This may explain why so many people have been afflicted with summertime colds this year.

The majority of people link colds and influenza with colder weather. However, this does not rule out the possibility of catching a cold during the summer. Some viruses are more prevalent during the summer than during the winter.

The influenza virus and respiratory syncytial virus (RSV) are more prevalent throughout the winter due to the cooler temperatures and more time spent indoors in close proximity to other individuals.

In contrast, enteroviruses and the parainfluenza 3 virus are significantly more prevalent in the summer, with infections from these viruses peaking in summer and early fall, when the weather is warm and humid.

Both viruses induce common cold symptoms, such as a runny nose, fatigue, muscle pains, cough, headaches, and sore throat. Parainfluenza can occasionally induce bronchitis and pneumonia in individuals with compromised immune systems.

While these symptoms are similar to those of allergies, the distinguishing characteristic is that allergies rarely induce fevers, body aches, or coughing. Some individuals may experience allergy symptoms throughout the entire summer, depending on the allergen that started the reaction.

It seems counterintuitive that certain viral illnesses are more prevalent during the warmer months, when we spend more time outside.

During the warmer months, however, we socialise and travel more, meaning that we interact with a larger number of individuals, sometimes from distant countries. When the weather is hot, many of us gravitate towards air-conditioned interior surroundings.

However, the nature of a virus may also explain why some are more contagious during warmer months.

For a virus to spread and infect healthy cells, it must be able to survive both within and outside of the body, as well as utilise the machinery of human cells (such as their DNA) to replicate itself.

Viruses are encased in a protein “coat” known as a capsid, which not only gives the virus its shape but also safeguards the genetic material within. Additionally, the capsid helps the virus connect to human cells in order to induce infections.

Some viruses (termed “enveloped viruses”) have a lipid (fatty acid) envelope. This viral envelope protects the virus from immune system destruction. Furthermore, it interacts with human cells to generate infection.

Enteroviruses don’t have an envelope. Photograph by: Kateryna Kon/Shutterstock
Many “winter” viruses are enclosed, including influenza and RSV. Envelopped viruses are typically more susceptible to heat and dryness than envelopeless viruses. This is one of the reasons why it is believed that these winter cold viruses are best able to survive in colder winter surroundings.

Some summer colds (such as enteroviruses) do not have an envelope, although others (parainfluenza virus 3) have. In fact, parainfluenza virus 3 is more prevalent when temperatures and humidity are high and low, respectively (though it can survive in a range of different humidities).

This shows that other components of a virus’s structure, in addition to the envelope, may have a role in determining the optimal conditions for its survival and dissemination; however, additional research is required to confirm this.

There may also be a function for the interaction between temperature and the immunological response to a virus. According to one study, mice exposed to temperatures of 36°C have a decreased immune response against influenza virus. However, additional research is required to confirm these results in humans.

Immune response

This year, numerous individuals have reported suffering from summer colds, prompting many to question why this is the case and whether the pandemic has had a role.

Immunity to common cold viruses is temporary. When we are exposed to new types each season, our immune system must adjust. During the pandemic, however, different lockdown measures, such as maintaining a safe distance and wearing masks, reduced many people’s exposure to these infections.

When we reassembled after the lockdown, cold viruses began to spread, but our immunity had not been enhanced by exposure to this virus the year before. Despite the fact that the predictability of seasonal viruses has changed since the emergence of COVID, the increases in summer colds observed this year are likely due to an increase in travel, an increase in social mixing, a decrease in mask wearing and distancing, and a decrease in previous exposure to respiratory viruses.

This year, numerous regions of the world have also experienced excessive heat and an abundance of heatwaves. These changes in temperature and humidity may have contributed to the propagation of common cold viruses this year. In the future, these elements will become increasingly more significant and may even alter the time of year when particular viruses are observed. Future climate change could exacerbate the spread of viruses.

Since there is no vaccination for summer colds, the best method to avoid catching one is to (if possible) avoid sick individuals, wash your hands frequently, and avoid touching your face. If you are unfortunate enough to have caught a cold, the same advice applies as if you caught it in the winter: drink plenty of fluids, get plenty of rest, and eat nourishing meals. Coughing or sneezing into your elbow or tissues is also recommended for the protection of others.

Additionally, as the weather cools in the approaching months, it may be prudent to consider how you might prevent illness. Each winter, the flu vaccine is advised for certain individuals, therefore it is prudent to determine if you are eligible for the shot this year. Australia has seen a very severe flu season this year, and forecasts indicate that the same will be true for much of the world this winter.

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