Social distancing strategies for curbing the COVID-19 epidemic

Stephen M Kissler; Christine Tedijanto; Marc Lipsitch; Yonatan Grad

The intensity (peak size) of outbreaks depends strongly on the degree of seasonal forcing of transmission (4) . The transmission of many respiratory pathogens, including the human coronaviruses that cause mild common cold-like syndromes, is seasonal in temperate regions, peaking in the winter months (4) . This variation in transmission strength may be driven by a variety of factors, including increased indoor crowding in the winter, the onset of the school term in the autumn, and climate factors (5) . If SARS-Cov-2 transmission is similarly subject to seasonal forcing, summer outbreaks would naturally have lower peaks than winter outbreaks. This aligns with observations from influenza pandemics, where relatively small spring and summer outbreaks are frequently followed by larger autumn/winter outbreaks (6) . Due to seasonal variation in transmission strength, it may be more difficult to flatten epidemic curves in the winter than in the summer. Moreover, a winter peak for COVID-19 will coincide with peak influenza (5) , further straining health care systems.