The word ‘endemic’ has become one of the most misused of the pandemic. And many of the erroneous assumptions encourage a misplaced complacency. This does not mean that COVID-19 will come to a natural end.
For an epidemiologist, an endemic infection is one in which overall rates are static – not rising, not falling. More precisely, it means that the proportion of people who can become ill offsets the virus’ ‘basic reproductive rate’, the number of individuals that an infected individual would infect, assuming a population where everyone could become ill. Yes, the common cold is endemic. The same is true of Lassa fever, malaria and polio. The same was true of smallpox until vaccines knocked it out.
In other words, a disease can be endemic and both widespread and fatal. Malaria killed more than 600,000 people in 2020. Ten million became ill with tuberculosis the same year and 1.5 million died. Endemic certainly does not mean that evolution has somehow tamed a pathogen so that life simply returns to ‘normal’.
As an evolutionary virologist, it frustrates me when political decision-makers invoke the word endemic as an excuse to do little or nothing. There is more to global health policy than learning to live with endemic rotavirus, hepatitis C or measles.
To say that an infection will be endemic says nothing about how long it can take to reach stasis, what the case is, morbidity levels or death rates will be or, crucially, how large a part of a population – and which sectors – there are will be receptive. Nor does it suggest guaranteed stability: there may still be disruptive waves from endemic infections, as seen with the US measles outbreak in 2019. Health policies and individual behavior will determine what form – out of many options – endemic COVID-19 takes.
Shortly after the Alpha variant appeared and spread in late 2020, I argued that unless infections were suppressed, viral development would be rapid and unpredictable, with the emergence of more variants with different and potentially more dangerous biological properties. Since then, public health systems have struggled under the highly transmissible and more virulent Delta variant, and now there is Omicron with its significant ability to evade the immune system, causing reinfections and breakthroughs. Beta and Gamma were also very dangerous, but did not spread to the same extent.
The same virus can cause endemic, epidemic or pandemic infections: it depends on the interplay between a population’s behavior, demographic structure, susceptibility and immunity, plus whether viral variants occur. Different conditions around the world can allow more successful variants to develop, and these can create new waves of epidemics. These seeds are linked to a region’s political decisions and capacity to respond to infections. Even if a region reaches an equilibrium – be it low or high disease and death – it can be disturbed when a new variant with new characteristics arrives.
Of course, COVID-19 is not the world’s first pandemic. The fact that immune systems have evolved to cope with constant infections, and the traces of viral genetic material embedded in our own genomes from ancient viral infections, is evidence of such evolutionary struggles. It is likely that some viruses went ‘extinct’ by themselves and still caused high mortality rates on the way out.
There is a widespread, rosy misconception that viruses develop over time to become more benign. This is not the case: there is no predetermined evolutionary outcome for a virus to become more benign, especially those such as SARS-CoV-2, where most transmission occurs before the virus causes serious illness. Consider that Alpha and Delta are more virulent than the strain first found in Wuhan, China. The second wave of the 1918 flu pandemic was far more deadly than the first.
Much can be done to shift the evolutionary arms race to the benefit of humanity. First, we must put aside the lazy optimism. Second, we need to be realistic about the likely levels of death, disability, and illness. Reduction targets should take into account that circulating viruses risk giving rise to new variants. Third, we need to use – globally – the formidable weapons available: effective vaccines, antiviral drugs, diagnostic tests and a better understanding of how to stop an airborne virus through masking, distancing and air ventilation and filtration. Fourth, we need to invest in vaccines that protect against a wider range of variants.
The best way to prevent more, more dangerous or more transmissible variants from emerging is to stop unlimited spread, and this requires many integrated public health interventions, including, all-important, vaccine equity. The more a virus replicates, the greater the chance that problematic variants will occur, most likely where the spread is greatest. The alpha variant was first identified in the United Kingdom, Delta was first found in India and Omicron in southern Africa – all places where the spread was violent.
To think that endemism is both mild and inevitable is more than wrong, it is dangerous: it sets humanity up for many more years of illness, including unpredictable waves of outbreaks. It is more productive to consider how bad it can get if we continue to give the virus opportunities to outsmart us. Then we might do more to make sure that does not happen.
The author declares no competing interests.