What does ‘endemic’ mean and would it be a good thing Covid-wise?

The coronavirus has a knack for dividing us and this week it opened up a new battleground: a scrap over the meaning of “endemic”. You won’t be surprised to learn that views on the subject are polarised.

“Covid will soon be endemic, thank goodness,” announced Dr Monica Gandhi, an infectious-disease physician and professor of medicine at the University of California, San Francisco, in the pages of the Wall Street Journal.

“Widespread immunity, vaccinated and natural, will bring control and a full return to normal,” she said.

Unfortunately for Dr Gandhi, her article was premature. Published in September, it came before the delta and omicron waves added another 145,000 deaths to the US total.

Prof Christina Pagel, at University College London, notes that “a virus isn’t endemic just because a government minister says it is and just because people want it to be”.

“The current pattern of waning vaccination, new immune evasive variants and minimal public health response seem set to doom us to massive surges once or twice a year,” she tweeted last week. Dr Helen Salisbury, a senior GP and Oxford academic, added that people may regret talking about Covid becoming endemic as a good thing. “TB and smallpox were once endemic in the UK – it doesn’t mean mild, it just means widespread,” she warned.

So what does it really mean for a disease to become endemic and where do we stand as regards SARS-CoV-2?

Prof Francois Balloux, at University College London, was one of the first to talk about Covid becoming an endemic disease and says, “in retrospect, we epidemiologists should have come up with a tighter definition”.

He says the dictionary definition of the word – a disease regularly found among people in a particular area – is misleading. For epidemiologists, the term is more technical and relates to a virus’s reproduction value settling at around one. “Essentially, it means that things are kept in check up to a point by the immunity in the population”, says Prof Balloux. “There is a stability and a predictability to an endemic pathogen but the complication is that they can still go up and down.”

Influenza is a good example. Its seasonal waves are largely predictable and kept in check through a mixture of natural immunity, vaccines and behaviour change. There are good years in which it kills very few and bad years where it can stretch health services. Prof Balloux is optimistic that Covid is heading in the right direction.

Things are “far from perfect”, he says, but it would be wrong to say there has not been progress. Vaccines have saved a huge number of lives. “With every new wave, the virus is left with less space to surprise.”

Prof Adam Kucharski, at the London School of Hygiene and Tropical Medicine, thinks it may take longer for things to become predictable. He notes Covid may not yet be settled into a gentle foreseeable drift, where new variants comes from a known lineage.

Omicron didn’t emerge from the delta lineage and delta didn’t emerge from alpha, beta or gamma, he says. “We’ve still got a period of uncertainty before we can predict the coming years with any confidence,” he said. “Covid evolution might pause in an omicronshaped corner for a while, or omicron might give rise to another variant, much like we’d see for seasonal coronaviruses and flu, or we could see another evolutionary surprise.”

For now, omicron cases appear to be plateauing in parts of the UK.

Yesterday, Dr Susan Hopkins, the UK Health Security Agency’s chief medical adviser, said the number of infections were flat in London and the South East and rising only slowly in the North.

“All of that means we are seeing a slowdown in the number of admissions to hospital but they are slowing down rather than reversing”, she said.

With hospitalisations still at 2,000 a day and regular NHS business still on hold, Dr Hopkins will hope SARSCoV-2 becomes endemic in the UK at a somewhat lower level than it is today.