disease; it’s the measles of mankind.”

The University of Michigan’s Markel, who chronicled in The New Yorker last month how trust in science had taken a serious hit since the advent of the polio vaccine in the 1950s, said the United States and other countries could eliminate or “very nicely suppress” Covid-19 but it would require people around the globe to place their faith in doctors and line up for the vaccine.

Markel understands why politicians would steer clear of mandatory vaccinations, but as a public health expert, he’d like to see them. Many experts describe worldwide vaccination as some sort of moon shot, he said, but the moon shot was developing and manufacturing safe and effective vaccines in record time.

“The moon shot happened. We’re on the moon,” he said. “I’m a vaccine man. If you counted up all the lives that have been saved and all the disease prevented over the last 100 years, you’re talking the top 9 out of 10 greatest hits of medicine.”

With widespread vaccinations across all eligible ages, regional elimination of Covid-19 is on the table, Harvard’s Hanage said, pointing to how New Zealand and Australia eliminated it with minimal immunity. Any solution, he said, would have to overcome the United States’ politics and its “balkanized” health care system — while aggressively tackling any reintroductions to keep those outbreaks small.

It will require commitment, discipline and unity — the latter being of chief importance in an age too often marred by provincialism.

“Even with a coherent response, it’s hard work,” he said. “You’re only as strong as the weakest link, especially if you’re trying to drive anything like eradication or elimination.”