Can acute upper respiratory tract viral infections be considered seasonal?
Yes, it seems that in the cold seasons of the year, the incidence of respiratory tract infections, especially upper respiratory tract infections, and especially viral types, is higher. Seasonal changes in weather are accompanied by heterogeneous changes in the lives of animals and plants. Humans adapt to seasonal changes as much as possible by making changes in their lifestyle (such as wearing clothes and heating and cooling the house), but they are still affected by these changes and experience changes in mood and health conditions. There is a general belief that the incidence of colds and flu increases with cold weather, and studies have also proven that in the northern and southern parts of the Earth (the North and South Poles), the incidence of these diseases increases significantly with cold weather.
According to studies, the incidence of respiratory infections increases significantly in the cold seasons, and more than 200 types of viruses have been implicated in the pathogenesis of these diseases, which belong to the following six viral families:
Orthomyxoviruses (influenza)
Paramyxoviruses (respiratory syncytial virus)
Parainfluenza
Coronaviruses
Picoronaviruses (cold virus)
Herpesviruses and adenoviruses
In addition, some viruses that cause general symptoms in the body but enter through the upper respiratory system, such as infections caused by paramyxoviruses such as measles, mumps, and chickenpox, also show an increase in winter prevalence.
Pneumonia: or lower respiratory tract infection that can be viral or bacterial in origin, also shows a seasonal prevalence (with an increase in winter).
The nose is the entry point for these lower respiratory tract infections that follow upper respiratory tract infections. Life-threatening pneumonias often originate from upper respiratory tract infections.
Annual influenza vaccination can protect people at risk of serious lower respiratory tract infections, but we still do not have protection against hundreds of types of cold viruses. Each year, upper respiratory tract infections, followed by lower respiratory tract infections, in susceptible individuals impose a heavy burden on the health system of communities. These diseases can even increase the risk of mortality from cardiovascular events in communities.
What is the reason for this seasonal prevalence of upper respiratory tract infections?
Until the late twentieth century, it was believed that the gathering of people in closed environments in the winter season provided the basis for the transmission of disease-causing organisms, especially viruses, among people and increased the prevalence of these diseases. However, this view was criticized by many scholars. Today, gatherings in urban communities and public places such as subways and …. occur at the same rate in winter and summer, but the incidence of upper respiratory infections is still significantly related to the decrease in air temperature in winter.
A newer theory that is proposed for the reason for the increase in the prevalence of upper respiratory tract infections in winter and cold seasons is the theory of a decrease in the temperature of the nasal mucosa. Today, scientists believe that a decrease in the temperature of the air passing through the nasal space. It causes the mucociliary movements of the respiratory cilia to be disrupted, thus causing the accumulation of pathogenic particles, and on the other hand, the phagocytosis of leukocytes (related to the immune system) is disrupted, making the body vulnerable and permeable to microorganisms.
The effect of cold air on the defense system of the upper respiratory tract:
On the one hand, cold air reduces the flow and continuous movement of mucus covering the respiratory tract, and on the other hand, it disrupts the function of phagocytic PMN cells and NKCELLs. Deficiencies in this part of the immune system make the body vulnerable to particles and very small particles that are present in the respiratory air.
On the other hand, cold air causes a decrease and disruption of mucociliary function, which disrupts the movement of pathogenic particles out of the body or into the digestive system (and consequently their removal by stomach acid), and pathogenic particles colonize in the upper respiratory tract and cause disease.
Acute changes of chronic exposure to cold air:
A theory that is being put forward these days about the increase in the prevalence of upper (and subsequently lower) respiratory infections is that, contrary to the long-standing belief that epidemic waves of respiratory infections occur when the weather suddenly becomes cold, in fact, prolonged exposure to cold air and a decrease in the function of the upper respiratory tract defense system lead to SUBCLINICAL infections (without visible symptoms) in the airways, which, following a significant decrease in temperature and further functional weakness of the upper respiratory tract ventilation system, these infections transform from a SUBCLINICAL and asymptomatic form to a CLINICAL and symptomatic form, and this is the same factor that causes a wave of respiratory infections to spread throughout society following the cold weather.
One of the challenges that this theory faces is that if cold weather causes colds, then why do people in tropical regions get colds? Studies conducted on an island in these areas have shown that a drop in air temperature also leads to the occurrence of such infections in these places.
Another evidence for this claim is that during respiratory infections, the warming of the air inside the respiratory tract, followed by fever and nasal congestion, helps to solve the problem of respiratory tract infection, and this is actually the body’s natural defense response and mechanism. For this reason, today we know fever as a beneficial and protective biological factor. For this reason, viruses that cause fever, such as the influenza virus, are less successful in spreading the infection than viruses such as the cold virus, which only trigger a local defense reaction (in the upper respiratory system), and this is the reason why colds are more common than flu.
Another of the body’s defense systems against upper respiratory infections is nasal congestion. Heat and nasal congestion prevent viruses colonized in the nose and sinuses from having the opportunity to multiply and cause disease. For whatever reason, if this heat and congestion do not occur, the person becomes vulnerable to these viruses.
During colds and upper respiratory infections, nasal congestion, by raising the temperature, causes a local war to be launched at the site of virus proliferation in the nose, and this transfer in the posterior parts causes one of the nasal passages to close, thus preventing the penetration of viruses into the more internal airways and localizing the infection in the same place (a defense reaction that we destroy by taking cold pills).
General Conclusion:
From what was described above, we realized that the ancient belief that colds are caused by cold weather is correct, although the spread of new and newly synthesized viruses in the warm seasons can also cause epidemics, because people do not have a history of immunity to them. But cold weather is a very important driving factor. Therefore, in addition to using personal hygiene methods, as we are required to observe all year round, such as: 1) washing hands regularly 2) avoiding shaking hands with people (if possible) 3) not being in public places when we are sick 4) using personal utensils and equipment or washing dishes thoroughly and properly in public places,
In cold seasons, the following precautions should be taken:
Using a scarf and mask causes the exhaled respiratory air to mix with the inhaled respiratory air, and as a result, the air we inhale becomes warmer.
Eating energy-giving foods increases the body temperature by increasing the speed of metabolism and prevents the development of such diseases.
Exercise also helps to fight these diseases by increasing the body’s metabolism and increasing the defense power of the immune system.
Elderly people and sensitive people (heart and lung problems, asthma, kidney failure, etc.) should take the recommendations for keeping the body warm and avoiding inhaling cold air (using a scarf and mask) seriously in addition to getting the flu vaccine.
When is the best time to get the flu vaccine?
The best time to get the flu vaccine is 2 weeks before the approximate time of the flu outbreak in the country, which is around mid-September.
In children aged six months to eight years, the flu vaccine must be given in two doses to provide adequate immunity, and the interval between the two doses must be at least 28 days. Therefore, these people should start vaccination earlier so that they receive the second dose around mid-September.
The latest recommendation for flu vaccination, made by the CDC (part of the ACIP (Advisory Committee on Immunization Practices))
, is to vaccinate everyone over six months of age.
