www.landesbioscience.com Dermato-Endocrinology 307
Dermato-Endocrinology 1:6, 307-309; November/December, 2009; © 2009 Landes Bioscience
Dermato-Endocrinology 1:6, 307-309; November/December, 2009; © 2009 Landes Bioscience
*Correspondence to: Asta Juzeniene; Email: email@example.com
Submitted: 01/21/10; Revised: 01/29/10; Accepted: 02/01/10
Previously published online: www.landesbioscience.com/journals/dermatoendocrinology/article/11357
Nearly all human diseases related to respiratory pathogens
exhibit seasonal variations.1,2 The reasons for the seasonality,
however, are still not known. Among the poorly tested hypoth-
esis are: Seasonality of low temperatures, of dry air, of crowding
together indoor in the winter, travelling patterns, vacations, sea-
sonality of ultraviolet (UV) radiation from the sun that might
kill pathogens, circannual rhythms of hormones like the “dark
hormone” melatonin, etc.3 Additionally, the question of whether
it is the host or the vira/bacteria that exhibit seasonality arises.
In the present work we have compared the seasonality of deaths
of inﬂuenza and pneumonia in Norway with those of vitamin D
photosynthesis and vitamin D serum levels.
Results and Discussion
Figure 1 shows the seasonal variation of vitamin D formation
in human skin calculated from the known, 1990–2000 average
levels of UV as earlier described,8 using the action spectrum of
Galkin and Terenetskaya.9 The small dip in the curve in June is
related to the cloud cover, which is taken into account. In agree-
ment with earlier work, the vitamin D level is maximal about one
month after the time of maximal formation, which occurs close
to midsummer. This is due to the fact that the vitamin D level
is here determined as the concentration of 25-hydroxyvitamin
D [25(OH)D or calcidiol] in serum and that the formation of
this metabolite from previtamin D, via vitamin D (mainly in the
liver), takes some time. The fact that many have their vacation
in July may also play a role. The observed delay is in agreement
with earlier ﬁndings11-14 and has been discussed in these papers.
Inuenza, solar radiation and vitamin D
Johan Moan,1,2 Arne Dahlback,2 LiWei Ma1 and Asta Juzeniene1,*
1Departm ent of Radiation Biolo gy; Institute for Cancer R esearch; the Norweg ian Radium Hospital; Osl o University Hospit al; Montebello, Oslo No rway; 2Institute of Physic s;
Universit y of Oslo; Blindern, Osl o Norway
Key words: vitamin D, influenza, pneumonia, solar radiation, UVB radiation
Abbreviations: 25(OH)D, 25-hydroxyvitamin D; TOMS, total ozone mapping spectrometer; UV, ultraviolet
In fact, the lowest vitamin D level is found in February, and aver-
age levels as low as 25 nmol/l have been observed among women
avoiding direct sun exposure.
In both periods studied (1980–1989 and 1990–1999, Fig. 1),
the death rates are very small in the season when the vitamin D
status is best. The agreement between the death numbers in the
two periods is good. This argues, although only weakly, for the
common assumption that the seasonal variation of inﬂuenza is
host related rather than caused by differences in the viral strains
from year to year. This is also in agreement with recent observa-
tions that the same virus strain seems to be present in the hosts
over longer periods, two years or more, but leading to manifest
disease only under favourable conditions, mainly related to host
immune weakening.1,15,16 One might expect variations in the
immune system to play major roles. Vitamin D interacts with
the immune system, essentially strengthening it, in several ways
as reviewed elsewhere.17-19 Other pathways of interaction, like
via circannual rhythms, cannot yet be ruled out.20 The preven-
tive effect of vitamin D supplementation has been demonstrated
also in intervention studies.21 Furthermore, Ginde et al. found
that the serum levels of vitamin D were inversely associated with
upper respiratory tract infections,22 and it seems that exposure to
artiﬁcial UVB radiation has a protective effect.23 We, therefore,
propose that the seasonal variation of inﬂuenza death numbers is
related to the seasonal variation of vitamin D status.
An argument against this proposal from our data might be
that the inﬂuenza death rates start to increase almost two months
after the vitamin D levels have reached their minimum (Fig. 1).
However, one should remember that our register gives time-
points for death, not for disease initiation. A delay of weeks is
quite likely. Similarly, the death numbers start to decrease several
The annual death numbers of inuenza and pneumonia in Norway were studied for the time period 1980–2000. No direct
relationships were found with the variations of the annual UVB uences, probably due to the fact that these variations
did not exceed 30%. However, there was a very pronounced seasonal variation of both inuenza deaths and pneumonia
deaths, the vast majority occurring during the winter. Vitamin D levels were also estimated from several publications.
The data support the hypothesis that a high vitamin D level, as that found in the summer, acts in a protective manner
with respect to inuenza as well as pneumonia. The ndings are discussed and compared with data from tropical and
308 Dermato-Endocrinology Volume 1 Issue 6
synthesis in human skin is about ﬁve times larger in late June
than in late December. Sometimes secondary peaks are observed
in June–August.15,3 3 These peaks may be caused by other cli-
matic factors than those directly related solar elevation.34 Thus,
cloud cover is likely to be involved since it will inﬂuence both
humidity and UV penetration through the atmosphere. It seems
that that many respiratory syncytial virus epidemics begin in
Variations of the ambient UVB level have sometimes been
proposed as factors related to inﬂuenza periods.36 -38 However,
we have earlier shown that inﬂuence of variations of cloud cover
on the annual UVB ﬂuence is more than three times as large
as that of variations of ozone levels.39,40 Furthermore, during a
sunspot cycle (about 11 years) the UVB ﬂuence varies less than
In conclusion, our data are in agreement with the assumption
that the high numbers of winter inﬂuenza and pneumonia deaths
in Norway are related to low vitamin D levels in this season.
