What does it mean for a disease to be endemic?

A disease is endemic when it is continuously present in a population in a given geographical area, as defined in Principles of Epidemiology in Public Health Practice, third edition. For example, malaria is endemic in many countries in Africa, Asia and the Americas.

What is the difference between endemic, epidemic and pandemic?

While an endemic refers to a constant presence of cases that do not exceed what is expected, an epidemic is an increase, often sudden, in the number of cases above what would be expected in that geographic area.

In a pandemic this phenomenon occurs across several countries.

Is covid-19 endemic?

The SARS-CoV-2 coronavirus is still in the so-called pandemic phase, according to the 2005 WHO model based on influenza pandemics, which was revised after the 2009 pandemic.

After phases 5 and 6 there is an inter-pandemic phase in which recurrent events may still occur. Finally, the post-pandemic phase is reached, in which the disease recovers its activity to typical seasonal levels.

This model is based on influenza and is not necessarily extrapolable to the current coronavirus pandemic, whose future is more uncertain.

"For covid-19, other theoretical models have been proposed that attempt to contemplate non-pharmacological interventions, such as a spiral one that proposes a gradual decrease in response actions," explains Adrián Aginagalde, director of the Public Health Observatory of Cantabria.

Will covid-19 be endemic one day?

"Not all acute respiratory infections need to become endemic," Aginagalde qualifies. "The phenomenon is common for influenza viruses, but we don't know it for the others, so the post-pandemic phase may contemplate other scenarios, not just endemicity."

"Beyond the decrease in risk and the phenomenon of novelty that accompanies the appearance of any disease, we have no precedents where a coronavirus has been epidemic and then endemic," says Aginagalde.

The case of the coronavirus responsible for MERS (Middle East respiratory syndrome) would be an exceptional example, "without sustained transmission between humans and with a complex zoonotic cycle".

What does it mean for a disease to be seasonal?

"We can speak of seasonal disease when its prevalence is linked to a time of year," explains health climatologist Dominic Royé, who has published research on this phenomenon in relation to SARS-CoV-2.

The most typical example is influenza: although it circulates all year round, the highest number of cases occurs in winter.

The factors that determine seasonality are very varied. "There are environmental factors such as temperature and humidity that influence transmission, because dry cold helps to increase it. There are also sociocultural factors, such as social contact and the longer stay indoors," Royé explains.

Is covid-19 seasonal?

So far, the coronavirus has caused peaks in cases, hospitalizations and deaths at different times of the year. In the case of Spain, and after the initial explosion in spring 2020, these have been observed in months such as November, January, April and July.

This CDC graph compares deaths from covid-19 and influenza in recent years. While the latter appears almost mathematically around week 50, SARS-CoV-2 has to date been much more chaotic.

"I currently consider it not to be a seasonal disease," Royé explains. "As long as high-impact waves are occurring, I don't see a trend toward seasonality."

In the case of the covid-19 pandemic, Royé has studied other factors at play. "The variation in the effective reproduction number explained by government interventions is six times larger than for the average temperature," he says.

In other words: "Environmental effects exist [in the case of SARS-CoV-2], but their effect is moderate and, in addition, our habits and government interventions have much more influence."

Will covid-19 ever be seasonal?

"Estimating the timing is still very difficult," comments Royé, who believes that this step will occur if covid-19 becomes endemic after the pandemic phase.

Aginagalde recalls that MERS "does not seem to have a clear seasonality and has not decreased its overall lethality or morbidity."

If it becomes seasonal and/or endemic, will that mean that covid-19 has become milder?

"Seasonality does not decrease severity," says Aginagalde. He explains that many acute respiratory infection pandemics start "off on the wrong foot" at a time of year that does not fit their typical seasonality, but subsequently become seasonalized and flare up in specific periods without diminishing their severity.

"A simple example of the fact that being epidemic and seasonal does not change the severity can be seen in influenza: the intensity of the 2017-18 season was considerable compared to previous years and that did not mean it was no longer epidemic or seasonal," he adds. In other words, seasonality is a label that describes the distribution of a disease over time without implications for its severity or mildness.

Something similar happens with the concept of endemism, which only refers to the constant presence of a disease in a geographic population. Dengue fever, chickenpox, malaria, Chagas disease, tuberculosis and smallpox are or have been endemic in many areas of the planet, sometimes for thousands of years, regardless of their severity.

But don't viruses attenuate over time?

The idea that parasites evolve towards a more or less peaceful coexistence with their host in an almost predestined way is a widely held dogma, but often criticized by evolutionary biologists. Counterexamples range from measles to polio to rabies to AIDS.

Paul W. Ewald, an evolutionary biologist specializing in parasitism, extensively criticized this dogma in his 1994 book Evolution of Infectious Disease. "Few ideas have been so entrenched in the literature [...] Few ideas in science have been so widely accepted with so little evidence. And few ideas are so at odds with the fundamental principles on which they are supposedly based," he wrote. By principles he was referring to the theory of evolution by natural selection.

Criticism is based on the fact that natural selection acts at the level of the individual, without thinking about the common good of the species or the future. Therefore, the parasite does not care what happens to the host once it has been successfully transmitted, so attenuation is not an inevitable fate. For example, in diseases such as AIDS and covid-19, the virus can be transmitted long before the patient reaches the hospital.

A separate issue is that the population immunity achieved against a new pathogen, through infections or vaccines, may give the impression that the disease is no longer as harmful as it was at the beginning.

So what is the future of the covid-19 pandemic?

"If it were to follow the model of the 1918 flu, there would be a seasonalization and a reduction in its spread and severity," comments Aginagalde, who considers that it is not "the best precedent" due to the extraordinary wartime circumstances in which this pandemic took place.

"Nor do the cases of the influenza pandemics of the 1950s and 1960s, which were of short duration, seem extrapolable; or the Russian flu of 1889, which caused excess mortality for many years afterwards without losing intensity in the different waves, as did most of the nineteenth-century flus."

"It could happen [that SARS-CoV-2 ends up being endemic and/or seasonal], but we have no data on previous coronaviruses to extrapolate from and influenza pandemic models have limitations to support the claim without a hint of doubt," Aginagalde adds. "We have to answer honestly that we want it, but we don't know."