Whole world affected with the spread of an epidemic the Corona virus disease 2019 (COVID-19), initially started in Wuhan, China in late December 2019 [17,16,20]. COVID-19 mainly caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)[9]. In first quarter of 2020, COVID-19 spread across the globe exponentially and the World Health Organisation (WHO) declared the pandemic on 11 March 2020 [32]. The first positive case of COVID-19 in India was reported in Kerala on 30 Jan 2020 in a student who had a travel history of flying from Wuhan, China. The positive cases crossed 500 by 23 March 2020 and then India faced a nation wide full lockdown for 21 days. The number of positive crossed 100,000 on 18 May, 2020 i.e. in 111 days from date of incidence. The present trend of the new infected cases shows exponential growing rate and it varies differently over different regions in world. As COVID-19 is a new disease, there is no evidence of the relation of weather parameters on the disease transmission or spread. There is a need of better understanding of the disease-climate parameter interaction by quantifying their relation so that weather parameter enabled modelling effort can be useful in predicting the COVID-19 incidence and the disease spread well in advance.
COVID-19 mainly causes the respiratory illnesses like flu which include influenza A and influenza B but it spreads more faster and easily than flu and also the death rate is higher in this disease. There are several studies related to transmission of seasonal flu ( influenza) virus which largely controlled by the seasonal climatic factors like temperature and relative humidity, population density, city size and its structure, etc.[10,8] . In the temperate regions, the seasonal variation of influenza transmission is controlled by the conditions of absolute humidity, levels of susceptibility and changes in population-mixing and contact rates [25] and past studies have shown seasonal patterns of human coronavirus species and annual variation in species [14]. Study by Chan et al. [7] showed in subtropical area during spring season the SARS coronavirus have better stability at low temperature and humidity. The specific climatic condition like the optimal temperature, humidity and wind velocity will help for the survival and transmission of SARS virus, whereas low humidity, high barometric pressure and daily temperature fluctuations will reduce the transmission [33].
Low absolute humidity during winter increases influenza virus survival and thus increases influenza virus transmission efficiency [27] . Relative humidity and temperature is sensitive to the aerosol route and insensitive to the contact route in the transmission of the influenza [19]. In a temperate region, influenza incidence increase with low temperature and low/high humidity and also with high diurnal temperature range [22]. The efficiency of transmission of influenza virus is affected by the humidity and temperature conditions and the transmission of influenza virus is less dependent on humidity and temperature in the absence of immunity[30].
After the widespread establishment of SARACoV-2 infection, the intensity, and timing of pandemic and post pandemic outbreaks will depend on the time of year. The transmission and the level of cross-immunity that present between beta corona viruses is less dependent on the magnitude of seasonal variation[15]. The seasonality of the SARS-CoV-2 has not been established so far but the climate might play a role in the spread of the disease[28, 29]. Some studies have reported that the COVID-19 spread can be affected by a large number of factors including the climate conditions[24,31]. Several recent studies is being carried out by researchers to find the relation of weather parameters and COVID-19 in different regions, countries and at global scale[1,2,3,4,5,6,21,23,33] but there is lot of divergence in the results as some studies indicated the expansion of COVID-19 is not driven by climate at global scale [6]. A recent study also finds the impact of environmental temperature on COVID-19 exponential growth for US and Italy at regional scale [18]. In few other studies the results shows insignificant correlation of temperature and UV radiation with COVID-19 transmission [23,33]. Similarly another time series study over mainland China explains the relation of temperature and humidity with the transmission of COVID-19 [24].
Our study is mainly focussed on finding the evidence of influence of the weather conditions particularly air temperature, relative humidity and solar radiation flux in the early stage spreading of COVID-19 epidemic in India using the data for the period March to May 2020.