What It Means for a Pandemic Like Covid to Become ‘Endemic’

For months, some American and European leaders have foretold that the coronavirus pandemic would soon become endemic. Covid-19 would resolve into a disease that we learn to live with. According to several governors, it nearly has.

But we are still in the acute phase of the pandemic, and what endemic Covid might look like remains a mystery. Endemic diseases can take many forms, and we do not know yet where this two-year-old disease will fall among them.


The coronavirus pandemic continues

Global Covid-19 boxes





40 boxes

per 100,000 per day

March 2020

WHO declares

Covid-19 a pandemic

40 boxes

per 100,000 per day

March 2020

WHO declares Covid-19

at pandemic


Source: Local governments; Center for Systems Science and Engineering at Johns Hopkins University; National Health Commission of the People’s Republic of China; World Health Organization

Note: Data as of April 5. Chart shows seven-day average.

At its most basic, an endemic disease is one with a constant, predictable or expected presence. It’s a disease that persists. Beyond that, there is no fixed definition.

Endemic diseases infect millions of people around the world each year, and some endemic diseases kill millions of thousands. Some we can treat and vaccinate against. Yet they can also cause unexpected outbreaks and significant suffering.

Interviews with two dozen scientists, public health experts and medical historians suggest the rush to recast Covid as endemic may be missing the point.

“There’s been a political reframing of the idea of ​​endemic as something that is harmless or normal,” said Lukas Engelmann, a historian of medicine and epidemiology at the University of Edinburgh. But epidemiologists use endemic to mean something we should watch carefully, he said, because an endemic disease can become epidemic again.

Endemic diseases can be mild or deadly

When people think of endemic disease, they often think of the common cold. Upper respiratory infections, including colds, are estimated to infect billions of people worldwide every year but kill several thousand. Other endemic diseases can be much more lethal. Malaria killed more than 600,000 people globally in 2019, and flu killed more than 200,000, though estimates suggest these tolls could be much higher.


Endemic diseases are not without suffering





New global cases per 100,000 in 2019

New global deaths per 100,000 in 2019

New global cases per 100,000 in 2019

New global deaths per 100,000 in 2019


Many scientists predict that endemic Covid may have a similar burden to other respiratory viruses.

“It will be no more deadly than seasonal flu, or may be mild like one of the cold-causing coronaviruses,” said Lone Simonsen, the director of the PandemiX Center at Roskilde University in Denmark.

“The reason for this is that we have a lot of immunity and we keep getting boosted from the infections that we run into,” she said.

Some scientists warn that immune protection from vaccination and infection may wane over time, and future variants might sidestep those defenses. And mutations are random, so there is always a chance a variant that causes more severe disease could arise in the future.

Endemic diseases can have epidemic periods

The common cold and the flu are widespread endemic diseases that persist year round, but their levels are not constant. Instead they cause seasonal epidemicswhere infections rise beyond baseline endemic levels, often in the winter when people gather indoors.


Influenza has seasonal epidemics

Percentage of specimens tested in the US positive for influenza Type A





2020

Covid-19

measurements

disrupt spread

2020

Covid-19

measurements

disrupt

spread


These patterns are predictable, but people can change them: The control measures used to blunt the Covid pandemic dampened flu and cold waves in recent years, too.

Scientists say that endemic Covid could be seasonal, but it could also have irregular and significant epidemic waves.

“Covid is much, much more transmissible than the flu,” said Jeffrey Shaman, an infectious-disease modeler at Columbia University. “Only a small portion of the population needs to be susceptible for an outbreak to foment, and that can happen at any time of year.”

The burden of endemic disease is unequal

One community’s experience with endemic disease can be vastly different from another’s, often depending on who is getting sick and whether they have access to tests, treatments and health care.

HIV, which has persisted across the globe for more than 40 years, is one example, though scientists and public health workers use both “epidemic” and “endemic” to describe the virus.

“One definition of endemic is defined by geographic location,” said Dr. Diane Havlir, an infectious disease specialist and professor of medicine at the University of California, San Francisco. “Through that lens, HIV is endemic in the United States, where approximately 1.2 million persons are living with HIV”

“But HIV is epidemic in subpopulations in the US,” she added.


HIV disproportionately affects certain groups

Estimated HIV incidence in the United States by race and ethnicity





Changes in data methodology

Changes in data methodology


Infectious diseases often remain in communities where poverty or discriminatory systems prevent access to health care, Dr. Havlir said.