Materials and Methods
Inﬂuenza and pneumonia deaths. The numbers of monthly
inﬂuenza and pneumonia deaths were calculated by Prof. ∅ystein
Kravdal from individual data provided by Statistics Norway. The
numbers are averaged over the years 1980–1989 and 1990–1999.
months before the vitamin D level starts to increase signiﬁcantly.
This is likely to be related to induction of better immunity. Thus,
inﬂuenza pandemies, starting outside the classical winter inﬂu-
enza season, have a duration of not more than a few months, but
also in the case of classical pandemies, like the Spanish ﬂu, 1918–
1919, there came a winter wave after the initial wave.24 The ﬁrst
wave peaked in 1918, in October 19th in Baltimore, in October
26th in Augusta and in November 5th in San Francisco (an east–
west wave), while the secondary wave came January 1919.
In future work other factors than vitamin D, essentially those
mentioned in the introduction, should be taken into account.
Out of these temperature and humidity may be the most impor-
tant ones.25 -28
Variation of summer UVB levels over the two decades, from
1980 to 2000, as shown in Figure 2, upper panel, are signiﬁ-
cant. For instance, from 1997 to 1998 there was a 20% decrease
in the level. Annual numbers of inﬂuenza deaths do not reﬂect
the variations of the UVB levels. This may be due to that the
UVB variations are too small, notably in view of the fact that
summer levels of vitamin D are not more than 30–50% higher
than winter levels. Thus, a 20% decrease in summer UVB
level would result in only a small overall decrease in vitamin D
level, probably too small to inﬂuence the immunity next winter
Norway is localized from 58 to 70 degrees north, vitamin
D being synthesized in skin exclusively in the summer months.
It is, therefore, of interest to compare our data with data from
tropical and subtropical regions. Generally, there is no distinct
seasonal pattern in the tropics.30 -32 In Singapore small peaks
were found in June–July and in November–January, coincid-
ing with the winter in the Southern and northern hemisphere,
respectively.30 At latitudes between 20 and 30 degrees north,
clear winter seasons of inﬂuenza are found.15,3 3 This is at ﬁrst
sight surprising. However, we have calculated by the methods
described above, that at 25 degrees north the rate of vitamin D
Figure 1. Seasonal variations of serum levels of vitamin D (▼) in the
Nordic countries, given as averages from three reports in ref. 9. Photo-
synthesis of vitamin D for Southern Norway (■) is calculated by use of
the action spectrum in ref. 7, UV measurements in Oslo and radiative
transfer calculations using a cylinder model as described in the text.
The results are presented in relative units (rel. u.). Inuenza deaths in
two periods (●, ○) in Norway are also given.
Figure 2. Upper: The UVB levels in Norway in the period 1980–2000.29
In this case the UVB uences were determined for a horizontal plane
with the CIE erythema spectrum, which is UVB weighted and very close
to the vitamin D spectrum used in the rest of this work. Lower: The av-
erage number of inuenza and pneumonia deaths per year in Norway
in the same period.
www.landesbioscience.com Dermato-Endocrinology 309
in converting 7-dehydrocholesterol to previtamin D.9 Br ieﬂy,
an efﬁciency spectrum is calculated by multiplying the inten-
sity of the solar radiation (wavelength by wavelength) with
the action spectrum for the vitamin D production for the cor-
responding wavelength. The vitamin D action spectrum was
taken from the publication of Galkin and Terenetskaya9 which
is similar to that measured by MacLaughlin et al. in ex vivo
The vitamin D levels in different seasons are taken from an
earlier publication11 and are averages from three different experi-
mental Nordic series described there.
We want to thank ∅ystein Kravdal from the Institute of
Economy (Oslo University, Norway) for calculating the number
of deaths of inﬂuenza and pneumonia. Direct overpass TOMS
data were provided by NASA/GSFC.
It may be difﬁcult know if the inﬂuenza deaths are caused directly
by inﬂuenza or by secondary diseases.
Solar exposure and seasonal vitamin D formation. The main
factors inﬂuencing the UV irradiances at ground level are solar
zenith angle (variable with season, latitude and time of day),
cloud and snow cover and the thickness of the ozone layer.4
In this study, the solar UV exposures were calculated using
a radiative transfer model.5,6 Total ozone amounts used in this
model were measured by the Total Ozone Mapping Spectrometer
(TOMS) satellite instruments. The daily cloud cover was derived
from measured reﬂectivities by TOMS instruments and ground-
based UV measurements in Oslo. A cylinder geometry of the
human body was used. The arguments for this choice have been
presented earlier.7, 8
The results are presented as vitamin D forming U V doses.
The efﬁciency spectrum for vitamin D production gives the
relative effectiveness of solar radiation at different wavelengths
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