Background:The first wave of COVID-19 in India began to decline suddenly in September 2020 and appeared to be nearly over by the end of January 2021. At the time, no models or papers predicted or explained this decline. The authors hypothesized in their previous study that the cases decreased due to increased Relative Humidity during Monsoon and forecasted that another wave would begin with the dry season in February 2021 and would be contained by monsoon humidity. The current study was carried out to put the seasonality hypothesis to the test in 2021-22. The study also included findings about the effectiveness of policy control measures on case decline.Methods: Humidity cycles in India were studied to determine the most humid periods, which corresponded to changes in daily cases across the country, on a zone-by-zone basis, and in smaller regions. The enforcement date and subsequent case decline (if any) were observed for the effectiveness of policy control measures. Results:In low humidity periods, there was a clear relationship between relative humidity and case decline and case increase. Policy controls have been found to be effective in reducing and halting case increase, resulting in a subsequent decline. Conclusion:In India, COVID-19 increases during the dry season around February and decreases during the monsoon season. Policy controls (lockdowns) are an effective way to halt the virus's exponential spread. The findings may be useful in planning local control and prevention activities.
Background:The COVID-19 pandemic was expected to affect India severely; cases rose exponentially from May-June 2020, but around mid-September reached their peak and started declining. It showed a sign of the wave's completion by the end of January 2021. This decline was not predicted by any models and the authors have not come across any explanation. Winter seasonality of influenza and similar viruses is well known and observed fact and that it has a direct correlation to the colder temperatures as well as lower humidity. Similarly, in low humidity, viruses are most viable, and they become ineffective as the humidity increases and reaches its maximum extent. This article hypothesizes and tries to explain the cause behind the first major decline and shows the subsequent rise of the second wave, and one short low humidity period followed by a high humidity period between the first and second waves. Methods:The humidity cycles in India were studied to find high and low relative humidity periods, which then corresponded to the daily cases in the country (macro-level), region (mid-level), and smaller regions (micro-level).Results: A definite correlation was observed between Monsoon-induced humidity and the incidence rate decline. This happens in 8 to 10 weeks. Incidence rates start declining about 4 weeks after the peak humidity is reached in a particular region. A decrease in humidity below 65% or 55% or lower causes an increase in the case increase/uptrend in about 3-4 weeks. Conclusion:COVID-19 has a seasonal peak in India, peaking in the middle of the monsoon season around mid-September and reaching its lowest levels in January-February. As humidity drops from February to June/July, a trend reversal and sharp rise are expected. The subsequent wave/case peak would be expected to be seen around mid-September 2021.
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