3.1 Description of Data
In order to identify the relation of UVB radiation and COVID–19 deaths, we constructed the dataset outlined in Table 1. We collected data covering 108 days from 22 January 2020 until 8 May 2020 across 183 countries of which 158 reported COVID–19 deaths prior to 8 May 2020 and of which 152 reported more than 20 COVID–19 infections prior to 8 May 2020. We focus on those 152 countries to ensure that the results are not biased by countries that are at a very early stage of COVID–19 outbreak, which would limit data points with respect to COVID–19 deaths. In addition, we drop the first 20 daily observations of every country after that country reported the first COVID–19 infection to further ensure that results are not biased by the observations at the very early stage of the COVID–19 outbreak.
The corresponding country level data consist of the cumulative daily COVID–19 deaths and infections, the daily ultraviolet index (UVI), which is closely connected to the daily UVB radiation, and a set of control variables such as daily weather parameters such as precipitation index, cloud index, ozone level, visibility level, humidity level, as well as minimum and maximum temperature.
Table 1: Summary of Dataset
Number of countries in the world
|
195
|
Number of countries in our dataset
|
183
|
… > 0 cumulated COVID-19 deaths before 8 May 2020
|
158
|
… > 20 cumulated COVID-19 infections before 8 May 2020
|
152
|
Covered time-period
|
22 January 2020 - 8 May 2020
(108 days)
|
Granularity of data
|
Daily
|
COVID-19 data source
|
https://github.com/ CSSEGISandData/COVID-19
|
Weather data source
|
https://darksky.net/
|
We present descriptive statistics of the dataset in Table 2. As of 8th of May, the cumulative COVID–19 deaths of these 152 countries were on average 1,808 and the growth rate of COVID–19 deaths on May 8 was on average 2.6% as compared to the average growth rate of COVID–19 deaths across countries and time which was 10.1%. The cumulative COVID–19 infections were on average 25.888. The case-fatality-rate (CFR), as measured by the cumulative COVID–19 deaths divided by the cumulative COVID–19 infections per country, was on average 4.3% on May 8. The growth rate of CFR on May 8 was on average –1.1% as compared to the growth rate of CFR across countries and time which was 2.6%. We use cumulative COVID–19 deaths as the main dependent variable to test our hypothesis linking UVB radiation to COVID–19 deaths and use the CFR to test the consistency of our results. On average, the first reported COVID–19 infection in each country happened 68.15 days before 8 May 2020. UVI is on average 6.81 representing a moderate to high risk of harm from unprotected sun exposure.
Table 2: Descriptive Statistics of Dataset
Variable
|
Number of Countries
|
Number of Observations
|
Mean
|
Std. Dev.
|
Min
|
Max
|
Cumulated COVID-19 deaths on 8 May
|
152
|
152
|
1,808
|
7,779
|
1
|
77,180
|
Growth rate of cumulative COVID-19 deaths on 8 May
|
152
|
152
|
0.026
|
0.051
|
0
|
0.4
|
Daily growth rate of cumulative COVID-19 deaths
|
152
|
6,589
|
0.101
|
0.259
|
-1
|
9
|
Cumulated COVID-19 Infections on 8 May
|
152
|
152
|
25,888
|
111,303
|
23
|
1,283,929
|
CFR on 8 May
|
152
|
152
|
0.043
|
0.037
|
0.001
|
0.206
|
Growth rate of CFR on 8 May
|
152
|
152
|
-0.011
|
0.057
|
-0.421
|
0.238
|
Daily growth rate of CFR
|
152
|
6,589
|
0.026
|
0.200
|
-1
|
5.935
|
Time-passed by from first reported infection until 8 May
|
152
|
152
|
68.15
|
17.59
|
29
|
108
|
Daily Ultraviolet Index (UVI)
|
152
|
7,471
|
6.81
|
3.05
|
0
|
14
|
Daily precipitation index
|
152
|
7,471
|
0.29
|
0.31
|
0
|
1
|
Daily cloud index
|
152
|
7,471
|
0.50
|
0.30
|
0
|
1
|
Daily ozone level
|
152
|
7,471
|
308.48
|
47.06
|
235.5
|
472.7
|
Daily Visibility level
|
152
|
7,471
|
15.29
|
2.17
|
0.117
|
16.09
|
Daily humidity level
|
152
|
7,471
|
0.63
|
0.20
|
0.04
|
1
|
Minimum temperature per day within a country
|
152
|
7,471
|
12.45
|
9.78
|
-23.45
|
30.87
|
Maximum temperature per day within a country
|
152
|
7,471
|
22.83
|
10.47
|
-16.41
|
45.82
|
3.2 Illustration of Ultraviolet Index (UVI) and COVID–19 Deaths
The graph on the top of Figure 2 shows the cumulative COVID–19 deaths and the associated daily growth rates for Italy from 26 February 2020 until 8 May 2020. As time progresses, the cumulative COVID–19 deaths increase but at a slower rate. Initially, the growth rate is high at 41.67% (growth rate from 26 February to 27 February) and it gradually slows to 0.81% (growth rate from 7 April to 8 May).
The graph on the bottom of Figure 2 shows the daily growth rates and daily UVI for Italy as well as the UVI values lagged by one, two, three, four and five weeks respectively. It is important to consider the lagged effect of UVI because synthesized vitamin D is cumulative and can be stored in body fat to be used later21. Therefore, it seems more plausible that an increase of UVI today will continue to support an individual’s immunity later i.e., two- or more weeks’ time. Furthermore, the likelihood of skin synthesis is low in severely infected people, while they are hospitalized, indicating the importance of lagged UVI values.
It is evident that the growth rates slow down over time, as counter-measures imposed by governments take effect, which results in lower infection rates and lower mortality rates. At the same time, the UVI is increasing due to seasonal changes in the northern hemisphere countries. In order to approximate the association of UVI, we need to isolate it from the underlying time-trends, which are potentially affecting both UVI as well as the growth rates of cumulative COVID–19 deaths.