The new coronavirus is not the first coronavirus to jump from animals to humans. What can we learn from previous experiences?
We usually divide colds into flu and common cold. Both types of colds are caused by viral infections, but apart from similar symptoms, they have nothing in common. Influenza is caused by influenza viruses. But almost a quarter of patients with the common cold are caused by the coronavirus. They are relatives of the culprit of the new crown pneumonia epidemic, and they also jumped from animals to humans. Moreover, when it first began to spread among humans, it was once violent, leading to mass deaths.
Therefore, understanding the origin, spread and characteristics of the coronavirus that causes the common cold can provide us with important clues to fight this epidemic and predict its development. Understanding these relatively mild coronaviruses may also help us avoid another epidemic.
coronavirus is a big family, mainly caused by livestock disease is known. The coronavirus in humans was only discovered in the 1960s. The two strains discovered at that time caused only mild cold symptoms, so they did not attract the attention of virologists.
This situation began to change in 2002, when the SARS virus, a new member of the coronavirus family, began to infect humans. During the period 2002-2003, the SARS virus infected nearly 8,000 people in 26 countries, with a mortality rate as high as 10%. The powerful lethality of the coronavirus has sounded the alarm for us, and it is from then that virologists have paid attention to them.
The SARS virus was quickly traced to its source, and finally the source was found in bats. Bats are animals whose unusual physiology allows them to coexist with a large number of coronaviruses without getting sick. When bats infected the civet cat with the virus and the virus transferred from the civet cat to humans, SARS broke out.
The parasitic of the coronavirus on the host is achieved by the protein spikes on its surface (the name of the coronavirus is derived from this). These protein spikes are like a key that can open the “lock” of certain cells in the host. Protein spikes can also change shape due to mutations, thereby opening the door to a new host that is parasitic. It’s like a key can’t unlock the lock for the first time, change its shape, try again, so and so, there will always be a success. In the past, the host of the coronavirus was mainly domestic animals, but the SARS outbreak showed that the host can also be humans. Realizing this danger, virologists embarked on a journey of coronaviruses, tracking them on people and wild animals, trying to understand how these changes occurred and assess future risks.
One of the common cold virus:
in SARS virus we know Shortly after, a virologist in children suffering from bronchitis found another coronavirus NL63. Subsequent studies have shown that the NL63 coronavirus is widespread in humans, and 1%-9% of respiratory infections are caused by it, especially in children in the first few years after birth. The typical symptoms are fever, cough, sore throat, bronchitis and pneumonia, similar to the symptoms of the common cold. In other words, NL63 is another coronavirus related to the common cold.
Later, relatives of NL63 were found in pigs, cats and bats. A genetic comparison between the NL63 virus in humans and its close relatives in bats revealed that they had a common ancestor between 563-822 years ago. This shows that the NL63 virus jumped to humans at some point in the 13-15th century. Virologists further speculated that when it first jumped to humans, it might have caused a pandemic at that time. Because like the SARS virus and the new coronavirus, the initial spread of the NL63 virus is also fatal among people who lack immunity.
From the 13th to the 15th century, Europe was in the Middle Ages. Virologists had expected to find evidence of the NL63 coronavirus epidemic in medieval historical documents. Unfortunately, this job is not easy. Because many fatal diseases were prevalent in Europe in the Middle Ages, even if there are records, it is difficult for posterity to judge whether it is a pandemic of the NL63 coronavirus.
However, for a more recent epidemic, we already have more conclusive evidence that the culprit is the coronavirus. That epidemic was called the “Russian Flu”.
The second common cold virus:
In 1889, a disease that broke out in Central Asia spread, and the following year triggered an epidemic that swept the world, killing an estimated 1 million people. It causes symptoms such as fever and weakness. Limited to the level of awareness at the time, people classified this epidemic as influenza and called it “Russian influenza.” However, existing evidence indicates that the real culprit is likely to be a coronavirus.
The clues were spliced together later. In the early 1960s, virologists first isolated a coronavirus OC43 from patients with the common cold. However, because it only causes symptoms of the common cold, it has been ignored for a long time since then. It was not until the SARS outbreak that people regained interest in it. Virologists sequenced the genome of the OC43 virus and compared it with strains found in other animals. They concluded that the OC43 virus originated in cattle or pigs. They also calculated that the time it jumped to people was around 1890. This time is familiar, isn’t it? Yes, this is exactly the year when the “Russian flu” prevailed.
