Stream A and Noru also have "new crown sequelae"? Will it get scarier?

Viruses such as influenza A and noro, which have recently caused some school closures, have actually existed in human society for thousands of years. After the pandemic, their epidemic performance is temporarily different from the past, but they will eventually return to their usual state. In the past, we were able to coexist with these common infectious diseases, and in the future, we are likely to coexist at a smaller cost.

Written by | Zhou Yebin

Recently, some schools in many places have taken measures to suspend classes or online teaching due to some students infected with the new crown or influenza. Having just experienced the peak of the new crown epidemic, many people have palpitations, and they are more wary of various infectious diseases, worrying about whether the second wave of the new crown epidemic is coming, or whether other viruses will affect our lives.

In fact, there are many infectious diseases that have existed in our lives for a long time, and the new crown should also join them and become a common disease that coexists with us. In the post-pandemic era, we should not only be aware of the possible re-emergence of some infectious diseases that have been neglected in the past few years, but also take a rational view of the actual risks of various common viruses and take reasonable but not overly aggressive responses.

1 Human society has always been a society that coexists with pathogens

In the three years of the new crown, the general public has been extremely impressed and even afraid of the word "virus". Therefore, when they hear that a child in a school is infected with influenza A or an outbreak of norovirus in a certain place, many people have a cold back, for fear that a new epidemic is coming. In fact, human society has been coexisting with many common pathogens, including the recent news of influenza A and norovirus, which are common pathogens and can be regarded as our long-term "companions". These viruses may sound unfamiliar to the name, but when it comes to common symptoms, such as suddenly getting a "bad cold" or "eating bad things" diarrhea in winter, most people have had similar experiences in their lives. For such "minor diseases", we will not pay too much attention to them, but if we really want to dig the root problem, behind these "small diseases" is likely to be those viruses that have recently been on the news and scared us.

The full name of influenza A is influenza A virus. There are many types of influenza A viruses, some mainly infecting birds or other mammals, such as H5N1, which has been spreading globally since the end of the 20th century, is a highly pathogenic avian influenza that mainly infects birds, and it is also classified as influenza A viruses. But the children infected with influenza A reported in the news refer to influenza A in seasonal flu. That is, influenza viruses that have existed in our society for a long time, are more common in winter, and form seasonal characteristics.

Seasonal influenza viruses are caused by two main influenza A strains, H1N1 and H3N2, and two influenza B virus lines, Victoria and Yamagata. In general, influenza A dominates seasonal influenza, although the proportion of influenza B in children is generally slightly higher than in adults. The specific circulating virus strains also vary from year to year. However, overall, it is normal to have a large number of cases of influenza A in winter. For example, the CDC estimates that from 2010 to 2020, the number of symptomatic illnesses caused by seasonal influenza is about 9 million to 40 million per year, which can be said to be very common [1].

Another recent news is norovirus, which is the most common pathogen causing infectious gastroenteritis [2]. The symptoms caused by norovirus are also known abroad as winter vomitting disease, reflecting the fact that it also prefers to have outbreaks in cold weather. Another common respiratory infectious disease that also likes to occur in winter is respiratory syncytial virus RSV, and symptoms often manifest as the common cold.

It's not hard to see that many viruses seem to use winter as a comfort zone. It really has to do with the behavior patterns of their hosts, which are us humans. During the cold season, we spend more time indoors, which gives these viruses more opportunities to spread and cause outbreaks. This is true of respiratory viruses such as influenza and RSV, and even the novirus that causes gastroenteritis is mainly transmitted from person to person, such as aerosol transmission after vomiting in an infected person. Therefore, it is still prone to outbreaks in winter when there are more crowds.

2 In the post-COVID era, common infectious diseases are also coming out of the epidemic

Many people may find it strange, experts often say that these infectious diseases are common, but they have not heard of it in recent years, what is going on?

