Mode of transmission of viruses may explain their preference for winter
Viruses are everywhere and have been around for millions of years. No form of life is spared, especially not man. From AIDS to SARS (= severe acute respiratory syndrome), passing through smallpox or hepatitis C, viral pathologies have decimated populations and constantly stir up the specter of health catastrophe. Others, however, are more common and less dangerous to health.
Veritable « stars » of winter, the flu, gastroenteritis and colds are the talk of the town at this time of year. Their epidemic threshold is systematically reached during this season, marked by cold and low levels of sunshine. But what role does the climate play in the emergence of these epidemic peaks? Are there more viruses in the air? Is our body more fragile?
Before answering all these questions, we must remember how vast the world of viruses is. Unknown until the end of the 19th century , it remains largely unexplored today, for lack of sufficient technical means. In fact, little research has been done on the viral ecology of the air, as well as how these entities interact with the environment. We know, however, that some viruses are transmitted mainly through the air, while for others, it is contact that is decisive. This is actually explained by the morphology of the viruses .
Basically, all have an identical mode of operation: the virus enters the body, enters a cell, then releases its genetic material inside it. This material then forces the parasitized cell to make hundreds of copies of the virus which will accumulate inside it. When the viruses are in sufficient number, they leave the cell in search of other prey. It is here that we can observe a major difference between two categories of viruses.
Two main categories of viruses
Some viruses, to leave the infected cell, will burst it by piercing the cell membrane. Others will leave it, by « budding », by wrapping themselves in this same membrane. In fact, these viruses are said to be « enveloped » by a cell membrane: the peplos. Paradoxically, this envelope does not protect them. On the contrary, it makes them more fragile! Far from being an additional armour, they are, on the contrary, the Achilles heel of viruses of the genre. The peplos inherited the fragility of the cell membranes from which it derives. However, a virus must be whole to be infective. There are thus two places where the envelopes will degrade rapidly: in the external environment and in the digestive tract. In these same places, “naked” viruses, that is to say without an envelope, will resist much longer. This difference is crucial in explaining the mode of transmission of certain diseases and the epidemics that result from them.
“Enveloped” viruses: the case of influenza
Influenza is an integral part of the viruses coated with a peplos. The viruses that are responsible for this well-known disease therefore do not survive long in the air, because they will be inactivated by two factors: temperature and drought.
Does this mean that one cannot be contaminated by the air? No, quite the contrary! It is even almost the only possible route of contamination because an influenza virus cannot survive very long in dust or on an external surface such as the hands. Contamination occurs through the air when two people are very close together: the healthy individual directly inhales the infecting microdroplets projected by the cough of the individual with the flu. Thus, the viruses do not pass for a long time in the air. But this brief survival in the air is favored when the air is humid and cold, the peplos fearing heat and drought. This peculiarity partly explains why the flu is most prevalent in winter.
“Naked” viruses: the case of gastroenteritis
But there are also winter epidemics caused by “naked” viruses. The latter are particularly resistant, both in the air and in the digestive tract. They are also transmitted by air, but above all, by indirect contamination involving the stool: 1g of faecal matter from an infected individual can contain up to 10 billion viral particles! For lack of hygiene, the infected individual then presents many viruses on the hands which he deposits on objects or food. A healthy individual, by touching the object or by ingesting the contaminated food, is then contaminated in turn. Water can also be an important vector because naked viruses are resistant to it. However, if gastroenteritis strikes above all in the winter period, this is not the case for all illnesses caused by this type of virus. Some, like poliomyelitis, occur even more… in summer! How to explain it? In fact, since each virus is different, so will their mode of transmission. Nevertheless, a number of hypotheses have been put forward to try to explain this winter phenomenon.
| One virus, and that’s the infection?It is not enough for a virus to enter the body for you to be infected. The pathogenic dose, i.e. the amount of virus required for infection, differs depending on the virus. For example, that concerning rotavirus (gastroenteritis) is estimated at around 10 to 100 viral particles. |
Hypotheses to explain winter epidemics
It’s not a myth: people die each year more often in winter than in summer. It’s no coincidence that the expressions “wintering” or “catching cold” have passed through history and are still used today. In 2006, a study evaluated the number of additional deaths occurring each year at this time of year at 15,000. This surplus is partly linked to an increase in the number of respiratory diseases such as the flu, colds or bronchitis. What explanation was first advanced by the scientific community? Promiscuity.
