@mvankerkhove @WHO (1/17) A few thoughts to add to the discussion about asymptomatic cases and asymptomatic transmission…
@mvankerkhove @WHO (2/17) Some people will get infected with SARS-CoV-2, and clear the infection after a period of time, and not experience any symptoms throughout the infection. These are called asymptomatic infections.
@mvankerkhove @WHO (3/17) Based on data I have seen on systematic PCR testing of persons exposed to infection, and in quarantine, I believe asymptomatic infections occur in a minority of infections, perhaps around 20%.
@mvankerkhove @WHO (4/17) Serological testing may be able to identify additional asymptomatic infections that PCR has missed, so in future we may come up with an estimate that exceeds 20%.
@mvankerkhove @WHO (5/17) For comparison, in influenza around 15%-20% of infections identified by systematic PCR testing of exposed persons are asymptomatic, but asymptomatic fraction is much higher if using serology to identify infections. ncbi.nlm.nih.gov/pmc/articles/P…
@mvankerkhove @WHO (6/17) Among persons infected with SARS-CoV-2, there is absolutely no doubt that those who have symptoms are more contagious, on average, than those who have no symptoms.
@mvankerkhove @WHO (7/17) Coughing, sneezing, runny nose, etc. are all ways that help virus to get out into the environment and then into other peoples’ respiratory tracts. Coughs and sneezes spread diseases!
nature.com/articles/s4159…
nature.com/articles/s4159…
@mvankerkhove @WHO (8/17) However, we need to distinguish /contagiousness/ from /transmissibility/. If symptomatic cases stay at home, as they are advised to do, they won’t have so much opportunity to spread. Asymptomatic cases could have more opportunities to transmit infection to others.
@mvankerkhove @WHO (9/17) Another issue is /pre-symptomatic/ transmission. We do have good evidence that contagiousness is high around the time symptoms appear. @mvankerkhove spoke about this back in March statnews.com/2020/03/06/wer…
@mvankerkhove @WHO (10/17) In this study, we estimated that 44% of onwards transmission in China occurred from cases before their symptoms appeared (“pre-symptomatic transmission”): nature.com/articles/s4159…
@mvankerkhove @WHO (11/17) This estimate should be interpreted within the context of aggressive case finding and isolation in China, which would reduce post-symptomatic transmission. Pre-symptomatic transmission might play a smaller role in other places where cases do not self-isolate.
@mvankerkhove @WHO (12/17) Going back even further, in this study we reported (in Table 1) that at least half of the first 425 cases in China did not recall any contact with a person with respiratory symptoms nejm.org/doi/full/10.10…
@mvankerkhove @WHO (13/17) Sometimes symptoms may not be recognised or reported. Some “asymptomatic” individuals may actually have a mild fever which they have not noticed, others may not want to reveal that they have lost their sense of smell because then they can’t go to work.
@mvankerkhove @WHO (14/17) Nevertheless, this is still chalked up to asymptomatic/pre-symptomatic transmission, and cannot be stopped by policies that focus on people recognising and reporting symptoms.
@mvankerkhove @WHO (15/17) It is clear that pre-symptomatic transmission can occur, and contact tracing should start a few days before illness onset, rather than on or after illness onset. cdc.gov/mmwr/volumes/6…
@mvankerkhove @WHO (16/17) I think it is less clear whether truly asymptomatic cases play a major role in transmission, and I think it is fair for WHO to say at this stage that asymptomatic cases seem to play a minor role in spreading infection.
@mvankerkhove @WHO (17/17) Physical distancing continues to be an important public health measure, and I think face masks should be worn in public transport and crowded areas where physical distancing is difficult because these can reduce transmission in the community thelancet.com/journals/lanre…