First of all, many common infectious disease viruses have been neglected by us for a long time. This can also be seen from the history of the discovery of some viruses. For example, the RSV virus was not discovered by scientists until 1956, and was originally named "chimpanzee upper respiratory pathogen" because it was found in chimpanzees, and later scientists realized that this respiratory viral infection is extremely common in the population, almost everyone was infected before the age of three, and those infected chimpanzees were infected by breeders, and then renamed "human respiratory syncytial virus" [3]. Norovirus is similar, its name comes from the 1968 outbreak in an elementary school in Norwalk, Ohio, USA, but in fact, similar outbreaks were recorded in Denmark in the 1930s, and from the genomic analysis of different strains, several strains have parted ways in the first or two thousand years BC, and these viruses have accompanied us before human civilization [4].

It can be seen that some viruses that have existed in human society for a long time have also been ignored by us, and it is normal that the general public has not heard of it. Even if seasonal influenza, an infectious disease that brings a large number of hospitalizations and even deaths at its peak every year, people who usually have less concern about infectious diseases or public health are likely to be confused with colds. However, after the new crown epidemic, the public has a deep experience of the threat of infectious diseases, and the media will report more cases of various infectious diseases, which will naturally give people the feeling that the virus is suddenly more visible.

Secondly, we should also note that as human society gradually comes out of the shadow of the new crown pandemic, some common infectious diseases are also "returning to normal". In the previous article, an important factor in the outbreak of infectious diseases in winter is the pattern of human behavior, and people gather more in the cold season, which gives the virus the best chance of spreading. However, during the past three years of the pandemic, human behavior patterns around the world have changed drastically for a short period of time, which has also affected the incidence of many common infectious diseases.

Refer to the change in the positive rate of seasonal influenza tests in the United States in recent years [5]:

Figure 1. Changes in influenza test positivity rates in the US since the 2015/16 flu season (source, citation [5])

2020/21 was the first influenza season after the global pandemic, but the influenza virus was nearly extinct that year, and it can be said that there was no influenza season at all that year. Behind the disappearance of influenza is precisely the dramatic change in people's behavior patterns that year. In the 2021/22 flu season, the United States gradually relaxed various new crown epidemic control with the increase in the new crown vaccination rate, and the flu also recovered slightly, but the number of cases was still much lower than usual, and there was a rare double peak: there were not many peak cases in the winter that year, and another small peak appeared in the spring. What happened in the winter of the turn of 2021/22? It happened that the first wave of Omicron hit the world, and people's behavior was difficult to recover from the epidemic, and influenza, a traditional respiratory infectious disease, was naturally affected.

But by the 2022/23 flu season, which began last winter, many parts of the world have begun to coexist with the new crown, and people's behavior patterns are becoming less and less different from before the pandemic: visiting relatives and friends during the festival, watching the World Cup, the audience is full... With the return of human society and human behavior to normal, influenza, a common infectious disease, has also begun to return to "normal". Like in the United States, the positive detection rate at the peak of infection is not much different from before the pandemic.

Not only the flu, RSV has experienced a similar "disappearance to return" in Europe and the United States. Different countries and regions have different times for epidemic control adjustments, so the recovery of these traditional epidemics will also be different. China only made adjustments to the coronavirus epidemic at the end of 2022, and we may only now face the "recovery" of viruses such as influenza. I have not seen these long-standing "old friends" in human society for three years, and now that I encounter them again, it is inevitable that more viruses will suddenly appear. However, the truth is not that a new virus suddenly appeared, but that the old virus, like our society, began to recover.

3 Pre-existing immunity and the "new crown sequelae" of the virus

Although infectious diseases such as influenza have also begun to emerge from the impact of the new crown epidemic, there will still be a transition period from the situation in Europe and the United States. In the short term, we will also see some anomalous manifestations of traditional epidemics.

