We know for sure that most respiratory viruses peak during the winter and drop off as the spring hits. Why? While the answer is complex, one part of the answer may want to make you start taking Vitamin D. So let’s dig in on this one.
Respiratory Viruses and Seasonality
In the 36 years that CDC has been collecting data on flu peaks (a respiratory virus), we’ve never had one in the spring and summer (2):
While the coronavirus is not the flu, both are enveloped respiratory viruses that are thought to spread via water droplets. Above you see the number of times that month was the peak of the flu season. Notice how in April and May you don’t see anything. Why is this happening? That’s a really complex answer.
The Really Complex Answer
First, we know that heat may be a factor. Viruses don’t survive as long on hot surfaces. For example, heat at 56C (132 F) kills the virus at a 10,000X reduction per 15 minutes (it’s 10,000X less) (3). For example, if it’s a hot summer day, the inside of your car will be that temp in about 30-60 minutes (4).
In addition to being killed off, viruses have temperatures they like and don’t like. This novel coronavirus likes temps between 41-51F (7).
Higher humidity lowers respiratory virus transmission because the water particles from coughs fall to the ground faster. For example, in a low humidity environment of 23 percent, 70-77 percent of the flu virus particles in water droplets were still able to cause an infection at one hour (5). When humidity is raised to 43 percent, just 14 percent of virus particles can cause an infection. In addition, there’s some thought that the lower variation in humidity in the summer is less conducive to the survival of viruses like coronavirus (enveloped) (6).
Latitude is how far north or south you live. One team of researchers put together the following model for COVID-19 based on combining latitude, temperature, and humidity (7). That map is below:
These are the conditions that the researchers believe can spread the virus:
- Places within the red band above which is most of the US and Canada
- Temps between 41F-51F (5-11C)
- Low humidity
Note that temps are more typical for Canada in April than most of the US. Meaning that by April, most of the US will move out of the average temperature that this virus likes. Obviously, none of the US South would qualify as “low humidity” once May comes around.
Vitamin D and Latitude
Where you live north/south also has an impact on your Vitamin D levels (8). Why? Because in the winter the northern hemisphere is tilted away from the sun. That means less direct sunlight and the sunlight is what helps your body produce Vitamin D naturally. The farther north you are, the worse the problem gets.
Researchers have also connected latitude and vitamin D deficiency and the risk for viral respiratory infections (9) For example, we know that vitamin D is involved in enhancing your innate defense mechanism against respiratory viruses (10). Hence, it makes sense that as vitamin D levels drop in the winter due to less direct sunlight, we see more respiratory infections.
Do You Need to Take Vitamin D?
At the end of winter, 2/3rds of healthy young adults in Boston were Vitamin D deficient. 60% of nursing home patients are also deficient (11). Hence, now that we’re exiting winter and have the threat of coronavirus, supplementing your Vitamin D is likely not a bad idea. The general recommendation is to supplement at 1,000 IU a day with D3, but others have recommended as high as 5,000 U a day.
Natural Sources of Vitamin D
As a country, we just don’t drink nearly as much milk as we used to. In fact, milk consumption in the US has fallen 40% since the ’70s (12). That’s a problem because we all know that milk is a great source of vitamin D.
What are other sources of vitamin D that might not be on your radar? Fatty fish like salmon, cheese, egg yolks, kale, soy, and white beans. Also, some foods are fortified with vitamin D, so check the label.
The upshot? I went out and bought a bottle of D3 for my family. If you already take a good multivitamin, you likely get more than enough vitamin D. However, given the circumstances, if you live in a northern part of the world, buying some D3 may not be a bad idea. In addition, if you tend to get outside less, which will happen more with the shutdown, supplementing is also a good idea.
(1) Beard JA, Bearden A, Striker R. Vitamin D and the anti-viral state. J Clin Virol. 2011;50(3):194–200. doi:10.1016/j.jcv.2010.12.006
(2) Centers for Disease Control. The Flu Season. https://www.cdc.gov/flu/about/season/flu-season.htm Accessed 3/8/20
(3) World Health Organization. First data on stability and resistance of SARS coronavirus compiled by members of WHO laboratory network. https://www.who.int/csr/sars/survival_2003_05_04/en/ Accessed 3/21/20.
(4) Heat Kills. How hot can the interior of a car get – and how quickly? http://heatkills.org/how-hot/ Accessed 3/21/20.
(5) Live Science. Higher Humidity Lowers Flu Transmission. https://www.livescience.com/27533-flu-transmission-humidity.html Accessed 3/21/20.
(6) Price, R.H.M., Graham, C. & Ramalingam, S. Association between viral seasonality and meteorological factors. Sci Rep 9, 929 (2019). https://doi.org/10.1038/s41598-018-37481-y
(7) Sajadi, Mohammad M. and Habibzadeh, Parham and Vintzileos, Augustin and Shokouhi, Shervin and Miralles-Wilhelm, Fernando and Amoroso, Anthony, Temperature, Humidity and Latitude Analysis to Predict Potential Spread and Seasonality for COVID-19 (March 5, 2020). Available at SSRN: https://ssrn.com/abstract=3550308
(8) Leary PF, Zamfirova I, Au J, McCracken WH. Effect of Latitude on Vitamin D Levels. J Am Osteopath Assoc. 2017 Jul 1;117(7):433-439. doi: 10.7556/jaoa.2017.089.
(9) Zittermann A, Pilz S, Hoffmann H, März W. Vitamin D and airway infections: a European perspective. Eur J Med Res. 2016;21:14. Published 2016 Mar 24. doi:10.1186/s40001-016-0208-y
(10) Hughes DA, Norton R. Vitamin D and respiratory health. Clin Exp Immunol. 2009;158(1):20–25. doi:10.1111/j.1365-2249.2009.04001.x
(11) Medscape. What is the prevalence of vitamin D deficiency in the US? https://www.medscape.com/answers/128762-54281/what-is-the-prevalence-of-vitamin-d-deficiency-in-the-us Accessed 3/21/20
(12) Statista. Milk’s Massive American Decline. https://www.statista.com/chart/2387/american-milk-consumption-has-plummeted/ Accessed 3/22/20.