#17,051
Although flu circulates at low levels year-round in the tropics, for most of the world influenza is primarily a seasonal threat, which appears to peak during times of of lower temperatures and humidity.
It's Not So Much The Heat, It's The Humidity
Influenza Virus Survival At Opposite Ends Of The Humidity Spectrum
NIH Study: Climate & Influenza Transmission
The irony here is that hospitals are normally kept cool and dry in order to curb the growth of bacteria, but may be unintentionally providing an environment conducive to the spread of respiratory viruses like influenza, SARS-CoV-2 & MERS.
This is not exactly a new idea, as the Chinese have boiled vinegar for centuries in their homes believing the pungent smell to `disinfect the air' during epidemics (including SARS). While vinegar is unproven to add any beneficial effect, vinegar is 95% water, and boiling it undoubtedly raises the humidity inside their homes.
Even though we have seen significant transmission of COVID during the more humid summer months (see graph below), they have been far outpaced during the winter months.
Other factors, including initially low levels of community immunity, masking & social distancing, the arrival of new variants, and the movement and clustering of people undoubtedly also contribute to increased transmission.
Understanding the seasonality of COVID (particularly as the virus enters an endemic phase), and identifying potentially new mitigation efforts, will be important for public health officials and policy makers going forward, as well as helping individuals quantify their risks of exposure.
All of which brings us to a new open-access study (warning: heavy statistical analysis ahead) that looks at the impact of absolute humidity (and the movement of people) on the spread of COVID.
Due to its length, and technical nature, I've only included the Abstract and some excerpts from the Discussion. Those inclined will want to the link to read the report in its entirety.
I'll have a brief postscript after the break.
Investigating the effects of absolute humidity and movement on COVID-19 seasonality in the United StatesGary Lin, Alisa Hamilton, Oliver Gatalo, Fardad Haghpanah, Takeru Igusa & Eili Klein
Scientific Reports volume 12, Article number: 16729 (2022)
Abstract
Mounting evidence suggests the primary mode of SARS-CoV-2 transmission is aerosolized transmission from close contact with infected individuals. While transmission is a direct result of human encounters, falling humidity may enhance aerosolized transmission risks similar to other respiratory viruses (e.g., influenza).
Using Google COVID-19 Community Mobility Reports, we assessed the relative effects of absolute humidity and changes in individual movement patterns on daily cases while accounting for regional differences in climatological regimes.
Our results indicate that increasing humidity was associated with declining cases in the spring and summer of 2020, while decreasing humidity and increase in residential mobility during winter months likely caused increases in COVID-19 cases.
The effects of humidity were generally greater in regions with lower humidity levels. Given the possibility that COVID-19 will be endemic, understanding the behavioral and environmental drivers of COVID-19 seasonality in the United States will be paramount as policymakers, healthcare systems, and researchers forecast and plan accordingly.
Discussion
As the COVID-19 epidemic continues in the US and given the surge of COVID-19 in the winter seasons, there is renewed interest in understanding the relationship between outbreaks and seasonal changes, especially climatological factors related to outdoor and indoor humidity. This is not the first study to investigate humidity impacts on transmission, which been associated with increased transmission of respiratory pathogens (e.g., influenza) and SARS-CoV-2.
While SARS-CoV-2 is a novel human virus, other pandemic coronaviruses (e.g., MERS-CoV and SARS-CoV-1)9,32,33,34,35 have also been associated with increased transmission in the winter, thus suggesting similar implications for SARS-CoV-2.
Here, we found that the relative effect of absolute humidity on transmissions has so far been significant and was greatest in the Western, upper Midwest, and Northeast regions of the United States, which were clustered into the driest climatological regimes.
These results support the hypothesis that falling rates of absolute humidity magnify the transmission risk of SARS-CoV-2, particularly in regions that are more arid and dry36. This effect was less noticeable for more humid regions, such as the coastal and southern counties of the US (Fig. 2).
(SNIP)
Transmission of SARS-CoV-2 will likely increase during the winters in the United States and other temperate regions in the northern hemisphere due in part to falling humidity. Studies of prior viruses and preliminary studies of the SARS-CoV-2 virus underpin the theoretical connection between humidity and transmission of droplet and aerosols. Nevertheless, mobility is still a significant driver of transmission.
(Continue . . . )
The idea that the COVID threat is diminishing has been largely born over the summer months, which has seen lower levels of activity than were reported during the summer of 2021.
Whether that trend carries on into this winter remains to be seen, as last year many people were still masking, and the uptake of COVID vaccines was more robust.
Last winter also saw the emergence of a new, far more transmissible Omicron variant, which - while milder than Delta - helped to propel case counts into the stratosphere.
But if the past two winters are any indication, we should expect another surge in COVID cases this fall as temperatures and humidities drop, and people gather indoors without masks. And what is conducive for the spread of COVID, is likely equally beneficial to the spread of flu.Unless and until COVID `settles down' - and the rapid replacement cycle of subvariants ends - the true `seasonality' of COVID may remain less than clear.