For example, the 2022/23 flu season in the United States, from the peak of the disease, is similar to the previous flu season, but it can occur much earlier than before, starting from the 40th week of 2022, that is, October, nearly two months earlier than in most previous years. This is also reflected in the number of flu hospitalizations [5]:

Figure 2. Weekly flu hospitalizations per 100,000 people in the United States over the last 6 flu seasons (from citation [5])

In terms of the number of weekly influenza hospitalizations per 100,000 people, the peak of the 2022/23 flu season in the United States is 560 or 600 per 100,000, similar to the 2018/19 and 2019/2020 seasons, which is within the normal range, lower than the 2017/18 particularly severe influenza season, but the peak came two months earlier than the past.

At the same time, the flu came and went early in the United States, and the number of infections fell sharply in February 2023, and if it does not rebound after that, it is equivalent to ending the flu epidemic at the peak of the past flu.

Since human society and other viruses are coming out of the shadow of the new crown, why is there such an "abnormality"? Because the return of human behavior patterns may not be consistent with seasonal patterns, in the short term, the activity patterns of these viruses will be closer to the time of human behavior changes, and it will take a process to return to the original seasonality. But another factor that cannot be ignored is that just after emerging from the new crown pandemic, human pre-existing immunity to many common viruses has not yet returned to normal.

Similar to the new crown, the severity of the epidemic caused by viruses such as influenza in human society is also affected by two factors: the pathogenicity of the virus and the basis of population immunity. For example, the influenza virus is constantly mutating, most of the mutations are subtle, for the human immune system is only a small change in antigens, that is, antigen drift, in this case, people after multiple influenza seasons of repeated infection, will have a better immune basis for influenza, limit the impact of influenza. However, when the flu undergoes major changes and antigen conversion occurs, the pre-existing immunity accumulated by repeated infections of the immune system in the past has limited effect, and it is easy to have a serious influenza season.

Under the influence of the epidemic in the past three years, the epidemic pattern of common viruses such as influenza and RSV has been abnormal for several consecutive epidemic seasons, and our pre-existing immunity has also deviated from normal levels. And the "abnormality" of pre-existing immunity also makes the virus more likely to behave "abnormalities".

For example, RSV, like influenza, generally peaks in winter, but in the summer of 2021, the United States experienced its first RSV peak after the pandemic[6]:

Figures. The US had an unusual peak in RSV infections in August 2021 (blue curve) (from citation [6])

Since October 2022, RSV infections in young children in the United States have also surged, with an unusual peak not seen in previous years [7]. The high-risk groups for RSV are at opposite ends of the age structure: newborns, who are vulnerable because they have never been infected and have no immune base; The other is the elderly, who are also at risk of severe disease because of their relatively weak body. In the past, young children were basically infected with RSV after the age of 2, and since then, with the immune basis of previous infections, reinfection with RSV is often mild. However, in the few years since the RSV epidemic disappeared, many young children did not have pre-existing immunity even after the age of 2, which led to the continuous appearance of RSV in terms of time or severity.

Similarly, although norovirus infection manifests as acute gastroenteritis with diarrhea and vomiting, increased social distancing and wearing masks during the pandemic will inhibit the spread of the virus. Therefore, many countries and regions rarely have norovirus outbreaks in the early stage of strict epidemic prevention and control [8-9]. But on the other hand, while norovirus spreads between communities, it also accumulates corresponding immunity at the population level, so after the new crown control, the population immunity pre-storage declines, and norovirus may appear to rebound stronger than before, which is also observed in the United Kingdom and the United States and other countries [10].

From this point of view, common viruses are also experiencing "new crown sequelae", and it takes a period of adaptation to fully restore the previous epidemic. Some viruses may experience more serious "coronavirus sequelae", such as the Yamagata strain, one of the two common B virus strains of seasonal influenza, which has rarely been monitored since the outbreak of the new crown epidemic, and even in the 2022/23 season, when the overall flu has recovered, this strain is still difficult to find. We cannot rule out the possibility that the strain will become extinct within the past three years due to the lack of a host [11].

