Will there be repeated "Yang Yang Yang" after "two Yang"? What is the final outcome of "new crown"?

Three years after the COVID-19 outbreak, many descriptions believe that it is no longer a "pandemic" but has gradually entered the "endemic" stage. The term endemic is misleading. It does not mean that a disease is confined to a certain place, but that the infection rate in a population remains basically constant. In other words, it coexists for a long time.

Copyright images in the gallery. Reprinting and using them may lead to copyright disputes.

Many people hope that it will become like other endemic diseases, such as the common cold, which everyone gets but has no serious consequences and even gets weaker and weaker until it disappears on its own. Is this possible?

In the history of the evolution of life on Earth, countless species have come and gone. Most of the stories they have told are unknown to humans and have disappeared forever in the long geological era. However, evolutionary biologists have still been able to interpret some of these stories through various methods.

Next, let’s use these stories as a blueprint to conduct a series of possible simulations on the future of the new coronavirus.

01

Disappeared suddenly

Floating clouds come and go, who can convey this meaning? - Liu Changqing

Diseases have indeed disappeared in history. The most recent famous case is probably SARS (Severe Acute Respiratory Syndrome). According to WHO statistics, as of December 31, 2003, SARS infected at least 8,096 people and caused 774 deaths [1], and then the disease disappeared.

Of course, the “disappearance” is not the result of its own initiative, but the result of the efforts of medical staff, public health workers and even the whole society. The success of these efforts is also inseparable from the characteristics of the SARS virus (SARS-CoV) itself. It is very deadly, but it has a long incubation period and it is difficult to infect others during the incubation period. Therefore, as long as the speed of epidemiological investigation is fast enough, all potential exposed people can be found and isolated before symptoms appear.

Copyright images in the gallery. Reprinting and using them may lead to copyright disputes.

In contrast, the disappearance of other diseases is more puzzling.

The 1918 influenza pandemic , commonly known as the Spanish flu, did not actually originate in Spain, but other major countries were at war at the time, and the initial news of the epidemic was not made public, so neutral Spain stood out from the crowd. The flu infected about a third of the world's population and killed tens of millions, with some researchers putting the death toll at 100 million.

But after 1920, the lethality and spread of this flu strain were significantly less than before, and it was soon forgotten by most people. Finally, in 1957, this strain completely disappeared and was replaced by other strains.

If we have to explain "why", this is probably the common product of increased immunity in the population after infection, strengthened public health measures during the epidemic, and competition from other strains , but the specific details have not yet been fully determined.

The influenza pandemic was relatively recent and widespread, leaving behind a lot of research and records, so we can make some guesses. In history, there are many infectious diseases that disappeared without a clear explanation.

Archaeologists have found traces of a variant of the smallpox virus in human bones from the 11th century that was very different from the smallpox that later spread around the world and was eventually eradicated by vaccines, and that apparently disappeared hundreds of years ago.[2]

In 1917, researchers discovered a disease called Encephalitis lethargica (not the same disease as today's epidemic meningitis). After wreaking havoc for about ten years, it mysteriously disappeared. Although it caused the deaths of about 500,000 people worldwide, the cause of the disease is still unclear, let alone why it disappeared.

02

Arms race

If you don't see the strange appearance, how can you know the god of creation? - Li Bai

However, we cannot assume that pathogens will weaken or even disappear if left alone. Rabbit myxoma virus (MYXV) is a very famous counterexample.

Sick rabbit with myxomatosis. Copyright image. Reprinting and using may cause copyright disputes.

In the mid-19th century, European colonists introduced rabbits to Australia for hunting. Soon, the rabbit population exploded due to the lack of natural enemies, causing serious ecological damage and economic losses. In 1950, the Australian government released the rabbit myxoma virus to control rabbits, which quickly reduced the rabbit population to 1% of its original number.

However, the problem does not end there.

Ten years later, the number of rabbits began to recover because the mortality rate of the virus itself dropped from nearly 100% at the beginning to 70% to 95%, and the mortality rate of variants in some areas was less than 50%. Researchers believe that this is because the virus is too deadly and will cause the host to die before it can spread after infection, so variants with lower mortality rates gradually gain the upper hand.

However, viruses are not the only ones evolving. Rabbits are also evolving. They are gaining stronger resistance. The same variant once killed 90% of rabbits, but seven years later the mortality rate in the same location was only 26%. For viruses, being too toxic will certainly lead to the premature death of the host, which is not a good thing, but being too weak will directly be killed by the host's immune system, which is also not a good thing.

As a result, the rabbits' increased immunity caused the virus's evolutionary direction to reverse, and its toxicity increased again. Variants with a mortality rate of less than 50% almost disappeared, and several variants showed a new ability to suppress host immunity.

By the 1990s, most variants had acquired the ability to induce fatal immune collapse and septic shock, something that the 1950s variants lacked, and the original myxoma symptoms became secondary.

Today in Australia, the evolutionary arms race between rabbits and rabbit myxoma virus is still ongoing.

03

Sudden rise

The sun and the moon change their colors, and the wind and clouds change from morning to night. ——Zhang Zai

Even if a disease seems to have weakened enough to coexist with humans for a long time, it may become strong again and harm humans due to environmental changes or accidental factors . This seems to be the fate of polio .

