Understanding the Transmission of Respiratory Winter Viruses

4 reasons why certain viruses — including coronaviruses — thrive in the winter environment.

A recent review published in the Annual Reviews of Virology explains why the winter environment helps certain viruses to thrive. Researchers call them winter viruses which include influenza virus, human coronavirus (HCoV), and respiratory syncytial virus (RSV).

The review was written by Miyu Moriyama, a postdoctoral fellow in immunobiology, with supervision from Akiko Iwasaki, a professor at the Departments of Immunobiology and Molecular, Cellular, and Developmental Biology at Yale University.

Factor 1: Viability & Transmission

Winter viruses are best viable at low RH whereas high RH favors summer or all-time viruses. Viable means that the microbe is active so they can be infectious. RH means relative humidity that rises with temperature. In other words, low temperature helps winter viruses to spread more efficiently.

“Low-temperature condition promotes the ordering of lipids on the viral membrane and contributes to the stability of the influenza virus particle,” Moriyama said. Animal experiments have, indeed, shown that lower temperatures (or low RH) promote transmission of winter viruses.

Researchers, thus, “predict that aerosol transmission predominates during the winter season in temperate regions…, while contact is the major mode of spread in the tropics,” she continued.

Factor 2: Mucous Clearance

Effective clearance of viruses depends on efficient mucous transport. Mucous that lines epithelial (e.g., respiratory) tracts trap incoming foreign particles, which are later expelled upon mucous clearance. Breathing in cold air — which is always dry — disrupts and slows the clearance of mucous. And it also delays the repair/replacement of airway epithelial cells.

“Disruption of airway epithelial integrity caused by inhalation of dry air might be involved in the winter epidemics of certain types of respiratory virus infections,” Moriyama emphasized.

Factor 3: Sunlight Exposure & Vitamin D

The killing of pathogens — via phagocytosis and reactive oxygen species (ROS) production — is impaired when animals are not sufficiently exposed to daylight. Researchers believe a shortage in vitamin D synthesis from sunlight impair our natural immunity — phagocytosis and ROS production — against viruses.

“These data suggest short daylight as a contributing factor in the impairment of the innate immune responses in winter,” Moriyama said in a statement.

Factor 4: Host Antiviral or Interferon Immune Response

At lower temperature conditions, viruses — such as rhinovirus that causes the common cold — trigger fewer interferon production in the nasal cavity. Interferon is a type of immune cell that interferes with virus replication. This means that a cold environment could enable viruses to replicate more silently, without alerting the host immune system.

“Conversely, keeping the nose warm during the winter might boost antiviral innate resistance to the common cold virus,” Moriyama wrote.

Unknown Factor(s)

An intriguing phenomenon is that even in highly controlled animal experimental settings, influenza transmission (from the same virus stock) is more efficient if the experiment was conducted in winter than in summer.

“Thus, in addition to temperature, lighting, and humidity, there may be other environmental factors controlled by the seasons that contribute to higher levels of influenza virus infections in the winter months,” Moriyama noted.

How Does This Relate to COVID-19?

“We speculate that the low humidity and temperature environment would promote the viability of SARS-CoV-2 in the droplets and impaired ciliary clearance and innate immune defense, for robust access to the deep lung tissue and rapid transmission between infected individuals,” Moriyama and her colleagues wrote.

A study in the New England Journal of Medicine reported that “SARS-CoV-2 remained viable in aerosols throughout the duration of our experiment (3 hours).” Not to mention that a single cough can spread particles — that could carry SARS-CoV-2 virions — to surrounding vicinities in minutes, according to a supercomputer 3D simulation of a grocery store by researchers specializing in fluid dynamics, aerosol physics and virology at Finnish institutions.

“Someone infected by the coronavirus, can cough and walk away, but then leave behind extremely small aerosol particles carrying the coronavirus,” says Ville Vuorinen, assistant professor at Aalto University who is involved in the supercomputer simulation.

But experts say that the simulation may not paint the complete picture. “Our risk of acquiring it and getting enough of that virus aerosol is completely unknown,” explains Kumi Smith, an assistant professor of epidemiology and community health at the University of Minnesota. The video “gives the impression that any shared airspace will lead to transmission,” but real-life demonstrations with SARS-CoV-2 have not been conducted, she said.

Dr. William Schaffner, a professor of preventative medicine and infectious diseases at Vanderbilt University Medical Center, commented that the Finnish supercomputer simulation is “reasonable,” but “if everybody did what the CDC recommended, that would address the issue.” The CDC has, indeed, started to emphasize the use of face masks more strongly this early April.

Moving on, Mike Ryan, a WHO Executive Director of Health Emergencies, told reporters recently that, “We still believe the main driver of this pandemic is symptomatic individuals coughing or sneezing or contaminating surfaces or contaminating other individuals.”

And Moriyama et al. have highlighted how conducive the winter environment is for the transmission of respiratory viruses. In winter times, the viability and transmission of winter viruses increase, while mucous clearance, sunlight exposure, and interferon antiviral response decrease.

Considering all these factors, no wonder SARS-CoV-2 has been spreading so successfully during the winter.

But Moriyama and her colleagues are also aware that COVID-19 has spread rather quickly in humid areas. “The precise relationship between temperature, humidity, and COVID-19 will become more evident as the Northern Hemisphere reaches the summer months,” they added.