The coronavirus outbreak was first referred to as an epidemic, then a pandemic. Now you might be hearing scientists describing Covid-19 as ‘endemic’.
So, what’s the difference between these terms? If an epidemic refers to a disease that spreads rapidly within a community or region, a pandemic is an epidemic that has spread worldwide – crossing international boundaries and affecting a large number of people.
The government’s chief science adviser, Sir Patrick Vallance, recently told a select committee on national security it’s “likely” that Covid-19 will become endemic. The definition of endemic, in the context of Covid-19, is a disease or something that is native to or commonly found within an area.
The suggestion here is that Covid-19 could become endemic in the UK, like the flu – with recurring outbreaks.
“Now of course we can’t be certain, but I think it’s unlikely we’ll end up with a truly sterilising vaccine – i.e. something that completely stops infection,” Vallance told MPs. “It’s likely that this disease will circulate and be endemic.”
Many experts on the Scientific Advisory Group for Emergencies (Sage) think this is “a likely outcome”, he added. “Clearly, as management becomes better, as you get vaccination which would decrease the chance of infection and severity of disease – or whatever the profiles of the vaccines are – this then starts to look more like annual flu than anything else. That may be the direction we go in.”
What would make Covid-19 endemic?
Researchers Jeffrey Shaman and Marta Galanti, from Columbia Mailman School, identified some of the key factors that would contribute to Covid-19 becoming endemic. These include: the risk of reinfection, how widely available (and effective) a vaccine is, as well as potential seasonality of the virus and interactions with other viral infections that may alter its transmission.
Their paper, published in the journal Science, explores a scenario where immunity to the virus diminishes within a year – a rate similar to another type of coronavirus that causes mild respiratory illness. If this were the case, there would be annual outbreaks of Covid-19, much like the flu, they said.
But if immunity to the virus lasted longer, we might experience what would initially appear to be an elimination of Covid-19 followed by a resurgence after a few years.
Either way, it’s probably coming back. “Should reinfection prove commonplace, and barring a highly effective vaccine delivered to most of the world’s population, SARS-CoV-2 will likely settle into a pattern of endemicity,” the authors said.
“Whether reinfections will be commonplace, how often they will occur, how contagious re-infected individuals will be, and whether the risk of severe clinical outcomes changes with subsequent infection remain to be understood.”
So, when might this happen?
Judging by what makes flu endemic, it’s likely we’ll know whether Covid-19 is endemic by Christmas next year, according to Dr Julian Tang, a consultant virologist at the Leicester Royal Infirmary and honorary associate professor in the Department of Respiratory Sciences at University of Leicester.
By this point, he envisages about 80% of people will have been infected with Covid-19 (if not more). By 2022, he predicts we’ll see some form of seasonality kick in – as with flu – where there are Covid-19 outbreaks over the winter months.
“If you look at the influenza pandemics, they’ve all become seasonal after it’s been through the whole world – and then the seasonality and endemicity is driven by the new birth cohort each year,” Dr Tang explains.
Adults get the virus, then recover or die, and all the while new babies are being born. “The massive first wave of the pandemic sweeps through the population that’s susceptible, then dies out – as we saw with the 2009 pandemic and as we’ve seen with previous pandemics of flu. Then you get a seasonal spiking.”
Flu viruses, seasonal coronaviruses and other respiratory viruses tend to spike in winter for a few reasons, but mainly due to human immunity and climate.
Dr Tang points out that cooler weather drives people indoors into crowded spaces. At the same time, immunity dwindles if we’re not getting enough vitamin D, and viruses tend to survive better in colder temperatures.
The hope is that vaccines will protect people from the most severe symptoms of the virus by boosting an immune response. Experts suggest we’ll probably need multiple vaccines (possibly yearly ones) to protect us from the worst outcomes of Covid-19.
Will Covid-19 ever disappear completely?
Total eradication of the virus is very unlikely, but not impossible. For example, smallpox, a virus that was very contagious and often deadly (three in every 10 people who got it died), was eradicated in 1980 thanks to vaccination programmes.
Professor Hans Heesterbeek, an expert in theoretical epidemiology at Utrecht University, in the Netherlands, wrote in a piece for The Conversation that if vaccines are found to prevent clinical disease, strongly reduce transmission and cause long-lasting immunity to Covid-19, eradication of the illness is possible.
“But realistically this is unlikely,” he said. “Eradication is notoriously difficult, even for diseases for which we have almost perfect vaccines and permanent immunity. Endemic disease is therefore the most likely outcome.”
This article originally appeared on HuffPost UK and has been updated.