Viruses account for over 80% of respiratory infections in infants and young children. Acute lower respiratory infections (ALRI), encompassing bacterial and viral pneumonias, acute viral bronchiolitis, and bacterial and viral bronchitis, remain the leading cause of global child mortality.
The leading causes of ALRI now are viral infections, with respiratory syncytial virus (RSV), influenza virus, parainfluenza virus and rhinovirus among the top viral causes of ALRI in children.
What is a Risk Factor?
A risk factor is a characteristic, condition, or behavior that increases the likelihood of getting a disease or injury. Risks do not occur alone. They often coexist and interact with one another. Knowing the risk factors is the first step in preventing or lowering a disease.
The risk factors for viral respiratory tract infections in children include the age, immunologic experience, prematurity, low birth weight, chronic cardiopulmonary diseases, immunodeficiency syndromes, malnutrition, exposure, other children in family, crowding, care setting, family versus day care, indoor and outdoor pollution, seasonality, lack of access to preventive and curative services, parental smoking and lack of breast-feeding.
Children are more active and engage in more outdoor activities than adults. They breathe more rapidly and their metabolic rate is higher. Children’s immune systems are not fully developed, so the incidence of respiratory infections is high. Their lungs are still growing and any deficit in growth will have an impact for the whole of the child’s life. Moreover, possible confounding or modifying factors, such as active smoking, occupational exposure to dust and smoke or medical treatment are largely absent.
Pneumonia can affect people at any age. Babies and young children are at higher risk of getting pneumonia because their immune systems are not yet fully developed.
RSV is the principal cause of bronchiolitis in young children and is globally the leading cause of hospital admissions in infants. It plays an important role in the hospital resources utilization. RSV is responsible for nearly 16 times more infant hospitalizations than influenza. The influenza viruses are highly contagious and can cause large epidemics. Age, prematurity, birth close to the start of the RSV season, and presence of chronic conditions have been commonly identified as potential risk factors for RSV-associated disease.
Young children under age 2 are among those at higher risk of developing flu complications. In children, the fever associated with influenza is likely to be higher, on average, than an adult's fever. In addition, children more often experience symptoms related to the digestive system, such as nausea, vomiting and diarrhea. Children younger than 18 are more than twice as likely to develop a symptomatic flu virus infection than adults 65 and older.
Intrauterine influences which retard fetal weight gain may constrain the growth of the airways. Low birth weight is closely associated with poor airway function independent of neonatal respiratory illness. Infants with LBW often experience respiratory problems. This is due to the incomplete growth and development of the lungs and weak respiratory muscles.
Low birth weight is associated with excess respiratory morbidity in adults, regardless of whether the low birth weight was the result of in utero growth retardation or premature birth. Premature birth is a risk factor for severe RSV infection.
Chronic Cardiopulmonary Diseases
Viral respiratory infections (VRI) are one of the leading causes of hospitalization during the first year of life. Children with congenital heart disease have altered lung mechanics, decreased lung compliance and increased pulmonary vascular resistance. Therefore, they are more susceptible to complications of VRI, especially when they undergo open-heart surgery. Viral infections are the most common etiology of myocarditis. RSV myocarditis has been reported as a cause of viral myocarditis in both the pediatric and adult populations. Recurrent acute lower respiratory tract infections often occur in children with a history of congenital heart diseases.
Primary immunodeficiencies (PIDs) are inherited disorders in which one or several components of the immune system are decreased, missing, or of non-appropriate function. These diseases affect the development, function, or morphology of the immune system. The group of PIDs comprises more than 200 different disorders and syndromes and the number of newly recognized and revealed deficiencies is still increasing.
Respiratory symptoms and complications present a significant cause of morbidity among patients suffering from different forms of PIDs and they are observed both in children and adults. RSV infection is more severe in immunocompromised infants than in healthy infants, as a result of impaired immune response unable to efficaciously neutralize viral replication, with subsequent increased viral shedding and potentially lethal lower respiratory tract infection.
Breastfeeding protects against respiratory tract infections in infants. Most studies confirm a protective role of breastfeeding against respiratory infections in the long term, as the outcomes are often measured after 6 months of age, or even at 1, 2, or 6 years, showing a persistent protective effect even after breastfeeding has been stopped. Breastfeeding a sick baby gives him or her a great chance of a speedy recovery. Breast milk contains antibodies, white blood cells, stem cells and protective enzymes that fight infections and may help with healing. Breastfeeding confers protection against both incidence and severity of RSV disease, particularly in those born prematurely, as well as the subsequent development of recurrent wheezing illness.
Pneumonia is a common co-morbidity in children presenting with malnutrition and can increase the risk of death 15-fold especially in children younger than 24 months of age. While most healthy children can fight the infection with their natural defenses, children whose immune systems are compromised are at higher risk of developing pneumonia. A child's immune system may be weakened by malnutrition or undernourishment, especially in infants who are not exclusively breastfed.
Malnourished children might have a blunted inflammatory response, leading to an inability to produce clinical sign-symptoms, despite having infections like pneumonia.
Seasonality of Respiratory Viral Infections
Annual epidemics of the common cold and influenza disease hit the human population like clockwork in the winter season in temperate regions. Moreover, epidemics caused by viruses such as severe acute respiratory syndrome coronavirus (SARS-CoV) and the newly emerging SARS-CoV-2 occur during the winter months. The seasonal nature of respiratory viral infections depends on environmental parameters and human behavior.
Studies have revealed the effect of temperature and humidity on respiratory virus stability and transmission rates. More recent research highlights the importance of the environmental factors, especially temperature and humidity, in modulating host intrinsic, innate, and adaptive immune responses to viral infections in the respiratory tract.
Respiratory virus infection can occur through direct/indirect contact, droplet spray in short-range transmission, or aerosol in long-range transmission (airborne transmission). Airborne transmission occurs as droplet spray of predominantly large droplets up to millimeters settling directly or by indirect contact on mucous membranes or by inhalation of either large respiratory droplets (>10 μm in diameter) or small airborne droplet nuclei (<5 μm in diameter).
The state of vapor equilibrium in room air, expressed as saturation ratio or RH, affects all infectious droplets with respiratory viruses, independent of their source (respiratory tract or aerosolized from any fluid) and location (in air or settled on surfaces). RH therefore affects all transmission ways but has the most pronounced effect on airborne transmission.
In-house overcrowding is associated with an increased risk of hospitalization for acute respiratory infections in young children. Aerosols containing germs and viruses like flu, SARS and rhinoviruses can remain infective for hours. Up to 48 hours in crowded rooms and busy areas, to be precise.
Eliminating the fraction of the acute respiratory infections burden due to in-house overcrowding will rely on increasing awareness regarding indoor air pollution and ventilation in the house and making efforts to avoid smoking in dwellings. Along with the management of crowding, child nutrition and exclusive breast-feeding requirements should be continued for a wide range of child health benefits.
The prevalence of smoking parents of children with acute respiratory tract infections is higher than in the general population. Exposing children to tobacco smoke is one of the risk factors for acquiring influenza and other respiratory tract infections. Quitting smoking can decrease the risk of infectious diseases.