Does Influenza Peak in Winter Just Because It’s Colder?

Check out this answer from Consensus:

While colder temperatures and low humidity levels are significant factors in the winter peak of influenza in temperate regions, they are not the sole determinants. The seasonality of influenza is influenced by a complex interplay of climatic conditions, urbanization, and population density. In tropical and subtropical regions, other factors such as precipitation and specific humidity play a more prominent role. Therefore, the winter peak of influenza cannot be attributed solely to colder temperatures; it is a multifaceted phenomenon shaped by various environmental and social factors.

Influenza, commonly known as the flu, is a highly contagious respiratory illness caused by influenza viruses. It is well-documented that influenza epidemics exhibit a strong seasonal pattern, particularly in temperate regions where they peak during the winter months. This article explores whether the winter peak of influenza is solely due to colder temperatures or if other factors also play a significant role.

The Role of Temperature and Humidity

Several studies have investigated the relationship between climatic conditions and the transmission of influenza. Research indicates that both low temperatures and low specific humidity levels facilitate the airborne survival and transmission of the influenza virus in temperate regions, leading to winter epidemics1 2. Laboratory experiments using guinea pigs as model hosts have shown that cold and dry conditions favor the aerosol spread of the virus, with increased viral shedding observed at lower temperatures2.

Climatic Variations Across Different Regions

While cold and dry conditions are associated with influenza peaks in temperate regions, this relationship does not hold in tropical and subtropical areas. In these regions, influenza epidemics often occur during the rainy season or transmit year-round without a well-defined season1 9. For instance, in Hong Kong, influenza A shows two seasonal peaks, one in winter/spring and another in summer, influenced by both cold and humid conditions8. This suggests that local climatic variables, such as specific humidity and precipitation, also play a crucial role in influenza seasonality.

Urbanization and Population Density

Urbanization and population density further modulate the intensity and duration of influenza epidemics. In smaller cities with less residential density, influenza epidemics tend to be more intense and concentrated within shorter periods, driven by climatic conditions. In contrast, larger, more densely populated cities experience more diffuse epidemics, likely due to higher rates of personal contact that make influenza transmission less dependent on climate variations5.

Mortality and Influenza

The impact of influenza on mortality rates also varies with climatic conditions. A retrospective study in New York City found a significant association between periods of low dew point temperature and increased pneumonia and influenza mortality, suggesting that cold, dry air may contribute to higher mortality rates during winter3. Similarly, in the United States, the winter increase in mortality is primarily attributed to influenza, with weather conditions influencing the timing and magnitude of this increase6.

Does influenza peak in winter just because it’s colder?

Anne Moore has answered Uncertain

An expert from University College Cork in Immunology, Vaccines

Partly; it’s believed that due to the cold weather people congregate more indoors, in closer surroundings with less ventilation. This “density” allows better transmission of the virus. There are also some discussions on relative humidity and how that might permit the virus to survive for longer. My understanding is that the weather affect is mostly due to humans responses to that weather. 

Does influenza peak in winter just because it’s colder?

Maria Sundaram has answered Uncertain

An expert from Marshfield Clinic Research Institute in Epidemiology

Winter seasons often coincide with respiratory virus seasons in general—not just influenza, but other respiratory viruses like rhinovirus, adenovirus, and respiratory syncytial virus. It’s likely that these viruses are more common in winter for several different reasons, including (but not limited to) temperature, humidity, and human behaviour (spending more time indoors and with larger groups of people around the winter holidays). It’s important to note, though, that influenza epidemics in particular are incredibly difficult to predict—a lot of these factors are constantly changing, and so is the virus itself!

Does influenza peak in winter just because it’s colder?

Stephen Kissler has answered Uncertain

An expert from Harvard University in Epidemiology

We still don’t fully understand why flu and other respiratory illnesses peak in the winter in temperate parts of the globe. If there is a direct link between influenza and the weather, it probably has more to do with humidity than temperature. A few studies show that influenza can thrive better in the air when absolute humidity drops, which usually happens at low temperatures. That said, we do still see flu spread in tropical regions of the globe, so this can’t be the whole story. Some of the wintertime spikes in influenza are probably also due to the fact that people tend to spend more time indoors!

Does influenza peak in winter just because it’s colder?

Patricia Foster has answered Uncertain

An expert from Indiana University Bloomington in Biology, Genetics

The colder temperatures do increase the spread of the flu, but only indirectly.  The flu virus spreads through exhaled water droplets.  Obviously, people spend more time together indoors when it is cold, and thus can more readily spread the virus.  There is good evidence that the flu virus survives and transmits infection better through the air when the humidity is low, which, in temperate regions, happens both outside and inside in the winter.  (This is not true of all viruses; for example, a recent study found that the Covid-19 virus was less infective in low humidity).  In addition, when the humidity is low people’s airways dry out and that can increase susceptibility to the virus.  Finally, there is evidence that people are more susceptible to infections when their vitamin D levels are low.  Since biosynthesis of vitamin D requires sunlight, blood levels of the vitamin will drop in the winter unless people take supplements.

Does influenza peak in winter just because it’s colder?

Ian Frazer has answered Unlikely

An expert from University of Queensland in Vaccines, Vaccinology

Its more to do with us living closer to each other as a result of the inclement weather.

Does influenza peak in winter just because it’s colder?

William  Rawlinson has answered Uncertain

An expert from UNSW Sydney in Virology, Microbiology, Molecular Biology

The cold is one thing, also the close cohorting of people allowing spread by droplets (close contact being usually 1 metre) and by fomites (dust or similar that can bind viruses). The virus does transmit in model systems better when it is cold and dry, possibly because of survival and also because the local environment created in the nose by these conditions allows more virus replication (growth). Similar to any respiratory virus, more close contact, indoors, is likely to increase spread – the features of winter. There is also research around some other possible effects – Vitamin D, and melatonin, both of which are influenced by sunlight.