  Limitations in the design of the experiment of Boulware et al[1] are
considered in Cohen[2]. They are not subject to correction but they are
reported for readers' consideration. However, they made an analysis for the
incidence based on Fisher's hypothesis test for means while they published
detailed time dependent data which were not analyzed, disregarding an important
information. Here we make the analyses with this time dependent data adopting a
simple regression analysis.
  We conclude their randomized, double-blind, placebo-controlled trial presents
statistical evidence, at 99% confidence level, that the treatment of Covid-19
patients with hydroxychloroquine is effective in reducing the appearance of
symptoms if used before or right after exposure to the virus. For 0 to 2 days
after exposure to virus, the estimated relative reduction in symptomatic
outcomes is 72% after 0 days, 48.9% after 1 day and 29.3% after 2 days. For 3
days after exposure, the estimated relative reduction is 15.7% but results are
not statistically conclusive and for 4 or more days after exposure there is no
statistical evidence that hydroxychloroquine is effective in reducing the
appearance of symptoms.
  Our results show that the time elapsed between infection and the beginning of
treatment is crucial for the efficacy of hydroxychloroquine as a treatment to
Covid-19.
