Authors' response.

We thank the authors for their interest and comments with regard to the use of chloroquine nasal drops in mild COVID-191. Therapeutic applications for COVID-19 has been a rapidly evolving area and it is quite true that the initial promise of chloroquine and hydroxychloroquine (CQN and HCQ) has not stood the test of evolving evidence over time. This particular study was conducted in April-May 2020 when Covid was just emerging in the world, and many of the insights of today were not available. It also is in tandem with other evidence accumulated over time to indicate that CQN delivered intranasally, is similar to systemically administered CQN and HCQ,2 in not being demonstrated as effective in patients with established COVID-19 infection.

The statement of conveniently understating adverse events is unwarranted. Of the 30 patients, seven had adverse events which were no worse than mere irritants (local nasal irritation from topical nasal drops instillation), and which did not deter them from continuing the treatment. One other patient was non-compliant to the treatment. These facts have been stated as observed.

The observations on toxicity and mortality of systemic HCQ as per the quoted systemic reviews,23 do not apply to nasal CQN.

The post-hoc analysis on Ct values as pre-exposure prophylaxis in non-infected patients receiving nasal CQN/ placebo was not a planned observation and the numbers are therefore not suitably powered for this observation. It was however, of significant promise at the time of the observations when no appropriate treatment or vaccines were available. The observation is only in the context of prophylactic treatment before infection, and can certainly not be of any greater relevance than as a hypothesis generating observation which needs further testing. In current times, the availability of effective and safe vaccines would seem to eclipse and override any potential role of nasal CQN for prophylaxis. This is the nature of parallel and competing advances in science.