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Covid-19: four fifths of cases are asymptomatic, China figures indicate

BMJ 2020; 369 doi: https://doi.org/10.1136/bmj.m1375 (Published 02 April 2020) Cite this as: BMJ 2020;369:m1375

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Re: Covid-19: four fifths of cases are asymptomatic, China figures indicate

Dear Editor

Coronavirus has shown remarkable variability in its effects, ranging all the way from asymptomatic carriers to viral pneumonitis and death.

Its most manifest and serious effects are local and these are at the alveolar level. There are also reports of anosmia and loss of taste https://www.entuk.org/sites/default/files/files/Loss%20of%20sense%20of%2.... I know of a case of unilateral anosmia, demonstrating the local effect of the virus. The case of Ria Lakhani who reports that she needs ‘to remember to breathe’ https://www.bbc.co.uk/news/uk-52204444 suggests a feedback mechanism from damaged alveolar or lower respiratory tract receptors to the respiratory centre, similar to damaged tongue taste buds which can take several weeks to recover.

The three routes of infection are reckoned to be via the mouth, nose and eye mucosa.

Asymptomatic infection
I have not heard of any significant local symptoms associated with infection via the eye mucosa and this route of infection may account for the large proportion of asymptomatic infections. It may also be the route through which the herd immunity that is surely building up in the community has developed. Local eye inoculation may have provoked an immune response before there was significant involvement of the respiratory mucosa.

These different routes of infection may suggest that mouth breathing is more likely to lead to viral pneumonitis than nasal breathing, and that face masks, scarves and bandanas in the community may direct any viral particles towards the eyes rather than the nose and mouth.

Research
Researchers could investigate whether the severe respiratory cases are predominantly mouth-breathers and whether eye inoculation has been the route for asymptomatic infection.

Competing interests: No competing interests

08 April 2020
Donald M. Boyd
Retired medical doctor
Inverness, U.K.