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With the arrival of autumn and winter in the northern hemisphere, most countries in the northern hemisphere ushered in the "flu season." In the context of the COVID-19 pandemic this year, people are even more worried about the double attack of influenza virus and new coronavirus. Mentioned that the fall and winter are more likely to suffer from respiratory viral diseases such as flu and colds. People may first think of it because the weather is cold, and most people's activities shift from outdoors to indoors, which increases the possibility of virus transmission. But is the reason really that simple?
A few days ago, Yale University professor, HHMI researcher Dr. Akiko Iwasaki and partners published a review in the Annual Review of Virology magazine, and conducted an in-depth inventory of the causes of seasonal respiratory virus infections. The authors stated that since industrialization, people have drastically changed the impact of environmental temperature on our behavior. In many parts of the world, the temperature of offices and homes remains constant and is not affected by the seasons. Moreover, people-to-person contact in the work environment will not decrease due to seasonal changes. In industrialized countries, most people spend 90% of their lives in a relatively closed environment with poor air circulation. Therefore, the reason for the high incidence of respiratory virus infection in autumn and winter is not just the change of people's behavior. So, what factors in autumn and winter increase the risk of respiratory virus infection? What enlightenment can this bring us to prevent the new coronavirus infection this winter? Today, the WuXi AppTec content team will introduce the highlights of this review. Types of seasonal respiratory viruses At present, there are at least 9 different types of viruses that can cause common respiratory infections, and epidemiological studies in temperate regions show that most of the peaks of respiratory virus infections fluctuate with the seasons. For example, influenza virus, human coronavirus, and human respiratory syncytial virus (RSV) peak in the winter months. Adenovirus, human Boca virus, human metapneumovirus (hMPV), and rhinovirus may appear throughout the year. Some types of enteroviruses peak in summer. An interesting phenomenon is that the peaks of most respiratory viruses do not overlap. For example, epidemiological studies have found that even though RSV viruses and influenza viruses are both circulating in winter, their peaks still do not appear at the same time. The prevalence of a virus can delay the prevalence of another virus. The underlying mechanism of this interference phenomenon has not been fully clarified. Possible hypotheses include the interference of cell surface virus receptors, cell death, and human interferon (IFN) response. The influence of environmental factors on the stability and spread of respiratory viruses https://www.2021hermes.com - The transmission modes of respiratory viruses can be divided into three categories: direct or indirect contact; short-distance transmission caused by droplets or droplets; and long-distance transmission caused by aerosols. The efficiency of virus transmission through these three ways will be affected by a variety of environmental factors. Among them, the factors that have the greatest impact on long-distance transmission efficiency include temperature and air humidity. They are thought to regulate the viability of the virus by affecting the characteristics of the virus surface protein and lipid membrane. Animal experiments show that when the relative humidity of the air is too high (>60%) or too low (<40%), the virus can remain active in the droplets, and when the relative humidity of the air is between 40% and 60% The virus will be inactivated instead. The influence of environmental factors on the anti-virus defense line of the respiratory tract Because of breathing, the mucous membranes on the surface of our respiratory tract are exposed to the volatile and non-volatile pollutants in the inhaled gas and a variety of potential pathogens all the time. Therefore, the respiratory tract has also set up multiple lines of defense to prevent foreign virus infection. Studies have shown that changes in the temperature and humidity of the inhaled air with the seasons can directly affect the protective ability of the respiratory mucosa on multiple levels. If a virus entering the respiratory tract wants to infect the host, it must first contact the epithelial cells on the surface of the respiratory tract. In order to prevent the invasion of the virus, the mucus secreted by the respiratory tract can not only serve as a barrier, but also capture the epithelial cells before the virus "finds" them. Moreover, the antibacterial protein contained in mucus can also play a bactericidal effect. In addition, the cilia on the surface of epithelial cells can push the mucus that wraps the pathogen to the throat through continuous whip, and finally be spit out (or swallowed). This process is called Mucociliary Clearance (Mucociliary Clearance, MCC), which is an important means of removing pathogens and pollutants from the respiratory tract. However, dry and cold air will significantly affect the operation of this protective mechanism. Mucus needs to contain enough water to be able to act as a barrier. Dry and cold air will cause the water in the mucus to quickly lose, and the mucus becomes more viscous, more difficult to be pushed by the cilia, and therefore more difficult to remove from the respiratory tract. In addition, studies have shown that ambient temperature will also affect the speed of cilia movement. When the ambient temperature drops below 20°C, the flagging movement of mucociliary begins to slow down, and when the environment temperature reaches 5°C, the flagging movement of the cilia stops. The influence of winter environment on immune response against virus The human body's immune response to viruses includes innate immune response and adaptive immune response. The innate immune response generated in the mucosa of the respiratory tract plays a key role in preventing virus invasion. Among them, a variety of interferons (IFNs) can activate the expression of hundreds of genes to produce antiviral effects at different stages of the virus replication cycle. In recent years, studies have found that environmental factors can also affect the innate immune response against viruses. For example, human rhinovirus is one of the main causes of common colds. It mainly causes illness in winter. In vitro culture, rhinoviruses are easier to replicate at 33°C, which is the temperature of the nasal cavity in cold weather. So, why is rhinovirus easier to replicate at 33°C? Studies have found that at 33°C, rhinoviruses only trigger low levels of type 1 IFN in infected epithelial cells. The results of this study mean that exposing the nasal cavity to cold air in winter may lead to higher levels of rhinovirus replication. In addition to temperature and air humidity, the duration of sunshine also affects the immune system. In animal experiments, a shorter day life environment reduces the phagocytic capacity of white blood cells and the production of reactive oxygen species (ROS) in hamsters. The shortening of sunshine time in winter is more likely to lead to the lack of vitamin D, and the study of vitamin D-deficient mice found that their macrophages malfunctioned. These data mean that shorter sunshine hours in winter may be one of the factors leading to innate immune response disorders. For the new coronavirus, which is of particular concern to everyone, the study of its seasonality has just begun not long ago. However, studies published on the preprinted website medRxiv have shown that seasonal factors affect the severity of COVID-19 and patient mortality, and the degree of dry air is one of the important reasons pointed out by researchers. The review authors also pointed out that the outbreaks of the new coronavirus and its close relative SARS-CoV-1 both occurred in the winter months. The receptor ACE2 of these two viruses is highly expressed on type 2 alveolar cells deep in the lungs, and droplets larger than 5 microns in diameter usually cannot directly contact the alveolar cells deep in the respiratory tract. The author speculates that the winter weather may increase the viability of the virus in smaller droplets and reduce the ability to clear the mucus cilia, thereby increasing the spread of the new coronavirus. What should be done to limit the spread of respiratory viruses in winter? What insights can we gain from these studies to help us prevent respiratory virus infections and related diseases in winter? The researchers said that in addition to vaccination and the use of antiviral drugs, non-drug interventions are also very important. Healthy lifestyle habits including healthy eating, sleeping more than 7 hours a day, and good hygiene habits including frequent hand washing and wearing masks have been proven to prevent the spread of the virus and improve resistance. In addition, the researchers pointed out that an important measure that can be considered is to control the relative humidity of the indoor environment. Previous studies have shown that keeping the relative humidity at 40-60% can greatly reduce the risk of influenza virus infection. The senior author of this review, Dr. Akiko Iwasaki, posted on her Twitter (Twitter) that another function of wearing a mask is to protect the moisture and warmth of the nasal cavity, which also helps limit the spread of the virus.
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