In fact, human efforts to control polio have been very effective. In 2022, there were only 30 cases of wild-type polio worldwide, and it is expected to become the second infectious disease to be eradicated by vaccines after smallpox. [3]

However, the history of polio is very different from that of smallpox in one very important way.

Smallpox has always been a terrible killer in human history, breaking out continuously for thousands of years; but polio has been relatively silent for most of history, with only sporadic records, until the late 19th century when it suddenly became a widespread and deadly disease. And contrary to common sense, large-scale polio outbreaks first occurred in industrialized countries in Europe and the United States where medicine and public health were making rapid progress, rather than in underdeveloped areas with relatively poor sanitary conditions.

Copyright images in the gallery. Reprinting and using them may lead to copyright disputes.

Researchers speculate that historically, polio has been spreading endemic among the population, infecting children under the age of 4 almost harmlessly, with most of the symptoms after infection being very mild and not noticeable. Moreover, this mild infection allows the infected person to gain immunity.

Improved sanitation in industrialized countries in the late 19th century disrupted the transmission of many deadly diseases, but also disrupted polio's original endemic routes. When older children and adults are infected with polio for the first time, their symptoms are much more severe and can even cause paralysis.

Still, there are many mysteries about polio, so this explanation may not be the whole story. Regardless, it is clear that the disease suddenly became deadly after a long period of obscurity.

04

Point burst

I only know that we are going to part, but I don’t know that we will meet again.

Not to mention long-standing “unknown” viruses like polio, even viruses that seem to have disappeared can resurface without warning. Ebola is a famous example in recent years.

Ebola virus. Copyright image. Reproduction may cause copyright disputes.

Since it was discovered in 1976, Ebola has maintained a cycle of "outbreak-disappearance-re-outbreak". There have been eight outbreaks since 2018 alone, resulting in more than 2,000 deaths.

The cycle itself is not mysterious:

The natural host of Ebola is certain fruit bats, which can infect humans through contact with animals and then spread from person to person through contact with contaminated blood. Because the mortality rate of Ebola infecting humans is too high (50% to 90%) and the late symptoms of patients are extremely obvious, every outbreak will trigger a large-scale medical and public health response and eventually eradicate it.

However, what can be eliminated is only the human transmission this time. It is impossible to eliminate all fruit bats carrying the virus, or even to drive them away from all human settlements. Moreover, the Ebola virus can also infect other wild animals besides fruit bats, and hunting and slaughtering these animals may also bring them into contact with the virus. Therefore, no matter how many times the virus is eradicated, the continued outbreak of Ebola is basically inevitable.

05

Conclusion

Only by sharpening the weapons and preparing the horses can one be ready for dawn.

In nature, the number of parasitic species is likely to exceed that of non-parasitic species. Therefore, being infected by pathogens and contracting infectious diseases is the norm for almost all life forms—even the pathogens themselves may become infected. In this sense, completely eliminating a pathogen is an extremely rare and uncommon scenario, and "long-term coexistence" is a more common scenario.

Of course, this coexistence is not "peaceful" at all. The host's immune system will do its best to completely eliminate the pathogen. Unfortunately, most of the time, the parasite accumulates mutations faster than the host. Putting aside new diseases that have just arrived, it is not impossible to rely on natural immunity to eliminate a disease that has already established a foothold, but it is also quite "surreal".

Copyright images in the gallery. Reprinting and using them may lead to copyright disputes.

On the pathogen side, the goal is certainly not to kill the host and cut off its own transmission route, but it is definitely not peaceful coexistence either.

In addition to constantly resisting the attack of the immune system, pathogens also face the "tragedy of the commons" : a host is parasitized by a large number of foreigners at the same time. Even if the host is gentle and kind, it may be killed by other pathogens - if this happens, it will be a big loss. Some rare environments can cause parasites to no longer cause disease and achieve symbiosis with the host, but in most cases, pathogens will not be infinitely weakened.

Fortunately, humans have something that is faster than our own evolution: modern medicine and public health. Advances in drugs, vaccines, care, prevention, infrastructure, and social systems can keep up with the pace of pathogens. We can train our immune systems to restrain new mutations of pathogens, change our behavior and the environment around us to block the spread of pathogens, and even try to manipulate natural selection to guide the evolution of pathogens.

Ultimately, evolution is not a law of physics, but the result of multi-party games under specific history and environment. Although we cannot accurately predict the future, we can guide evolution by influencing the environment and push the future in a better direction. The new crown may never disappear, but we have the tools in our hands to reduce its influence. The key lies in whether we can use them effectively.

References:

[1]https://www.who.int/publications/m/item/summary-of-probable-sars-cases-with-onset-of-illness-from-1-november-2002-to-31-july-2003

[2]https://www.science.org/doi/10.1126/science.aaw8977

[3]https://polioeradication.org/wp-content/uploads/2023/05/weekly-polio-analyses-WPV-20230103.pdf

Author: Fang Gang, PhD student in evolutionary biology; Xu Lai, popular science author

Reviewer: Zou Zhengting, Researcher, Institute of Zoology, Chinese Academy of Sciences

The cover image and the images in this article are from the copyright library

Reprinting may lead to copyright disputes