One of the most well-known aspects of influenza, more commonly known as the ‘flu,’ is that it occurs in yearly, seasonal outbreaks, with infections increasing as much as ten times from their summer minimums during the dark days of winter. This is also true with the common cold, which is not a specific virus like the influenza virus but is an umbrella term for a mild upper-respiratory infection caused by over 200 virus types. The fact that these viruses favor winter has been attributed to people spending more time indoors and thus facilitating person-to-person transmission of the virus, and the colder, drier weather may also be a more favorable environment for the viruses themselves. The cold/dry weather and reduced UV radiation allows viruses to survive longer on exposed surfaces, and these same cold and dry conditions dehydrate mucous membranes and make them more susceptible to infection. Finally, kids are in school during the winter months, and if there’s anybody who knows how to spread germs around, it’s those little rascals!
Coronaviruses are relatively common and are responsible for approximately 15% of all cases of the cold. But the novel coronavirus that has resulted in thousands of deaths, hundreds of millions quarantined, and a record sell-off in global equities has proven to be a particularly virulent strain. This novel coronavirus, known officially as “Coronavirus disease 2019” (COVID-2019), has approximately a 1-2% mortality rate, approximately 5 times less than the 2003 SARS outbreak in China, another coronavirus, but 10-20 times higher than the mortality rate for the seasonal flu.
One significant question regarding COVID-19 is whether the disease will abate with the turn to warmer weather this spring and summer. According to the CDC, “it is not yet known whether weather and temperature impact the spread of COVID-19.” To be clear, I’m taking the CDC’s position here; this is a novel virus with unique characteristics, and it is impossible to know if warmer weather would ameliorate its spread at this time. However, I thought it would still be worthwhile to study the seasonality of some past pandemics and how they differ from the ebb and flow of the endemic viruses that traverse the hemispheres through the seasons, and if we could at least draw some conclusions about their spread rate relative to the seasons.
The Greatest Pandemic in Modern History: The Spanish Flu of 1918
The greatest viral pandemic in modern history was the Spanish Flu of 1918. According to the CDC, approximately 1/3 of the world’s 1.8-1.9 billion people contacted the virus, and at least 50 million people died from it. To be sure, the Spanish Flu was an extraordinarily virulent and contagious version of the influenza virus, but the pandemic was substantially amplified by World War I, as the combination of close quarters, unsanitary conditions, and mobile troops spreading the virus worldwide created a perfect storm for a global pandemic to ensue.
There were three “waves” of the Spanish Flu - the initial outbreak in spring/summer 1918, the second, deadliest outbreak during autumn 1918, and a third outbreak in winter/spring 1919 that was somewhere between the first and second outbreaks in severity. The Spanish Flu did not follow the typical “seasonal flu” profile; the second, deadliest wave began in August, peaked in late October/early November, and was over by December. As mentioned earlier, it is true that influenza viruses last a much shorter time on warm, humid surfaces and are easily killed by UV radiation, and that the behavioral shifts in winter (such as spending more time indoors) are more efficient at spreading contagious diseases. However, the Spanish Flu was far more contagious than the seasonal flu since most people had little-to-no-immunity to the virus and was able to easily overcome these seasonal barriers.
The 1918 Spanish Flu was so virulent that young, healthy adults could die less than 12 hours after showing their first symptoms. Most viruses of this virulence do not grow into a pandemic, as they kill their hosts before their hosts can spread the virus to others. The Spanish Flu thrived in large part because it was sustained in the trenches; the trenches were a perfect environment for the virus to spread among pre-existing soldiers, and new, uninfected soldiers were constantly being deployed to replace those that had died, either from battle or from the virus. Soldiers then interacted with the local population, spreading the virus and allowing it to subsequently ravage other communities.
COVID-19 is the first coronavirus pandemic, but many of us remember two other coronavirus epidemics - SARS from November 2002 to June 2003, and MERS in 2012 and 2015. SARS has been completely contained, while MERS continues to circulate at low levels. Because SARS lasted only one season, we can’t really infer any seasonality from it, and MERS does not have a predictable seasonal pattern.
When we take a holistic look at all 20th/21st century influenza/coronavirus pandemics and epidemics, we do not find a clear, seasonal factor like we do with endemic influenza and the common cold. I can only infer that this is because these influenza/coronavirus strains are ‘novel’ viruses with which there is little or no pre-existing immunity, and they can be easily spread throughout the population regardless of the season. To reiterate the CDC: we do not know enough about COVID-19 yet to know if it has a seasonal cycle. But if past upper-respiratory (namely, influenza and coronavirus) pandemics are any clue, seasonality might not play that big of a factor.
One thing is for sure: how countries respond to the virus has a much, much greater impact on its spread than the season. Just compare the graphs of confirmed cases in China vs the U.S. After initial exponential growth, the Chinese government issued draconian but highly successful lockdowns on major cities and got testing kits to millions of people, and the virus is now effectively contained with now only dozens of new cases per day. But even with months of lead time, the US was woefully unprepared, and cases continue to grow at an exponential rate. Australia is also currently seeing an exponential increase in cases, even though it is late summer there.
This virus has exposed how healthcare is not a service, or even a right; it is a national security issue that holds the fabric of our economy and society together. The US may have been able to sidestep a severe recession if we had the resources to provide free, public COVID-19 tests to all citizens, as South Korea has done. Unfortunately, we were not prepared, and we now must practice isolation and social distancing instead, which is far more damaging to our economy and society.
Please take care of yourselves, and focus diligently on washing your hands, not touching your face, and limiting social interaction/exposure over the next several months. Life is already beginning to return to normal in China, and if we do this right – and I have faith that we will – our lives should eventually return to normal within several months.
Charlie Phillips, a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist. Check out his weather website at charlie.weathertogether.net.