“Disease disparities increase over time unless they are addressed at the outset,” she said. “And that raises the question: Are we addressing those disparities with Covid or are we on that same trajectory?”

With one-third of the global population unvaccinated against Covid and life-saving treatments not available to all, the virus’s burden will likely continue to be unequal, experts say, even as parts of the world decide their levels are endemic.

Endemic disease is all about control

Among the many forms endemic disease can take, one thing is clear: Endemic does not mean the end of the disease.

Instead, it means living with, and often managing, a disease that has not been, or cannot be, stamped out. Health experts say that countries must use control measures, like testing, treatments and vaccinations, to keep endemic diseases in check.

Countries with endemic malaria aspire to eradicate the mosquito-borne disease and rely on interventions like insecticides and preventative treatments to reduce its incidence. These control measures can drastically alter the course of endemic malaria, as they have in South Africa.


Malaria control programs can reduce disease transmission

New malaria case notifications in KwaZulu-Natal, South Africa





2000 epidemic

DDT use resumes

2001

New treatment

introduced

2000 epidemic

DDT use resumes

2001

New treatment

introduced


Source: South Africa National Department of Health, Barnes et al.

In addition to environmental controls, vaccination programs can reduce cases and deaths. But when communities do not adhere to vaccination recommendations, outbreaks can happen.

Measles, for example, remained endemic in the United States for 40 years after the introduction of vaccines. During that period, unvaccinated people remained vulnerable, fueling occasional outbreaks. In 2019, two decades after the disease was declared eliminated in the United States, several outbreaks, many associated with unvaccinated travelers, infected more than a thousand people.


Outbreaks can happen even after a disease reaches endemic levels

New measles cases reported in the US





1963

Measles vaccinate

licensed in US

1989

many meals

outbreaks

2000

Measles declared

eliminated in US

1963

Measles vaccinate

licensed in US

2000

Measles declared

eliminated in US

1989

many meals

outbreaks


Source: US National Notifiable Disease Surveillance System via Project Tycho

Note: Chart excludes cases reported in the Virgin Islands, American Samoa, Northern Mariana Islands, Puerto Rico and Guam.

Unlike malaria or measles, public health experts say that Covid cannot be eradicated, so control measures will help determine the size and course of future waves. (We have eradicated just one human disease: smallpox, which behaved quite differently from Covid.)

Keeping up with Covid means staying focused on vaccinating, treating and updating vaccines, said Dr. Monica Gandhi, an infectious disease specialist and professor of medicine at the University of California, San Francisco. “It’s going to take constant vigilance to keep it — not to eradicate it, which would be what humans want — but to keep it under control.”

When will we know what Covid’s endemic phase looks like?

Probably not for a while. Scientists usually determine a disease’s endemic pattern after observing it for many years.

Pandemics can take years to settle, and the consequences of widespread illness can last long after new infections fade.

Much of what we know about the transition out of pandemics comes from flu — humans have witnessed four influenza pandemics in the last 100 years. The 1918-19 pandemic, which killed more than 50 million people globally, dwarfs them all.


Pandemics take time to resolve and can return again

Estimated influenza deaths in the US





1957–58

and 1968

pandemics

Change in data methodology

Change in data methodology

1957–58

and 1968

pandemics

Change in data

methodology

Change in data

methodology


Source: Doshi 2011 (data prior to 2004); Centers for Disease Control and Prevention (data after 2004); US Census (population data for years after 2004).

Note: Data prior to 1930 used different diagnosis criteria that is not consistent with later reporting methods. Data after 2004 is shown as yearly values, while data from 2004 and earlier are monthly values.

It took the 1918 flu pandemic three years to settle into a more regular pattern, and the United States had a significant 1920 wave that killed more people in some cities than previous waves had. In the years that followed, some seasonal outbreaks were larger than others.

The assumption about Covid’s endemic period is that it will look meaningfully different from the pandemic of the last two years. But endemic Covid, in the worst-case scenario, could look something like where we’ve been.

“You can imagine a situation where Omicron-like events happen every year,” said Trevor Bedford, a virologist at the Fred Hutchinson Cancer Research Center in Seattle.

“That can be the endemic state,” Dr. Bedford said. “And it doesn’t mean that it’s mild, and it doesn’t mean that it’s easy to deal with.”

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