Curious Case of Why Flu Dies Out in Summer

& Will Covid-19 Do Likewise?

By Dale Dauten, Syndicated Columnist

Why is there a flu season? And why winter, when we have enough problems already? Even the name “flu” comes from the Italian influenza di freddo or “influence of the cold.” So how does cold actually influenza the virus?

There have been plenty of theories. One blamed children – flu season coincides with kids being in school. (Speaking of which, STChealth’s Medical Advisor Scott Hamstra once told us, “We think of toddlers with nasal secretions streaming down their faces as ‘little walking bioterrorists.’”) Another theory blamed diminished immune systems from lack of sunshine/Vitamin D. Then, in 2007, researchers at Mount Sinai School of Medicine were actually able to demonstrate how cold and humidity affect the ability of the virus to be transmitted.

Flu transmission seems like a pretty simple matter to test, but then again, it would be illegal to give some test subjects the flu and then put half in a cold room with healthy people and half in a warm room with healthy people and see what happens. What to do instead? You could give mice the flu, but they don’t pass it along to other mice. So that conundrum stopped lab testing until 2007, when Dr. Peter Palese of Mt. Sinai happened to be reading a journal article from 1919 about a flu outbreak in New Mexico and the author noted that during the outbreak all his laboratory guinea pigs died. So Palese got himself some guinea pigs, injected some with a flu virus and, voila!, it spread… or not.

For his testing, Palese put four infected guinea pigs in a cage and then four uninfected ones in a neighboring cage, with a fan circulating air between them. Then he repeated the test at different temperatures and humidity levels.


At 41°, all four of the healthy guinea pigs caught the flu.

At 68°, just one got infected.

At 86°, none.

Palese also tested humidity.

At 41° and at low humidity (35%) the transmission rate was 100%.

But, at high humidity (80%), the rate was cut in half.

So, we had the answer – it was winter itself that created flu season (Cold air can’t hold as much water, so cold and low humidity run together.)

Other research also tested how long the virus can survive: most of the virus survived at 23 hours in cold, dry conditions, but in hot, damp conditions, only a small fraction survived an hour or more.

All these variables come together in the human sneeze, cough or even breath — each sends droplets that contain flu virus into the air. Then, in cold, dry air, the droplet begins to evaporate, making it smaller and lighter and able to float on air currents for hours, waiting to breathed in. However, in warm, humid weather the droplet merely falls to the ground and soon expires.


It would be welcome news if the Covid-19 virus would go away for the summer and offer the opportunity to do more research and preparation, including vaccine development. Possible? We put that question to Dr. Peter Palese, the man who put the guinea pigs to work, and who Chairs the Department of Microbiology at the Icahn School of Medicine at Mt. Sanai in New York where he’s been a leader in genetic mapping of influenza viruses and is at work on a universal flu vaccine.

Asked about Covid-19, he offered a statement reminding us that his work was with animals and flu and “thus I am reluctant to make meaningful comparisons between these agents [flu and COVID-19]. It’s really apples and oranges (if not apples and bananas).”

However, he concluded with this:

“One might look at Covid-19 as a form of influenza, but much less common. This puts the Covid-19 story into perspective and hopefully reduces the hysteria so common today.”

So let’s take a deep breath, preferably of warm, humid air, and then get back to work on preparations.

Sources: Gina Kolata, New York Times, 12/5/2007; Hannah Foster, Harvard University Science in the News, 12/1/2014; Emily Elert, Popular Science, 1/17/2013; Dr. Peter Palese, Scientia, 9/21/2017.

Photo credits: Lone tree by Fabrice Villard, Hamsters by Bonnie Kittle, Blowing snow by Freestocks.

NOTE ON COVID-19 FROM STChealth CEO Mike Popovich:

We learned from Hurricane Katrina, the Avian Flu, the H1N1 Pandemic and even the recent Measles Outbreaks that an important component of reducing the impact of each of these events was the ability to share information quickly, putting intelligence in the hands of providers, pharmacists, and public health.

A new event has become the focal point that will allow the STChealth team to accelerate the expansion and performance of existing communication networks and data systems. These are expected to play a role in the current outbreak, but also to increase opportunity to support improvements in immunization compliance.

STC established a COVID-19 Task Force to focus our attention on exactly this goal. If a new COVID-19 vaccine becomes available, the value of this network becomes even more important and since disease challenges every day, investments to better prepare for the current outbreak has significant long term benefit.

In response to questions from around the country as to how data and its analysis could help in efforts against COVID-19, we put together a PDF showing where and how data can be deployed. Please let us know if you have comments or questions:

Vax Stats of the Month

The Last Mile: 106,000 More Children Immunized a Day in 2024 — Possible?

by Bill Davenhall, Geomedicine Analyst, STC Health Analytics (

More than a 100,000 more children immunized every day in 2024? Audacious goal? That is what it would take to get the job done. Estimates are that about 27 million children will go without the CDC recommended seasonal influenza immunizations in 2024 – and that translates to 106,000 additional immunizations each of the 252 workdays in 2024!

Where children have historically received their seasonal influenza vaccinations might identify the greatest opportunity exists for getting more children protected in the future. The CDC’s 2018-2019 Seasonal Influenza Report on Children offers some interesting observations about where children receive their annual influenza immunization. Despite having one of the best years in term of reaching a larger number of children, the report’s 2018-19 observed rates yield an estimate that about 37.4% of children between 6 months and 17 years of age failed to receive a seasonal influenza vaccination. Projecting to 2024, that’s about 27 million children. The report also reports that the primary place where children received their influenza shot was the “Doctors Office” (66.7%), followed by Clinic (15.3%), Hospital (3.9%), Health Department 2.1%, Retail Pharmacy (6.6%), School (3.9%), and Other places (1.6%). Determining where the greatest opportunity will exist to reach the estimated 27 million children in 2024 who will likely not receive a seasonal influenza vaccination may require a different analytical unit of analysis.

Ultimately, immunization programs emanate from a CDC-supported state and county health department programs and thus operational data is typically analyzed by traditional geographies (e.g. states, counties, zips). But, what if a different geographical unit is used that reflected the established provider networks that already dominate various regions of the US? Does a geographic “network of providers” offer any greater ability to tackle the problem of getting a sizable portion of the 27 million unvaccinated children immunized? To look at this idea, we explored the use of the Hospital Referral Regions, the 306 delineated geographies inspired by the work of the Dartmouth Atlas project. The HRR geographical definitions are comprised of zip codes of the origin of patients (Medicare only) that receive most of their hospital care within a well-defined geography that extends across traditional geographies. The implications of such an analysis include issues of accountability due to the multiple public health jurisdictions involved and the various State regulations that guide public health activities. The HRR’s are not perfect but they may represent a better way to understand a regional health market area than what has been used in the past.

The map below shows the 306 HRR’s of which 135, or 44%, will encompass 89% of the unvaccinated children in 2024. It would suggest that the places where children can be vaccinated could rapidly becoming part of more highly organized health systems, health plans and retail pharmacies who will market to consumers across these larger regions. In fact, Doctors’ Offices and Retail Pharmacies together are the place of service for 73% of all childhood seasonal influenza vaccinations. The map also reveals the geographic extent of the 306 HRR’s and the relative number of additional “new” child vaccination “opportunities” each workday in 2024. As these 306 regional health and wellness markets (HRR’s) go, so goes immunizations. The Top 100 HRR’s will certainly have some economies of scale in getting to tackle the “last mile” of unvaccinated children. (A list of the Top 100 HRR’s is also available.) Stay tuned.