Test for Covid-19 seasonality and the risk of second waves.

Ten months into the Covid-19 pandemic it remains unclear whether transmission of SARS-CoV-2 is affected by climate factors. Using a dynamic epidemiological model with Covid-19 climate sensitivity in the likely range, we demonstrate why attempts to detect a climate signal in Covid-19 have thus far been inconclusive. Then we formulate a novel methodology based on susceptible-infected time trajectories that can be used to test for seasonal climate sensitivity in observed Covid-19 infection data. We show that if the disease have a substantial seasonal dependence, and herd immunity is not established during the first peak season of the outbreak (or a vaccine does not become available), there is likely to be a seasonality-sensitive second wave of infections about one year after the initial outbreak. In regions where non-pharmaceutical control has contained the disease in the first year of outbreak and thus kept a large portion of the population susceptible, the second wave may be substantially larger in amplitude than the first if control measures are relaxed. This is simply because it develops under the favorable conditions of a full autumn to winter period and from a larger pool of infected individuals.