“Winter is coming!” was the famous (and rather anticlimactic) refrain in “Game of Thrones”.* But this ominous and obscure warning may be relevant today. There is a theory, currently supported by multiple new studies, that Covid-19 will be a seasonal disease, one that becomes more contagious and more deadly in winter. There are two reasons for this theory.
First, vitamin D levels tend to be higher in the warmer months, when people are outdoors, with skin exposed to sun. Vitamin D is called the sunshine vitamin for good reason. When ultraviolet light (UVB) from the sun strikes your skin, a compound produced by the human body, 7-dehydrocholesterol, is converted into pre-vitamin D3, which then slowly becomes vitamin D3 itself over the course of a couple of weeks. “Exposure to light through windows is insufficient because glass almost completely blocks UVB light.” 
In regions of the world with cold winters and warm summers, the population tends to spend significantly more time outdoors, with exposed skin, leading levels of vitamin D to rise . Then, in the winter, they spend more time indoors, reducing their vitamin D levels. This causes vitamin D levels in the blood to rise and fall, in a cyclic manner. But there is also an 8-week lag between the time spent in the sun and the rise of vitamin D levels. The lag occurs because it takes time for the body to produce vitamin D after skin exposure.
During warmer months, higher levels of vitamin D in the blood results in lower incidence of certain diseases. Then in cooler months the fall of vitamin D levels increases the incidence of those diseases. This increase in disease, due to falling levels of vitamin D can be corrected with supplements. For example, a randomized placebo-controlled trial of school-aged children in the U.S. showed that children given vitamin D supplements in winter were 42% less likely to contract influenza A .
But is Covid-19 one of those diseases which are influenced by vitamin D levels? Yes. Nearly a dozen studies have concluded that severity and/or mortality in Covid-19 are influenced strongly by levels of vitamin D in the blood. See “Vitamin D versus Covid-19” at Covid.us.org In addition to the studies, there are over a dozen scientific papers reviewing the evidence and opining that higher levels of vitamin D in the blood has a protective effect against Covid-19.
The evidence so far is weighty, but not yet irrefutable. Even so, vitamin D is an essential nutrient, known to strengthen the immune system; therefore, the benefits to vitamin D supplementation, especially in winter, already far outweigh any risks. Vitamin D deficiency is common in the elderly , the same population most at risk of death from Covid-19, according to the U.S. CDC:
“Overall, 31% of cases, 45% of hospitalizations, 53% of ICU admissions, and 80% of deaths associated with COVID-19 were among adults aged ≥65 years with the highest percentage of severe outcomes among persons aged ≥85 years.” 
If the U.S. and other nations undertook a program of vitamin D supplementation, especially in the elderly, the reduction in severity of Covid-19 could save many lives. A program to increase the vitamin D levels of an entire nation was successful some years ago in Finland, causing average blood levels of vitamin D to increase from 48 nmol/l to 65 nmol/l . This was accomplished by an increase in vitamin D supplement usage, from 11% to 41% of the population, and by increased vitamin D fortification of food . However, a much more rapid program would be needed if vitamin D levels are to be increased in time for this coming winter (2020-2021). Absent such a program, vitamin D levels will fall in the coming winter, and this could lead to a so-called “second wave” of Covid-19 cases and deaths.
But the cause of this second wave would not be from lower vitamin D levels alone. A couple of studies have now shown a correlation between climate, especially temperature, and Covid-19. So the second reason that Covid-19 may become a seasonal disease is that lower temperatures in winter results in a higher growth rate for the pandemic, while summer weather slows the spread of the disease.
One study concluded that lower temperatures increase both the infection rate and the death rate for Covid-19:
“A definitive association between infection rate and death from COVID-19 and ambient temperature exists, with the highest risk occurring around 9°C.” 
The risk is highest at 48 degrees F (9°C) perhaps because at very low temperatures people stay indoors, where it is warm. The study also found that “80% of all COVID-19 related cases and deaths occurred between 4.29°C and 17.97°C” . These temperatures correspond to 39.7 and 64.3 °F. The study authors explained that a number of factors are behind this result:
“The reasons for this enhanced viral spread during colder weather may be due to enhanced shedding of the virus during colder weather, stability of the virus suspended in air droplets or increased viability on nasal mucus membranes cooled by surrounding air. Additionally, cold weather can impair the immune system by inhibiting nasal mucociliary clearance and phagocytosis, diminishing the protective properties of mucus in upper airways, or predispose to vitamin D deficiency due to lower exposure to ultraviolet radiation making people more susceptible to the infection.”
A second study agreed that Covid-19 may be a seasonal disease, but cautioned that in the initial stages of the pandemic, other factors prevail.