4 Rational response to common viruses in recovery

Having just experienced the peak of the new crown epidemic, it is normal to have a certain degree of worry in the face of the "return" of various common viruses. However, today, when "influenza A" and "norovirus" have frequently become hot words, we can still refer to the accumulation of scientific experience in dealing with these common viruses, face them rationally, and reduce anxiety.

First of all, in the short term, we need to be mentally prepared for some common viruses to behave abnormally. As mentioned earlier, after European and American countries began to coexist with the new crown, both influenza and RSV have shown different manifestations from the previous epidemic rules, because the two aspects of human behavior patterns and pre-existing immunity have not fully returned to normal. China's epidemic prevention policy has not yet been adjusted, and it is likely to encounter similar anomalies. The large-scale travel and circulation of people has just resumed at the end of 2022, which may lead to the rise of influenza now, when the influenza epidemic in the United States is nearing its end.

The "abnormal" performance of common epidemics in China in the post-new crown era does not mean that common viruses will change drastically, but only the performance of the transitional period when everything gradually returns to normal, and we do not need to be too afraid.

Second, some deficiencies in pre-existing immunity can be artificially compensated. Below-average pre-existing immunity may make us more vulnerable to some common viruses, but it is by no means irreversible. Like the influenza A that has recently caused some schools to change their teaching plans, according to the tracking of the 2022/23 influenza season in Europe and the United States, the influenza vaccine in the current influenza season has a better protective effect [12]. Getting the flu vaccine as soon as possible can make up for the pre-existing immunity deficiency caused by the previous pandemic to some extent, and may also better cope with the resurgence of the influenza virus.

Third, those common viruses still have their own characteristics, and the pathogenicity will follow the original law. As common viruses, the pathogenicity of influenza, RSV, norovirus, etc. is relatively stable, and there are also corresponding high-risk groups, most of which are concentrated in infants and the elderly. We can prepare accordingly according to such laws. For example, healthcare systems can prepare for increased likelihood of high-risk populations seeking care. As well as the use of additional protective measures in places where high-risk groups such as schools and nursing homes gather, and appropriate improvement of indoor ventilation in the cold season, the threat of some influenza and RSV will be reduced. Norovirus can also be prevented through measures such as personal hand hygiene and enhanced hygiene in places such as schools and restaurants.

Finally, the level of science and medicine is constantly improving, so even if various common viruses do not disappear from our lives, we will have more and more ways to deal with them. For example, with breakthroughs in the design of vaccine antigen structures (the same technology is also used in some of today's highly effective COVID vaccines), two RSV vaccines for the elderly may soon be available [13]. In the past, these common viruses have not stopped our human society from moving forward, and in the future, our methods of dealing with these viruses will only become better with the development of technology. In the past, we were able to coexist with these common infectious diseases, and in the future, we are likely to coexist at a smaller cost.

Resources

[1] https://www.cdc.gov/flu/about/burden/past-seasons.html

[2] https://www.cdc.gov/hai/organisms/norovirus.html

[3] https://pubmed.ncbi.nlm.nih.gov/23575961/

[4] https://pubmed.ncbi.nlm.nih.gov/30619155/

[5] https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2023-02/slides-02-22/influenza-02-Grohskopf-508.pdf

[6] https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2022-06-22-23/03-RSV-McMorrow-508.pdf

[7] https://www.cnn.com/2022/10/21/health/rsv-hospitals-what-to-know-wellness/index.html

[8] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8662166/

[9] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8888229/

[10] https://www.vox.com/health/2023/2/17/23603035/norovirus-stomach-flu-vomiting-nausea-diarrhea-outbreak-norwalk

[11] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9524051/

[12] https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2023-02/slides-02-22/influenza-03-Olson-Lewis-Tenforde-508.pdf

[13] https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2023-02/slides-02-23/RSV-Adults-04-Melgar-508.pdf

This article is supported by the Popular Science China Starry Sky Project

Produced by: Science Popularization Department of China Association for Science and Technology

Executive producers: China Science and Technology Press Co., Ltd., Beijing Zhongke Galaxy Culture Media Co., Ltd