  The existence of an exponential growth phase during early stages of a
pandemic is often taken for granted. However, for the 2019 novel coronavirus
epidemic, the early exponential phase lasted only for about six days, while the
quadratic growth prevailed for forty days until it spread to other countries
and continued, again quadratically, but with a larger coefficient. Here we show
that this rapid phase is followed by a subsequent slow-down where the
coefficient is reduced to almost the original value at the outbreak. This can
be explained by the merging of previously disconnected sites that occurred
after the disease jumped (nonlocally) to a relatively small number of separated
sites. Subsequent variations in the slope with continued growth can
qualitatively be explained as a result of reinfections and changes in their
rate. We demonstrate that the observed behavior can be described by a standard
epidemiological model with spatial extent and reinfections included.
Time-dependent changes in the spatial diffusion coefficient can also model
corresponding variations in the slope.
