It is a well-designed study with a fatal flaw.
The conclusion I can draw is HCQ does not prevent *self-reported* COVID19 from developing if taken 1-4 day post exposure.
The authors have a parallel study that I hope can address this question soon with higher quality data.
a quick look at the problem in the NEJM paper today.
Their use in early (within 24-48hrs of symptom onset) treatment is still perhaps the best approach if their efficacy is proven.
Plus, its impact to HCQ supply is smaller than that of prophylaxis.
1. The low infection rate (11-12%) of unprotected exposure is quite surprising, given the hospital/family clusters we have seen
2. the cohort between Mar17-23 include many people who were not exposed, since +ve PCR test results were not required for exposure