Epidemic influenza and vitamin D
To the Editor:
We were very interested to see this very thorough article [1] and welcome its particular acknowledgement of the pioneering work of Hope-Simpson.
Vitamin D deficiency states are associated with both inadequate diet and limited exposure to sunlight and there are clearly risk populations especially among the elderly who are likely to become vitamin D deficient. In consequence, they might have an increased likelihood of infections generally and of the more severe manifestations of influenza in particular. However, the converse argument is not so persuasive. Only a minority of persons who experience influenza are likely to be vitamin D deficient and certainly not enough to materially affect herd immunity thereby interfering with transmission of influenza viruses. We would like to use this opportunity to air an alternative hypothesis on the seasonality of influenza: this is offered as a development of ‘Proposition Number 1’ as articulated by Hope-Simpson, which concerned the lack of spread of influenza [2].
Influenza is clearly a seasonal condition which very rarely reaches epidemic proportions during summer months, whether north or south of the equator. Accepting that influenza is transmitted (along with many other respiratory virus infections) mainly by respiratory droplets, then it follows that anything interfering with the transmission of viruses in such droplets will inhibit spread. Perhaps ultraviolet (UV) radiation effectively reduces the likelihood of transmission of influenza viruses by substantially reducing the number of viruses which are transmitted in this way because they are ‘killed’ by UV exposure. Laboratory experiments have demonstrated the inactivating properties of UV radiation against a number of viral pathogens including influenza [3, 4]. The Weekly Returns Service of the Royal College of General Practitioners has recorded a declining trend in the incidence of respiratory infections (particularly influenza-like illness) over the last two decades and it is possible that this may, in part, be due to increasing levels of environmental UV radiation [5, 6]. This hypothesis on the transmission of influenza in relation to UV differing levels of exposure would need to be tested in an appropriate animal model.
The hypothesis is consistent with many features typically associated with influenza.
Historically it has long been observed that winter weather conditions precipitate epidemics of infection. These conditions include reduced daylight, increased particulate matter in the atmosphere and often overcast and cloudy conditions all of which reduce the penetration of UV radiation. A small increase in the potential of one person to pass the illness onto another would accelerate the spread of influenza in a susceptible community. There has to be a minimum value of the reproduction number (R0) in order to sustain transmission at all. Perhaps in conditions of high UV exposure this number at a community level falls consistently below that minimum.
Pandemics have always been described as coming in waves. Why does one wave peter out in circumstances where the population immunity is low? The virus has not disappeared and there are still substantial numbers in the community who are susceptible. Since this petering out invariably comes during periods of increased light intensity, the virucidal effects of UV light may be curtailing spread. The Sydney strain of influenza A/H3N2 appeared in March 1997 in the United Kingdom and although it had a sharp impact in close communities such as the elderly in nursing homes, it did not spread substantially to the population until the following winter.
Influenza viruses presenting in one hemisphere do not get to the other at the same time of the year, even though they appear in east/west differing populations the same distance away. Excess winter mortality is largely linked to the incidence of respiratory infections (not only influenza) as the major cause. However, excess winter mortality levels differ in ways that are not easily understood [7]. Part of the difference must relate to the density of population and level of personal interaction which might favour the spread of respiratory droplet infections and weather conditions may be relevant, in particular the levels of UV exposure. Fifty years ago it was common practice to send persons with tuberculosis to sanatoria in the mountains in Switzerland. Perhaps the benefits they obtained in regard to the management of tuberculosis were chiefly because they were living in an environment of clean air and high UV exposure resulting in a reduced likelihood of secondary respiratory infections.
Further support is presented in the study published by Yusuf and colleagues who investigated the relationship between meteorological conditions and respiratory syncytial virus (RSV) infection [8]. UVB radiation was inversely related to the incidence of RSV in most locations investigated, although they considered the effects were attributable to a combination of meteorological conditions which included UVB radiation. Whilst these authors concentrated on the importance of UVB radiation in the synthesis of vitamin D, their final sentence is particularly significant: ‘UVB irradiation may also be useful in reducing the survival of RSV in closed spaces.’
Declaration of Interest
None.
References
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