What it means for a pandemic like Covid to become ‘endemic’

For months, some US and European leaders have predicted that the coronavirus pandemic would be soon endemic† Covid-19 would dissolve into a disease we learn to live with. According to several governors, the time has come.

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 don’t yet know what this two-year-old disease will fall under.


The coronavirus pandemic continues

Global Cases of Covid-19





40 cases per day

per 100,000

March 2020

WHO explains

Covid-19 a pandemic

40 cases per day

per 100,000

March 2020

WHO declares Covid-19

a pandemic


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

Note: Data is as of April 5. The chart shows the seven-day average.

Basically, an endemic disease is one with a constant, predictable or expected presence. It is a disease that persists. Furthermore, there is no fixed definition.

Endemic diseases infect millions of people around the world every year, and some endemic diseases kill hundreds of thousands. Some we can treat and vaccinate against. But 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 not be the point.

“There has been a political reformulation 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 need to pay close attention to, he said, because an endemic disease can become epidemic again.

Endemic diseases can be mild or fatal

When people think of endemic disease, they often think of the common cold. Upper respiratory infections, including the common cold, are estimated to infect billions of people around the world every year, but the death of several thousand. Other endemic diseases can be much more deadly. Malaria killed more than 600,000 people worldwide in 2019 and flu killed more than 200,000, although estimates suggest this toll 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 could have a similar burden to other respiratory viruses.

“It won’t be more deadly than seasonal flu, or maybe 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 a boost from the infections we come across,” she said.

Some scientists warn that immune protection against vaccination and infection may wane over time, and future variants could evade those defenses. And mutations are random, so there’s always a chance that a variant will develop in the future that causes a more serious disease.

Endemic diseases can have epidemic periods

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


Influenza has seasonal epidemics

Percentage of samples tested in the US that are positive for influenza type A





2020

COVID-19

measures

disrupt distribution

2020

COVID-19

measures

to disturb

scatter


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

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

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

The burden of endemic diseases is uneven

One community’s experience of endemic disease can be vastly different from another, often depending on who gets sick and whether they have access to tests, treatments and healthcare.

HIV, which has been around the world for more than 40 years, is one example, although scientists and health professionals use both “epidemic” and “endemic” to describe the virus.

“One definition of endemic is defined by geographic location,” says 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 about 1.2 million people live 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





Data Methodology Changes

Data Methodology Changes


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

“Disease differences increase over time unless they are addressed from the start,” she said. “And that begs the question: are we tackling those differences with Covid or are we on the same trajectory?”

With a third of the world’s population not vaccinated against Covid and life-saving treatments not available to everyone, the burden of the virus is likely to remain uneven, experts say, even if parts of the world decide their levels are endemic.

Endemic disease is all about control

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

Instead, it means living with, and often coping with, a disease that has not been or cannot be eradicated. Health experts say countries should use control measures, such as testing, treatments and vaccinations, to control endemic diseases.

Countries with endemic malaria strive to eradicate the mosquito-borne disease and rely on interventions such as insecticides and preventive treatments to reduce its incidence. These control measures can drastically change the course of endemic malaria, as in South Africa.


Malaria control programs can reduce disease transmission

New reports of malaria cases in KwaZulu-Natal, South Africa





2000 epidemic

DDT use resumes

2001

New treatment

introduced

2000 epidemic

DDT use resumes

2001

New treatment

introduced


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

In addition to environmental controls, vaccination programs can reduce cases and deaths. But when communities fail to follow vaccination recommendations, outbreaks can occur.

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 eliminated in the United States, more than a thousand people were infected by several outbreaks, many of which have been linked to unvaccinated travelers.


Outbreaks can occur even after a disease has reached endemic levels

New measles cases reported in the US





1963

measles vaccine

license in the US

1989

a lot of measles

outbreaks

2000

measles explained

eliminated in our

1963

measles vaccine

license in the US

2000

measles explained

eliminated in our

1989

a lot of measles

outbreaks


Source: US National Notifiable Disease Surveillance System via Project Tycho

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

Unlike malaria or measles, public health experts say Covid cannot be eradicated, so control measures will help determine the size and course of future waves. (We only eradicated one human disease: smallpox, which behaved very 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 will take constant vigilance to keep it — not to eradicate it, which would be what people want — but to keep it under control.”

When will we know what the endemic phase of Covid looks like?

Probably not for a while. Scientists usually determine the endemic pattern of a disease after many years of observation.

It can take years for pandemics to resolve, and the effects of widespread disease can persist long after new infections clear up.

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


Pandemics take time to resolve and may return

Estimated deaths from flu 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


Sources: Doshi 2011 (data before 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 are inconsistent with later reporting methods. Data after 2004 are presented as annual 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. In the years that followed, some seasonal outbreaks were bigger than others.

The assumption about the Covid endemic period is that it will look significantly different from the pandemic of the past two years. But endemic Covid, at its worst, 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 could be the endemic condition,” said Dr. bedford. “And it doesn’t mean it’s mild, and it doesn’t mean it’s easy to deal with.”

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