Endemic COVID?
I discussed much of this in response to a question on a post on my Facebook page. There is an excellent Twitter thread on Endemicity, and why COVID is not endemic, but it brought up more questions.
“Endemic” comes from the Greek words “demos” = people.
“en” is of the people or in the people or “endemios” — native.
(I don’t know Greek, but this is how every public health student learns it)
A disease is “endemic” if it persists in a population, usually in a defined geographic region, with a stable rate of transmission. We have a reasonable expectation of the degree of infection and illness in a population because it is always there or at least is seasonal.
Endemicity has nothing to do with severity. Endemic does NOT mean “Mild.” It essential means we know to expect it and have it under control. This is the best analogy I’ve seen:
Influenza circulates yearly. It’s fair to say it’s endemic in much of the world. We have surveillance to monitor its spread. Most years, it follows an expected pattern. Some years, influenza can lead to a much larger outbreak — more cases than usual and more severe than expected. This could be called an epidemic.
The 2003–2004 flu season was such a year. I remember it well as I was extremely ill. I had had a significant flu like illness when living in Thailand in September (it was also the time of SARS-1), and then over Christmas in the US had flu (confirmed, I was also unvaccinated — last time for that!).
We had another epidemic in the 2017–2018 Flu season.
We “live with” endemic diseases but that doesn’t mean we do nothing to prevent them. On the contrary, we work constantly to prevent them. Influenza’s seasonality means we try to prevent outbreaks with an ever changing yearly vaccine as well as control efforts. It is not often necessary and we try to avoid it, but we can and do close schools on occasion (generally it’s only one campus).
Malaria is endemic in Sub-Saharan Africa. Dengue is endemic in parts of Asia. There’s a region in Sub-Saharan Africa known as the “Meningitis Belt” where that disease is “hyper-endemic.” The bacterium persists in a low level, there are seasonal outbreaks and every 8–12 years we see an epidemic. There are vaccines for 5 strains of meningitis (MCV4 and MenB). Widespread vaccination in the Meningitis Belt could potentially eradicate it, but the vaccine is not universally available due to cost. This is an excellent example of the “simmering pot.” (MCV4 is required to attend Texas public schools and universities. It’s not endemic in the US but there are occasional — very deadly — outbreaks.) This is also an excellent example of vaccine inequity….
How does herd immunity fit in?
Ideally, if we have herd immunity, a disease will NOT be endemic.
Measles is still endemic in some parts of the world, though herd immunity through vaccination has changed that, mostly. A vaccine and booster push in the US in the 1990s sought to eliminate measles by 2000. Vaccination rates greater than 95% led to near-herd immunity. As a result, measles was no longer endemic in the US — we didn’t have regular on-going transmission. So many were immune due to vaccination that the virus didn’t circulate. The “herd” protected the few who were unable to vaccinate or for whom the vaccine didn’t take. But because it existed elsewhere, we’d get the odd imported cases. Transmission of measles was so low that even 1–2 cases prompted an intense public health response.
That has changed in the last 15 years or so with the increase in anti-vaccine groups. MMR rate of vaccination has dropped. Though still relatively high in the US, there are clusters of anti-vaccine groups where vaccination rates are low and one infection can lead to many, as in the Disneyland outbreak. Many think “outbreak” must means hundreds or thousands of cases. It just means above the norm. So 20 cases of measles in the US is an outbreak. Thanks to widespread vaccination, such an outbreak is generally short lived.
In 1993, there was an outbreak of a deadly disease in the Four Corners region of the US. It had a mortality rate of 50%, though only 24 people were infected. Eventually a novel Hantavirus was isolated, later to be called Sin Nombre virus. Thanks to intense mitigation efforts, the outbreak ended. The virus is now considered to be endemic, because it persists in its rodent host, but constant surveillance and control efforts have mostly kept human infection to a minimum. It is still deadly when it occurs.
The often invisible to the public role of Public Health is keeping that endemic simmering water from boiling over. We do that through constant — when funded - surveillance.
Will we get there with COVID? Probably, but that doesn’t mean it’ll be a mild disease.
This isn’t endemic. This isn’t seasonal. This is still explosive illness we can’t quite control. This rate of hospitalizations is not sustainable.
As noted above, influenza is essentially endemic with seasonal bursts that are usually manageable. Influenza IS deadly. It is a severe illness even in otherwise health people. However, because influenza viruses have persisted in the human population for forever, there is generally *some* immunity to most strains. We get flu pandemics when a novel strain appears. The H1N1 pandemic of 2009–2010 was such a strain, but because it had some similarity to earlier strains that it wasn’t as deadly in the elderly population who are thought to have had some immunity to it.
Until late 2019, to our knowledge, no humans had immunity to SARS-CoV-2. Every human on the planet was susceptible. And like some pathogens, natural infection does not need to long lasting immunity.
Can we eradicate COVID-19?
Only one human pathogen has been eradicated: smallpox. And we didn’t do that through natural infection, we did it via vaccination. (There are many reasons why smallpox was ideal for eradication and many other pathogens are not.)
A vaccine doesn’t need to be perfect to control a disease.
The vaccine for influenza changes year to year in response to the potential new strains circulating. We mostly keep flu outbreaks under control through annual vaccination.
The mumps vaccine is only 88% effective and we’ve managed to stall most outbreaks — that’s because most people are vaccinated and there is sufficient herd immunity. So even when we have breakthrough infections, they don’t spread very far into a population.
We’re not going to eradicate COVID, at least not any time soon. We can go from the current “vat of boiling water” overflowing to a “simmer” through intense control efforts — vaccination. Because we can’t vaccinate fast enough and because efficacy varies, we need additional mitigation efforts to control COVID. Masks, limiting large social gatherings, quarantine and isolation when necessary, all help to control the spread. Above all, vaccine equity is essential.
COVID endemicity won’t mean we can ignore it, just that we have the tools — and use them — to control it.
*Note: It may seem like endemicity is linked to severity because often severity will decrease. There are multiple reasons for this. In part it’s due to the ability of healthcare systems to respond quickly and comprehensively to infection before it becomes severe. It also means we can prevent infection in those most likely to develop severe illness. The fatality rate of measles has decreased not because the virus changed, but our ability to respond to it improved. If hospitals were overwhelmed with measles cases, the fatality rate would climb again.