The first wave of COVID-19 in India declined suddenly in September 2020 and seemed to be almost over by end of January 2021. This decline was neither predicted nor explained by any models / papers then. In their previous study, the authors had hypothesized that the cases declined due to increased Relative Humidity during Monsoon and had forecasted that another wave would start with dry season in February 2021 and would be contained by monsoon humidity. The present study was conducted to test the seasonality hypothesis in 2021-22. Additionally, the study also included observations about effectiveness of policy control measures on case decline.
Humidity cycles in India were studied to find the most humid periods and corresponded to the change in daily cases in the country, zone-wise manner and in smaller regions. For effectiveness of policy control measures, the enforcement date and subsequent case decline (if any) were observed.
Definite association was observed between Relative Humidity and Case Decline, and Case Increase in low humidity periods. Policy controls have been found to be effective in reducing and halting case increase and causing subsequent decline.
In India COVID-19 shows a seasonality of increase starting the dry period around February and decline due to monsoon season. Policy controls (lockdowns) is an effective measure to arrest exponential spread of the virus. The findings may be helpful to plan control and preventive activities at local level.