HISTORY has shown us that every global pandemic has inevitably evolved into an endemic disease, writes our regular expert contributor professor Deborah Briggs.
Clearly, every one of us could write our own definition of the word pandemic as it relates to what we have personally experienced living with Covid-19 for the past two years.
However, it might be useful to define the difference between what constitutes a pandemic state and what constitutes an endemic state of a disease.
Pandemics are defined as being sudden disease events that spread rapidly across international borders causing large-scale social disruption, economic loss and general hardship.
On the other hand, endemics are defined as being the normal conditions of diseases in society. During the endemic phase of a disease, infections become relatively constant across time with occasional increased outbreaks.
A perfect example of an endemic disease we all live with is seasonal influenza. However, when influenza viruses suddenly change their components, a pandemic will often happen. It is also important to remember that influenza is not the same disease as Covid-19.
So what might the future look like for all of us currently living in the middle of the Omicron wave of Covid-19 infections?
One thing that we can be sure of is that SARS-Cov-2, the virus that has caused yet another wave in this pandemic, is unlikely to be eliminated from the world.
There is only one documented case of global eradication of a human virus and that was the virus causing smallpox. There are unique reasons why the smallpox virus was able to be eradicated globally that are not present for SARS-Cov-2, including the fact that the smallpox virus only infected humans and had no animal reservoir.
SARS-Cov-2, on the other hand, is a coronavirus that does infect animals, for example cats, minks as well as other animals are susceptible to this virus. SARS-Cov-2 will continue to exist in nature and thus it is likely to continue to pose a threat to humans.
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In addition, the smallpox eradication programme was organised and launched as a massive and highly coordinated global vaccination campaign; there was a high level of political will from countries across the world that were all working toward the same goal of global eradication; and vaccines were made readily accessible to everyone in remote corners of the world. Unfortunately, the same cannot be said for SARS-Cov-2 and the current pandemic.
If we cannot eliminate SARS-Cov-2 from the world, then what happens next?
Historical evidence has shown us time and time again that every virus that has caused a pandemic will inevitably become endemic in the human population.
When SARS-Cov-2 switches from being in a pandemic state to an endemic state, the infection rates would remain constant across time with perhaps seasonal outbreaks and occasional flare-ups, as we see with seasonal flu.
We will not see that happen across the world with SARS-Cov-2 until we have achieved herd immunity globally.
When we achieve herd immunity locally or even nationally, SARS-Cov-2 will not be able to cause severe illness and death on such a scale as we initially experienced in the UK. However, populations living in other regions with limited access to vaccines will continue to suffer and travel to and from infected areas will still be very challenging.
What will herd immunity look like and how can we achieve that goal?
The daily numbers of Covid-19 infections and the percentage of the population that are fully vaccinated are certainly important factors in determining when we will reach herd immunity, but they are only part of the equation.
It is critically important to consider the number of hospitalisations, percentage of hospitalised patients that are fully vaccinated, partially vaccinated, or unvaccinated, existing underlying conditions in hospitalised patients, as well as deaths.
I have mentioned previously that no vaccine is 100 per cent protective but recent data has provided strong evidence that Covid-19 vaccines are saving lives and preventing serious illness even in patients with breakthrough infections.
Prior to 2019, SARS-Cov-2 was unknown. The fact that SARS-Cov-2 viral infection was novel to humans meant that our immune systems became quickly overwhelmed when faced with such a new, rapidly spreading virus.
Without mass vaccination of a population, the only way that herd immunity can be achieved is through natural infection spreading through an entire population to build up herd immunity over time. That method of achieving herd immunity ultimately results in a huge death toll.
For example, there were no influenza vaccines available during the 1918 Influenza pandemic and the number of recorded deaths during three successive global waves of that pandemic is estimated to be well over 50 million.
Without a doubt, achieving herd immunity through mass vaccination of a population and by using physical control methods such as effective masks, hand washing, effective ventilation spaces, etc. until the majority of a population can be vaccinated is a more acceptable approach.
As mentioned above, SARS-Cov-2 is a new coronavirus to infect humans. However, there are four other human coronaviruses that have been infecting humans for possibly hundreds of years and have already reached global endemicity.
Those four endemic human coronaviruses might provide some clues about what SARS-Cov-2 infection might look like in the future. Two of the four endemic human coronaviruses are responsible for about 15 per cent of respiratory illnesses that we know as ‘the common cold’.
Studies have shown that most children come in contact with these four coronaviruses before they reach the age of six, are mildly infected, and develop immunity to them. The early immunity to the four endemic coronaviruses apparently wanes and is not sufficient to block later reinfection completely but it does protect adults from becoming seriously ill.
That might also be the case for SARS-Cov-2 and might provide some indication as to the road ahead. We will inevitably reach herd immunity and Covid-19 infection will become an endemic disease that we will live with.
However, there is still a long way to go and the next several weeks/months are most likely going to be difficult.
Nature 16 February 2021: https://www.nature.com/articles/d41586-021-00396-2
BMJ2021; 372:n494, published 18 February 2021: http://dx.doi.org/10.1136/bmj.n494
Professor Deborah Briggs has served as an expert to the World Health Organisation in the field of rabies prevention. She has provided expertise in the field of vaccine production and clinical trials for major pharmaceutical companies and continues to teach an online graduate level course in Global Public Health for Kansas State University. She lives in Bridge End, Burra.
More from Professor Briggs:
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