Why the Flu Spreads More in the Winter

The flu is a viral disease caused by different types of influenza viruses. In general, cases of infection tend to increase during the winter. Here, we'll tell you why the flu spreads more in the winter.
Why the Flu Spreads More in the Winter

Last update: 12 October, 2020

Nowadays, COVID-19 has monopolized worldwide media and research. Even so, other viruses still circulate that, although less dangerous, cause exceptionally severe symptoms and discomfort. This is the case with the flu, a disease that often spreads during the winter.

You may be wondering why the flu has its peak prevalence in autumn and winter and practically disappears in spring. In this article, we’ll answer this question.

One clinical picture, multiple forms

The flu is an infectious disease caused by different types of influenza viruses. They’re single-stranded RNA viruses, which are covered by an outer lipid layer that gives them their characteristic rounded appearance. According to the World Health Organization, there are four types of seasonal influenza viruses:

  • Influenza A viruses are classified into subtypes based on the combinations of two proteins on their surface. All known flu pandemics have been caused by type A viruses.
  • Type B viruses aren’t classified into subtypes, but the ones that are currently circulating can be divided into two lineages.
  • Type C viruses are less frequently detected and usually cause mild infections. Thus, they lack public health importance.
  • Type D viruses mainly affect cattle and aren’t known to infect or cause illnesses in people.

The global incidence is estimated at 20%. In other words, 20% of the world’s population suffers from it at some point in their lives. Furthermore, it’s a selective pathology, since it can affect up to 50% of certain population groups.

Due to its epidemiological importance, influenza has been the subject of multiple studies.

A woman with the flu.

This article may also interest you: 6 Habits that Can Help You Recover from the Flu

The flu and the weather

A study published in the scientific journal Plos Pathogens explains the relationship between influenza and climate factors and why the flu spreads during the winter:

  • The researchers used 20 experimental replicas with guinea pigs. They placed a number of guinea pigs sick with influenza in various compartments. In other compartments contiguous to the sick guinea pigs, they placed healthy ones.
  • After that, the researchers subjected different sample groups with healthy and sick guinea pigs to different relative humidity and temperature ranges.
  • At a constant temperature, relative humidity and flu transmission were found to be inversely correlated. At a relative humidity of 20%, most of the healthy guinea pigs were infected by the airborne particles of the diseased ones. At 80% humidity, no transmission was observed.
  • Also, they saw that guinea pigs infected in environments of 5 degrees of temperature expelled more viral loads in their airborne particles, and for longer than those infected in environments of 20 degrees.

It seems that this study concludes that low temperatures and dryness promote the spread of the virus.

But why?

The flu spreads with low relative humidity

Various hypotheses try to explain why low relative humidity favors the spread of the flu:

  • First, dry air could damage and deteriorate the host’s nasal mucous membranes, leaving the host more unprotected against viral respiratory infections such as influenza.
  • Secondly, the stability of the virus in airborne particles seems to vary with humidity, with the virus remaining active longer in less humid environments.
  • Thirdly, the virus could lose its ability to spread at greater humidity. At high relative humidity, the airborne particles the host expels could quickly adhere to water molecules in the environment, thus increasing their volume and precipitating earlier. This decreases the speed and distance traveled by the virus.
A woman holding a tissue as she's sneezing.-
Low temperatures provide an enabling environment for the transmission of viruses that cause influenza.

The flu spreads with low temperatures

In this case, the explanation may be a little simpler.

At low temperatures, the nasal mucous membranes cool down when they breathe in air. This creates a microenvironment that’s more conducive to the virus, which could replicate better. As a result, this translates into a higher viral load with each sneeze and spray, thus promoting transmission.

Read this article to learn more: What to Include in Your Diet When You Have the Flu

There’s a reason for the flu season

In short, influenza seasonality makes clear scientific sense. In the northern hemisphere, this virus begins to spread in October, reaching its peak between December and February. These data perfectly agree with what the researchers concluded during the study since those months are cold and dry.

Fortunately, there’s an annual flu vaccine, so its potential problem is almost nullified. This vaccine is highly recommended for the elderly and immune-suppressed.

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