As Covid-19 is dominating this respiratory illness season, the flu season has disappeared. I covered this fact recently here, but my curiosity has gotten the better of me since then. I dug quite a bit deeper into the influenza virus to see what the possible explanation could be. I learned that influenza outbreaks are very different from the usual epidemics like Chicken Pox, Small Pox, Measles, Mumps, etc. I found much of what I learned in "The Transmission of Epidemic Influenza," a book by Dr. R. Edgar Hope-Simpson that raised all of these questions back in 1992. You can read it for free here. I want to thank Ivor Cummins for raising my awareness of this book and making it accessible. You can also read the paper entitled The Epidemiology of Influenza as it covers many of the book's points in a condensed format.

There are several puzzles surrounding influenza:

(1) Why is influenza both seasonal and ubiquitous, and where is the virus between epidemics?

(2) Why are the epidemics so explosive?

(3) Why do they end so abruptly?

(4) What explains the frequent coincidental timing of epidemics in countries of similar latitude?

(5) Why have we not figured out when the virus gets transmitted to new hosts?

(6) Why is the secondary infection rate so low?

(7) Why did epidemics in previous ages spread so rapidly, despite the lack of modern transport?

(8) Why does experimental exposure fail to cause illness in all the volunteers?  (1)

Hope-Simpson explained that a seasonal trigger causes many of influenza's seemingly strange behaviors, and that trigger is Solar Radiation. As we now know, vitamin D production relies on solar UV exposure. There is not enough sunshine in the northern hemisphere in the colder months to allow us to produce vitamin D. Vitamin D deficiency results in a weakened immune system. A weakened immune system leads to disease. In 2007, Aloia and Li-Ng found that a low dose of 800 international units/day reduced reported incidence and abolished reported colds and flu seasonality. A higher amount of 2000 IU/day, given during the last year of their trial, virtually eradicated all colds or flu. (1)

Many of you may be surprised to hear that the Sun has other measurable cycles of higher versus lower irradiance. The most talked-about one is the roughly eleven-year cycle shown below. The sunspot activity is low when the Sun is less active and higher when it is more active.  Scientists believe the alignment of the planets is the leading cause of this phenomenon. As you can see, Dr. Hope-Simpson was aware that the eleven-year cycle corresponded to significant flu outbreaks.

The main reason for the close connection is the UV-Vitamin D relationship. Dr. Hope-Simpson touches on other potential links. When the Sun is more active, the additional UV light destroys viruses. When the Sun is less active, it allows much more cosmic radiation to reach earth. There is a theory that the radiation mutates viruses, potentially causing them to become more virulent. Another observation (by NASA) is that genetic material originates in space and survives entry into our atmosphere. (2,3) Some believe space sends us killer viruses. There is little evidence available to support this last claim, however intriguing it is.

There is another much longer solar variation that appears in two to four hundred year intervals. When the sun cycles into its lowest activity, it is called a Grand Solar Minimum. Three grand solar minima on record – the Sporer minimum (1450-1550AD), Maunder minimum (1650-1700AD), and the Dalton minimum (1800-1830) are all associated with pandemics – Small Pox, English Sweats,

Plague and Cholera. Read this study linking low sunspot activity with pandemics. As far back as three years ago, a warning of an upcoming solar minimum was the topic in the paper entitled "Sunspot Cycle Minima and Pandemics: A case for vigilance at the present time." They noted, "The sunspot numbers recorded for the present period 2002-2017 include the deepest sunspot minimum (Cycle 23-24) since records began and a trend to declining numbers throughout the cycle.  The same period has seen the resurgence of several pandemics – SARS, MERS, Zika, Ebola, Influenza A.  We consider it prudent to take note of these facts while planning future strategies for pandemic surveillance and control".

 The last three years' worth of sunspot data shows that we are now in another solar minimum some call the Eddy Grand Solar Minimum". SpaceWeather.com was quoted in a recent Forbes article "This is a sign that a solar minimum is underway." "So far this year, the Sun has been blank (no sunspots) 76% of the time, a rate surpassed only once before in the Space Age. Last year, 2019, the Sun was blank 77% of the time. Two consecutive years of record-setting spotlessness adds up to a very deep solar minimum, indeed." This current minimum could be partly to blame for the current pandemic. Low solar irradiance and low vitamin D play a dominant role in Covid-19 infections. I have covered this aspect before here if you are interested.

But why has the flu almost vanished? Scientists have a reasonable explanation. The flu virus does not readily transmit from person to person as other viruses do. If one child gets chickenpox, the whole house may become infected. This does not happen with the flu. (1) Hope-Simpson postulated that only a select few (unidentifiable to date) individuals spread influenza for short periods throughout the year, even when they have no symptoms. When lower solar activity conditions appear, the people already "infected" become symptomatic due to a weakened immune response. We have seen severe strains in the past suddenly disappear for decades, only to return due to competition between strains.

The current coronavirus may have out-competed recent influenzas. Covid-19 does exhibit traditional person-to-person transmission; the flu does not. Currently, Covid-19 is everywhere, infecting many, and there are potentially more asymptomatic people with it. If there is competition, and Covid-19 is transmissible by traditional means, it may have already won the viral battle for world dominance.

 Another reasonable explanation may be that the susceptible patients have already gained immunity or have perished in the previous year. Hence, the at-risk population for the flu is meager. Whatever the cause of the non-existent flu this year, let's all rejoice. But whatever you do, make sure your vitamin D levels are optimal!