The reconsolidation of dates is not the only evidence. During that pandemic, many patients had obvious symptoms of central nervous system involvement. Today, although the OC43 virus is mainly associated with mild cold symptoms, it is also believed to infect nerve tissue. It is suspected of causing neurological diseases such as chronic demyelinating diseases and multiple sclerosis.
If the OC43 coronavirus was indeed the culprit responsible for the “Russian Flu” of 1889, then it has obviously lost its edge and become mild in the past 130 years. It may have lasted quite a long time in history, but as people adapt to it, its power is not as good as once, and now it can only cause mild cold symptoms.
Common cold virus three:
Another 229E coronavirus was discovered in the mid-1960s. Subsequent experiments showed that half of subjects infected with the 229E virus developed cold symptoms 2-5 days later. But because it only caused mild cold symptoms, it was ignored for a long time. It was not until 2007 that a close relative of the alpaca was found on its body, that it became the focus of attention again.
In 2012, a mysterious and deadly new coronavirus broke out in Saudi Arabia for the first time, causing the so-called Middle East Respiratory Syndrome (MERS). When people were tracing the source of the MERS virus, an unexpected discovery was that 5.6% of dromedaries in Arabia and Africa were infected with a virus similar to 229E.
A genetic comparison of the human 229E virus and the suspected 229E virus of the dromedary camel showed that they both originated from African bats. Their common ancestor jumped to dromedaries, and then jumped from dromedaries to humans sometime between 1686 and 1800. Like the MERS virus, when it first jumped to the human body, it may have triggered a fatal pandemic. It is a pity that historical evidence in this regard is lacking.
The fourth common cold virus:
In 2005, virologists discovered the fourth common cold coronavirus HKU1 in a 71-year-old patient with pneumonia. The elderly are particularly susceptible to this virus. A close relative of the HKU1 virus appears to be a coronavirus in rodents. We don’t know when it started to infect humans. However, compared with 229E and OC43, coronavirus HKU1 has milder symptoms. From this we can guess that the history of its spread in humans may be longer than the two coronaviruses mentioned above.
What inspiration can it bring us?
The four common cold coronaviruses have an interesting feature: their spread fluctuates. For example, for NL63, there are years with a high infection rate and a year with a low infection rate, approximately every two years.
In addition, they seem to be in competition. For example, between 2000 and 2010, NL63 and OC43 infections were more common than 229E and HKU1 infections, but in recent years, it seems that 229E and HKU1 have gained the upper hand. Virologists speculate that perhaps our immune system has a certain period of immunity to them. After being infected, it protects us for several years, and then it fails, so it is easy to be infected again. If the new crown virus that ignited this epidemic is also the case for us, the prospects are worrying.
A darker fact is that an experiment in 1990 found that subjects who had been infected with the 229E virus were easily re-infected a year later; moreover, when they were re-infected, they had no symptoms but could spread the virus everywhere. When asymptomatic people spread the virus without knowing it, you can imagine what this means for vulnerable people. Considering that a large number of asymptomatic infections have also appeared during this epidemic, we must promptly investigate people who have been infected with the new crown virus and then recovered to see if this is also the case.
However, research on the common cold virus also provides us with some reasons for optimism. The Coronavirus family consists of four subfamilies, of which two subfamilies can infect humans. The NL63 and 229E coronaviruses belong to the alpha subfamily. OC43 and HKU1 belong to the β subfamily. Beta subfamily also includes MERS virus, SARS virus and this new coronavirus. Antibodies to a virus are sometimes effective against viruses belonging to the same subfamily. Therefore, those who have just been infected by the OC43 virus or the HKU1 virus may also have a certain degree of immunity to the new coronavirus. If the situation is true, gaining immunity to the new coronavirus by deliberately infecting the OC43 virus or the HKU1 virus may be a strategy to fight the epidemic. Of course, this has yet to be further verified.
In the long run, the new coronary pneumonia virus is likely to be like the other four common cold coronaviruses, becoming mild over time, and even one day it will become another common cold coronavirus. It is also in its own interest to become mild. If the virus has been fatal all the time, and the infected hosts die one by one, then it has no chance to spread again. But this process of reducing toxicity may take many years. During this period, it is still a huge threat to us.