“We find that while variations in weather may be important for endemic infections, during the pandemic stage of an emerging pathogen the climate drives only modest changes to pandemic size.” 
So climate has had an effect on the Covid-19 pandemic, but initially that was not the main driving force. High susceptibility is the initial impetus for an epidemic or pandemic, as no segment of the population has an immunity. However, as the pandemic continues, the balance shifts toward a greater influence from climate. Unfortunately, this implies that, once the pandemic is well-established, the following winter may bring a second wave:
“However, this does not mean the climate is not important in the longer term. Endemic cycles of the disease will likely be tied to climate factors and seasonal peaks may vary with latitude…. Furthermore, weather and near-term climate forecasts could be helpful for predicting secondary outbreaks after the initial pandemic phase has passed.” 
The variation in latitude is also found for vitamin D levels. At higher latitudes, a region will have colder winters and warmer summers. In addition, the solar radiation levels themselves will be higher in summer, as there will be more daylight hours and fewer nighttime hours. So this seasonality to Covid-19 will be most pronounced in nations or regions at those higher latitudes.
Summer vs. Winter
If the above-discussed studies and theories are correct, Covid-19 will be seasonal. The tendency to be outdoors in summer, combined with the current push to end the lockdown, should see many more persons in the sun. This increases the blood levels of vitamin D across many populations. Vitamin D levels rise in warm sunny weather, and illnesses like flu and possibly Covid-19 decline. However, in winter, vitamin D levels fall and colder weather favors the spread of disease. The result could be a very deadly, and mostly unexpected, second wave of Covid-19 cases in the winter of 2020 to 2021.
In the U.S., the lockdown is slowly ending. As summer nears, the benefits of the seasonality of Covid-19 should result in little harm from this attempt to return to normalcy. But this could lead us into a false sense of security. By the time autumn begins, a vaccine will be closer than ever, treatment modalities will improve (by additional studies and the first-hand experience of health care providers), and the case rate and death rates will hopefully be lower or at least flat. Most persons will become convinced that the worst is over.
Then winter will come. Vitamin D blood levels fall, with an 8-week lag , and colder weather favors the spread of disease. Without a vaccine and absent a major vitamin D supplementation and food fortification program, we may see a second wave of cases and deaths from Covid-19. And as the disease will already be well established in every nation, climate will drive the pandemic into a much worse state .
To avoid a second wave of Covid-19 cases and deaths, a vitamin D program must be widespread. And it must not be politicized, such that liberals support the program and conservatives oppose it, or vice versa. Broad political and social support should be sought, with medical experts taking the lead.
Edited on 26 May 2020 to Add: A new study, “Slower COVID-19 Morbidity and Mortality Growth at Higher Solar Irradiance and Elevation”, found that solar elevation (how high the sun is in the sky) explains pandemic growth differences from place to place, as does degree of cloud cover (“cloudopacity”) — “irradiance and in particular solar elevation angle in combination with cloudopacity explain COVID-19 morbidity and mortality growth better than temperature: a reduction of mean solar elevation of 9 degrees led on average to a 2500% increase in COVID-19 case growth over the following two weeks.”  Solar elevation is lower in winter and higher in summer, so this study, too, indicates that the pandemic will lessen in summer and worsen in winter.
Another study , published in JAMA Network Open, found that Covid-19 is a seasonal disease. This suggests that the virus will have a resurgence in winter, esp. in regions at higher latitudes.
“In this study, the distribution of substantial community outbreaks of COVID-19 along restricted latitude, temperature, and humidity measurements was consistent with the behavior of a seasonal respiratory virus.” 
Then there is the Edridge study  also discussed in another article here at Covid.us.org, which found that beta-coronaviruses which cause upper respiratory tract infections are also seasonal. Here’s the chart of only the coronaviruses covered by that study:
What you are looking at, above, is cold-type coronaviruses in their seasonality, but Covid-19, being the same type of virus, could well follow that same pattern. July, August, and September could be relatively restrained, for a deadly pandemic. But October, November, and December may see sharply increasing case rates and fatality rates in many nations. Then things will remain bad until late spring and summer.
What could stop that pattern from playing out? A massive vitamin D supplementation program: 10,000 IU of vitamin D per day for adults, but twice that for the elderly, starting as soon as possible and continuing until summer (when dosages can drop to half of those values).
Ronald L Conte Jr
Note: the author of this article is not a doctor, nurse, or healthcare provider.
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9. Backer, Alex. “Slower COVID-19 Morbidity and Mortality Growth at Higher Solar Irradiance and Elevation.” Available at SSRN 3604729 (2020).
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11. Edridge, Arthur WD, et al. “Coronavirus protective immunity is short-lasting.” medRxiv (2020).
* I should have gone with “Summer Me, Winter Me”.