Containment and lack of ventilation
In 2009, in France, the Institute for health monitoring on the physiology of cold decided on this hypothesis to explain winter epidemics . « The tendency to concentrate the population in confined and poorly ventilated spaces (when it is cold) increases the risk of cross-infections », he indicates. In other words, if we fall ill in the winter, it is because we huddle together near the fireplace or because we go more to places of community such as shops, restaurants, public transport, offices… Confinement and lack of ventilation are certainly decisive elements but cannot fully explain epidemic episodes.
More viruses in the air?
Another explanation has been put forward: there would be more viruses in the air in winter. This hypothesis is particularly difficult to prove given the microscopic size of viruses and our limited tools in this area. A South Korean team has nevertheless managed to analyze the air around us and has published its results . These researchers worked on three different sites: a residential district of Seoul, a forest and an industrial complex. Their experiment consisted in filtering all the elements of the air lower than the micrometer, then to extract the possible DNA and to study the sequences. The result is stunning: in one cubic meter of air, there are between 2 and 40 million viruses! So, without knowing it, we inhale up to 400,000 viruses per minute. Do not panic however, since the extreme majority of these viruses do not concern us. We must not forget that viruses are specific to the species they attack: there are viruses against trees, fungi and even…against bacteria! The main lesson of this study lies in fact in the variations observed. And these are not linked to the collection sites but to the seasons in which the readings were taken. Thus, the rate of the virus in the air undeniably peaks in January, before gradually declining after spring.
We gave a first explanation for this phenomenon in the first part of this article with the increased survival of « enveloped » viruses in cold weather. In 2008, researchers had tried to demonstrate it with an experiment on sick and non-sick guinea pigs . To do this, they studied the spread of the virus by varying temperature and humidity. It turned out that a temperature of 5°C and a degree of humidity of 20% were ideal for viral transmission. Other researchers have tried to shed light on the phenomenon. Philip Rice, of St. George’s Hospital, London, suggested in a study the role of UVin reducing epidemics. According to him, these rays would naturally degrade viruses. However, during the winter, sunshine is minimal, and therefore the exposure of viruses to these rays as well. This would explain why they are present in greater numbers in the air…
| Effective the mask?If viruses are more numerous in the air in winter, why not wear a mask when going to a confined public space? The idea is attractive on paper (although unsightly) but in reality turns out to be disappointing in practice. Firstly because there are many counterfeits, and care must be taken to only buy approved masks (FFP2 or surgical). Then, because they are never 100% effective, viruses being largely capable of getting through . The advantage is that they effectively block the « droplets » of saliva projected by coughing. Finally, their duration of protection is short: three to six hours . In fact, they should be reserved in the event of an influenza pandemic. |
A weaker immune system in winter?
Viruses would therefore be more numerous in winter… What if, in addition, we were more vulnerable at this time of year? This historical theory, but never completely proven by the scientific community, is still popular.
There are many mechanisms to protect us from viruses, especially those that are transmitted mainly through the air. Thus, the nasal cavities and sinuses are lined with a lining (the nasal mucosa) which makes mucus. Its role is to trap bacteria, viruses or other microbes that would take this path. But this barrier would be lessened in winter. When we inhale cold air, many blood vessels warm it by transmitting the heat of the blood to it. It is because of this phenomenon that we have a red nose when it is cold. But this heat transfer would humidify the air to the detriment of the nasal wall, making access to viruses and bacteria easier. This is not the only hypothesis of its kind: some believe thatthe cold would slow down the immune system or make it less reactive. Others think that the cold would irritate the nasal and bronchial passages, while the lack of light characteristic of the winter season would have a negative role on immunity. To date, it remains difficult to prove the validity of these hypotheses, but it is a safe bet that the years to come will be rich in lessons in this area…
| Vitamin C to strengthen your body?Taking vitamin C to avoid colds, a received idea? Not sure ! A study showed that doses of 250 mg to 1 g of vitamin C per day, from 3 to 8 weeks just before and during winter, could reduce the risk of catching a cold. On the other hand, it would be ineffective in reducing its duration or severity. |
