  By characterising the time evolution of COVID-19 in term of its "velocity"
(log of the new cases per day) and its rate of variation, or "acceleration", we
show that in many countries there has been a deceleration even before lockdowns
were issued. This feature, possibly due to the increase of social awareness,
can be rationalised by a susceptible-hidden-infected-recovered (SHIR) model
introduced by Barnes, in which a hidden (isolated from the virus) compartment
$H$ is gradually populated by susceptible people, thus reducing the
effectiveness of the virus spreading. By introducing a partial hiding
mechanism, for instance due to the impossibility for a fraction of the
population to enter the hidden state, we obtain a model that, although still
sufficiently simple, faithfully reproduces the different deceleration trends
observed in several major countries.
