How might India's monsoon season impact COVID-19 ?

With the onset of the COVID-19 pandemic, India has suffered enormous ill-health, with over 6.14 million cases and almost 100,000 deaths, besides unparalleled economic disruption. A new study, by two Goethe University Frankfurt researchers, published on the preprint server medRxiv* in September 2020 explores the relationship between the monsoon season, the chances of exposure to ultraviolet B (UVB) radiation, and the COVID-19 mortality.

Vitamin D, UVB and the Monsoon

Recent observations have demonstrated the possibility that vitamin D levels are linked to COVID-19 incidence and severity, and the death rate. If so, UVB light plays a vital part in the biosynthesis of vitamin D. In India, the southwest monsoon is the rainy season, and many infectious diseases are known to cause outbreaks at this time, peaking just after the monsoon. This includes influenza, diarrhea, cholera, dengue, typhoid, and respiratory illnesses.

However, it is not clear if this will be the case with COVID-19 as well. The effect of a surge in infectious disease coupled with ongoing COVID-19 is also unknown.

In the current study, the researchers aimed to determine if the monsoon and the immediately succeeding months would be associated with an abrupt rise in contagious disease in India, which would further burden the already inadequate healthcare system.

The researchers considered the hours of sunlight, the density of the cloud cover, and the overall intensity of UVB radiation in the monsoon months. This is set against the lower degree of social mixing and outdoor activity associated with the torrential rain, restricting the transmission of infection. However, in the absence of actual studies, the data is limited.

Possible Impact of the Monsoon on Vitamin D Levels via UVB

The researchers estimated the usual impacts of the monsoon season, such as traffic jams, extra cases stretching healthcare facilities, on the negative side versus the positive side, with restricted mobility and lower viral transmission due to less time spent outdoors. Another important factor is cloud cover and lower UVB exposure, resulting in lower vitamin D levels. While considering these factors, they also adjusted for other factors like humidity and temperature.

This is an important consideration in India with low skin exposure levels, below the 10-15 minutes at solar noon for the type V skin, which is most common in India, and well below the 10-45 minutes in winter. India also has a lower overall intake of non-vegetarian foods that have significant vitamin D levels. Moreover, UVB has been shown to inactivate viruses in fomites and thus reduce transmission.

In earlier studies, both UVB and vitamin D levels were shown to have a beneficial effect on COVID-19 via the modulation of the immune response. Vitamin D also modulates the renin-angiotensin system and the inflammatory response, thus preventing a cytokine storm. It also plays a role in antimicrobial peptide release. Vitamin D in the form of the active metabolite 1,25-dihydroxyvitamin D [1,25 (OH)2D] is involved in all these roles. Its deficiency is thus possibly a risk factor for higher COVID-19 incidence, severe disease, and mortality.

Covid-19 Deaths Drop During Monsoon

The researchers say, “We find substantial and significant associations of monsoon season, UVI, and their interaction with COVID-19 deaths.” They found that the daily growth rate of COVID-19 deaths drops by ~13 percentage points in association with the monsoon season. However, on the other hand, the UVI in this period declines by 1.3 percentage points, which reduces the protection due to this factor, since each unit of increase in UVI is associated with a daily death rate drop of 2 percentage points.

Relative to the daily growth rates of COVID-19 deaths in India, the growth rate fell by ~60% within two weeks of the beginning of the monsoon. Each permanent unit of increase in UVI is linked to a drop in growth rates of ~40% compared to the average daily growth rate. The monsoon-linked drop in UVI protection was ~16% over 2 weeks. The corresponding drop was 65% over 8 weeks.

There was no significant change in the estimates when a longer or shorter period of evaluation was adopted, or when the effect of government measures was incorporated concerning the mobility and outdoor behavior patterns of individuals.


The overall conclusion is that both monsoon and UVB have independent negative associations with the daily growth rate of COVID-19 deaths in India. The researchers point out that other mechanisms of UVB protection may be at work, such as nitric oxide or cis-urocanic acid, with their effects on immune and vascular function. Similarly, the effect of UVB on virus content in fomites was not considered.

The current observational study describes the reduction in COVID-19 death growth rates and the protection afforded by UVB in India against COVID-19 deaths. Thus, the protective effect of the monsoon is at the cost of a trade-off with UVB exposure.

While these findings are not sufficient to guide health policies in India, they may prompt further research into the right amount and mode of solar exposure as well as vitamin D levels that can effectively bring down COVID-19 deaths during monsoon season. The low cost and high scalability make such policies and interventions ideal for India, with its vast population of over 1.3 billion, with the majority living under restricted financial conditions.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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