Thread by @jeffgilchrist

[Part 1 of 2] COVID-19: Why respiratory viruses surge in winter

Thanks to @LongDesertTrain for bringing my attention to this excellent article explaining the seasonality of respiratory infections ( annualreviews.org/doi/10.1146/an… ). 🧵1/

In temperate regions, annual epidemics of the common cold and influenza hit the human population like clockwork in the winter season (barring pandemics) so why is that? 2/

There are multiple factors that affect respiratory virus transmission including seasonal environmental factors which modulate host airway immune response and affect viability and transmission of viruses. Human behaviours also affect rates. 3/

During winter, indoor heating causes a major difference of indoor and outdoor temperature and relative humidity (RH) but does not affect absolute humidity (AH). 4/

The number of people-to-people contacts significantly increases on weekdays compared to the weekend while local weather conditions (rain, sun, cold) have only minor effects on contacts. 5/

"In the industrialized world, most people interact, work, sleep, commute, and spend 90% of their lifetime in enclosed spaces, where they share a limited amount of breathing air. This implies that the overwhelming majority of person-to-person transmission events happen indoors."6/

There are at least 9 distinct viruses that have been identified that commonly cause respiratory tract infections. Some viruses like influenza, human coronavirus, and respiratory syncytial virus (RSV) clearly show peak incidences during the winter months. 7/

Adenovirus, human bocavirus, hMPV, and rhinovirus are detected all throughout the year. Some enteroviruses and parainfluenza virus (PIV) case numbers increase in summer, and rhinovirus disease severity increases in winter despite infection rates peaking in spring & fall. 8/

Temperature and humidity (RH) play an important role in the stability of respiratory viruses. High RH (> 60%) and low RH (< 40%) seems to allow viability in influenza droplets while intermediate RH (40%-60%) inactivates the virus. 9/

It is assumed that temperature and humidity modulate the viability of viruses by affecting the properties of viral surface proteins and lipid (fat) membranes. 10/

In experiments with guinea pigs, researchers found that at 20C transmission of influenza through the air was not observed at high RH (80%) while transmission was highly efficient at the same temperature but with low RH (20%-35%). 11/

They also found that virus transmission was more efficient at 5C than 20C. At 5C virus transmission occurred 50% of the time even at high RH (80%). 12/

There is little seasonality in tropical regions. No aerosol transmission was observed at 30C at any humidity despite contact transmission rates being similar at 20C & 30C. High ambient temperature likely negates the effect of humidity on influenza transmission. 13/

It was also observed that aerosol transmission was inefficient at 50% RH but more efficient at both low (20%-35%) and high (65%) RH at 20C. 14/

Exposing mice to aerosolized influenza virus at levels high enough to kill all mice at 23% RH allowed 77.5% of mice to survive the same virus levels at intermediate RH (40%-60%). 15/

An ideal humidity for preventing aerosol viral transmission at room temperature is between 40% and 60% RH. 16/

Based on these results, researchers predict that aerosol transmission predominates during the winter season in temperate regions because dry and warm indoor climate allows stability of influenza viruses in aerosols that stay airborne for prolonged periods. 17/

In tropical regions, contact is the major mode of spread because of warm and humid climates, droplets evaporate less water and readily settle on surfaces. 18/

This hypothesis has implications for proper precautions and public health measures against respiratory virus infections in different parts of the world and in different seasons. Part 2 will focus on the effect of environmental factors on the host airway antiviral defense. 19/

Part 2 is now available which focuses on the effect of environmental factors on the host airway antiviral defense (

https://twitter.com/jeffgilchrist/status/1454781113881317376

). 20/