by Gertrud U. Rey
Have you ever wondered why some viruses circulate primarily in the winter and others are more prevalent in the summer? Although we don’t have a clear answer to this question, a combination of factors is likely responsible.
Work done in the field of respiratory viruses suggests that these viruses can be transmitted in four ways: 1) through direct contact with an infected person; 2) through indirect contact by touching an inanimate object contaminated by an infected person; 3) through inhalation of large virus-containing droplets that drifted over a short distance; and 4) through inhalation of small virus-containing droplets that drifted over a long distance (i.e., “airborne” transmission). The same body of work also indicates that the seasonality of viral transmission is likely driven by changes in the environment, changes in human physiology, and changes in human behavior, all of which affect the stability and spread of viruses.
One of the environmental elements that influences the viability of respiratory viruses is UV light, which increases in intensity in the summer and typically inactivates viruses. Studies have also shown that the viral envelope, the outermost layer of many respiratory viruses, is more stable in cool weather and more likely to break down in warm temperatures. However, the biggest environmental contributors to the seasonal nature of respiratory viruses are temperature and absolute humidity. Absolute humidity is a measure of the exact amount of water vapor in a certain volume of air (expressed as grams of water per cubic meter of air) and it is independent of temperature. This feature distinguishes absolute humidity from relative humidity, which is a ratio (expressed as a percentage) of the absolute humidity to the amount of water vapor that can exist in the air at its current temperature. When the air is hot and absolute humidity is high, expelled respiratory droplets tend to sink to the ground or settle on nearby inanimate objects. In contrast, cool dry air causes droplets to partially evaporate and form smaller aerosol particles that remain suspended longer, float further, and are thus more likely to be inhaled by another person. Seasonal changes in absolute humidity, marked by high levels in the summer and low levels in the winter, both indoors and outdoors, are opposite to the seasonal patterns of circulation of influenza viruses, coronaviruses, and respiratory syncytial virus, which peak in the winter and drop in the summer. This inverse correlation suggests that absolute humidity is a key player in the overall transmission of these viruses.
However, the correlation between absolute humidity and incidence of infection does not explain the circulation of respiratory virus infections in tropical regions, where it occurs continuously, even though both indoor and outdoor absolute humidity remain high year-round. The most likely explanation for this disparity is that transmission in these regions occurs mainly through direct and/or indirect contact between individuals. The high absolute humidity probably causes expelled droplets to settle on nearby surfaces, where they evaporate less water than they would under dry conditions, thereby causing any viral particles they contain to remain stable until they are taken up by the next person.
Changes in human physiology may also alter the rate of infection, susceptibility, and severity of disease. Shorter winter days lead to low levels of vitamin D and melatonin, both of which are generated through a cascade of enzymatic reactions that require exposure to sunlight. This deficiency compromises certain innate immune responses like macrophage activation, which in turn decreases an individual’s ability to fight a viral infection.
Seasonal changes are also marked by changes in human behavior, which may further impact the frequency of contact between infected and uninfected individuals. In the winter, people spend more time indoors, where viral transmission is more likely as they come into close contact with infected individuals. The start of the school year also typically coincides with an increase in the incidence of respiratory infections among children, which is then often followed by transmission to their family members.
Meanwhile, many viruses that spread by the fecal-oral route predominate in the summer. There are various reasons for this phenomenon. Prior to rollout of the poliovirus vaccines, poliovirus infections often occurred in the summer, because children and adults would gather at picnics where they consumed cold, uncooked foods, which, compared to cooked foods, are more likely to be contaminated with viable stool-derived viruses. Many outbreaks also originated at public swimming pools containing inadequately treated water. Likewise, enteroviruses like hand, foot, and mouth disease virus also circulate more during the summer, but the reason why is less clear. It is possible that an interference mechanism between viruses, where infection by a first virus reduces the likelihood of infection by another virus, causes one group of viruses to predominate during one season.
You can obviously still become infected with a “winter” virus during the summer months, and vice versa. Washing your hands frequently is the most effective method for preventing infection with any virus, and based on what we know; using a humidifier in your home might lessen your chances of becoming infected with a respiratory virus.