select search filters
roundups & rapid reactions
before the headlines
Fiona fox's blog

expert reaction to a study looking at the timing of SARS-CoV-2 transmission

Research, published in Nature Medicine, report on the timing of SARS-CoV-2 transmission.


Prof William Keevil, Professor of Environmental Healthcare, University of Southampton, said:

“This new study in Nature Medicine adds to the growing evidence around the world of there being significant numbers of asymptomatic, presymptomatic and also low level infection patients showing minor symptoms that are difficult to recognise, trace and isolate without rapid diagnostic virus tests such as RT-PCR being available. This testing has been proving difficult to scale up in the UK to sufficient numbers to get a true impression of carriage and spread in the population. 

“The presence of significant numbers of infected people without showing symptoms has important implications for public health, building pressure on governments that everyone should wear masks. If governments pursue this approach then it is essential that everyone wear appropriate high quality, close fitting Class 2 or 3 surgical masks or FFP2 (N95) an FFP3 (N99) respirators rather than the many flimsy masks which do little to prevent virus transmission other than to perhaps shorten the distance of infectious droplet and aerosol travel. Moreover, even this may not be sufficient if the eyes are not protected which is why front line healthcare workers and emergency responders wear goggles and/or full face visors. Key to all of this is the need for adequate supplies of PPE and high quality education and training for the general public in how to fit, wear, safely remove and dispose of masks followed by immediate hand hygiene; easier said than done.”


Prof Babak Javid, Principle Investigator, Tsinghua University School of Medicine, Beijing and Consultant in infectious diseases at Cambridge University Hospitals, said:

“This new study in Nature Medicine by Leung and colleagues confirms that for the vast majority of patients, most virus shedding of SARS-CoV2 occurs at, or before symptoms arise.  Using these data and modelling approaches, they estimate that just under half of transmissions from cases where the most likely source of transmission was known, occurred before the source patient showed symptoms.  These data have important implications for public health control measures such as whether masks should be worn by well people.

“This study confirms, in a larger cohort, data presented in the study by Woelfel et al in Nature ( that maximal shedding of SARS-CoV2 appears to be before symptom onset.

“This is important because current public health control measures advised e.g. by the WHO and UK government assume that maximum contagion is after symptom onset.  Hence one reason masks are not advocated for wearing by asymptomatic members of the public. 

“Pre/Asymptomatic transmission has been suggested to account for 40-80% of Covid cases (discussed in our BMJ Editorial:

“Given that several studies have now confirmed these findings, I think the scientific consensus would be that these are likely to be real and robust findings with important public health control measures.”


‘Temporal dynamics in viral shedding and transmissibility of COVID-19’ by Xi He et al was published in Nature Medicine on Wednesday 15th April. 

All our previous output on this subject can be seen at this weblink:


Declared Interests 

None received 

in this section

filter RoundUps by year

search by tag