https://www.bloomberg.com/news/articles/2020-04-24/why-a-second-wave-of-covid-19-is-already-a-worry-quicktake
3. So how does a virus come back?
There are a number of possibilities. In the case of influenza, there’s the onset of cool weather, a factor that may affect
the coronavirus, too. Or the pathogen can mutate. This is another
feature of flu, which mutates more or less constantly. In the fall of
1918, a second wave of the historic influenza outbreak occurred and
caused most of the deaths in the pandemic. Some researchers believe it was brought about by a mutation
that made the virus again unrecognizable to most people’s immune
systems. Another important variable is the movement of the virus to
populations that haven’t been exposed before and don’t have immunity. In
addition, the World Health Organization said on April 24 that there’s
no evidence yet that people who have recovered from Covid-19 and have
antibodies are protected from a second infection.