Why Some Viral Infections Show Seasonality?

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Why Some Viral Infections Show Seasonality?

Why do certain viruses primarily spread during the winter while others become more prevalent in the summer?

While we don’t have definitive answers to this question, it is likely the result of multiple factors working in concert.

Research in the field of respiratory viruses suggests that these viruses can spread in four ways:

1) through direct contact with an infected person;

2) indirect contact with inanimate objects contaminated by an infected person;

3) through inhaling larger droplets containing the virus that travel short distances;

4) through inhaling smaller aerosolized droplets containing the virus (i.e., “airborne transmission”).

Studies also indicate that the seasonality of viral transmission may be driven by changes in the environment, human physiology, and human behavior, all of which affect the stability and transmissibility of the virus.

Why Some Viral Infections Show Seasonality?

One environmental factor affecting the survival of respiratory viruses is ultraviolet (UV) radiation. UV radiation intensifies during the summer, making it more likely to deactivate viruses. Research also shows that the viral envelope (the outermost layer of many respiratory viruses) is more stable in cooler weather and more prone to degradation in warmer conditions.

However, the most significant environmental factor contributing to the seasonality of respiratory viruses is temperature and absolute humidity. Absolute humidity measures the exact amount of water vapor in a given volume of air (expressed in grams of water per cubic meter of air) and is independent of temperature.

This sets it apart from relative humidity, which represents the ratio of absolute humidity to the maximum amount of water vapor the air can hold at the current temperature, expressed as a percentage.

When the air is warm and absolute humidity is high, respiratory droplets expelled tend to fall to the ground or on nearby surfaces. In contrast, in cool, dry air, droplets partially evaporate, forming smaller aerosol particles that remain suspended in the air longer, increasing the likelihood of inhalation by another person.

Absolute humidity exhibits a seasonal variation, with higher levels in the summer and lower levels in the winter, inversely matching the seasonal transmission patterns of influenza viruses, coronaviruses, and respiratory syncytial virus, all of which peak in the winter and decline in the summer.

This negative correlation suggests that absolute humidity is a key factor driving the overall transmission of these viruses.

However, the correlation between absolute humidity and infection rates doesn’t explain the transmission of respiratory infections in tropical regions, where respiratory viruses persist throughout the year despite high indoor and outdoor absolute humidity. The most plausible explanation for this difference is that transmission in these regions primarily occurs through direct and/or indirect contact between individuals, rather than relying on airborne transmission. High absolute humidity might lead to droplets being deposited on nearby surfaces, where the reduced evaporation in these conditions keeps any viral particles they contain stable until they are absorbed by the next person.

Changes in human physiology may also alter infection rates, susceptibility, and disease severity. Shorter winter daylight hours result in lower levels of vitamin D and melatonin, both of which are produced through a series of enzymatic reactions that require exposure to sunlight. This deficiency impairs certain innate immune responses, such as macrophage activation, reducing an individual’s ability to combat viral infections.

Seasonal changes are also reflected in human behavior, which may further influence the frequency of contact between infected and uninfected individuals. During the winter, people spend more time indoors, increasing the likelihood of close contact with infected individuals and, consequently, the potential for virus transmission. The start of the new school year typically coincides with an increase in respiratory infections among children, which then often spread to their family members.

Additionally, many viruses transmitted through the fecal-oral route dominate during the summer. Several reasons account for this phenomenon. Before the widespread use of polio vaccines, poliovirus infections typically occurred in the summer because children and adults gathered for picnics where they consumed cold, uncooked foods that were more likely to be contaminated with live virus from fecal sources compared to cooked foods.

Outbreaks of many diseases have also been traced back to public swimming pools with inadequately treated water containing fecal matter. Similarly, enteric viruses like those causing hand, foot, and mouth disease also spread more during the summer, although the reasons for this are less clear. There may be an interference mechanism among viruses, whereby infection with one virus reduces the likelihood of infection with another, resulting in a group of viruses dominating during a particular season.

Clearly, even during the summer, you can still be infected with “winter” viruses, and vice versa. Frequent handwashing remains one of the most effective ways to prevent any viral infection, based on what we know. Using a humidifier indoors during the winter may also reduce your chances of contracting respiratory viruses.

Why Some Viral Infections Show Seasonality?

References:
1. Jeffrey Shaman, et al., “Absolute humidity modulates influenza survival, transmission, and seasonality,” Proceedings of the National Academy of Sciences, 2009, 106(9), 3243-3248.
2. Moriyama, Miyu; et al., “Seasonality of Respiratory Viral Infections,” Annual Review of Virology, 2020, 7(1): annurev-virology-012420-022445.

(source:internet, reference only)

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Tags: Virus