An interesting study by Johansson and others published in the Journal of American Medical Association January 7, 2021 explored this question.
This study assessed the relative amount of transmission from presymptomatic, never symptomatic and symptomatic individuals across a range of scenarios in which the proportion transmission from people will never develop symptoms
i.e. remain asymptomatic and the infectious period varied according to published best estimates.
For all estimates, data from a meta-analysis was used to set the incubation period at a median of five days.
The infectious period duration was maintained at 10 days, and peak infectiousness was varied between three and seven days.
The overall proportion of SARS-CoV-2 was varied between 0 and 70% to assess a wide range of possible outcomes.
Results showed that the baseline assumptions for the model were that peak infectiousness occurred at the median of symptom onset and that 30% of individuals with infection never develop symptoms and are 75% as infectious as those who do develop symptoms.
Combined, these baseline assumptions imply that persons with infection who never develop symptoms may account for approximately 24% of all transmission.
In this base case 59% of all transmission came from asymptomatic transmission, comprising 35% from presymptomatic individuals and 24% individuals who never developed symptoms.
Under a broad range of values for each of these assumptions, at least 50% of new SARS-CoV-2 infections were estimated to have originated from exposure to individuals with infection but without symptoms.
Concluding, the authors suggest that transmission from asymptomatic individuals is estimated to account for more than half of all transmissions, but in addition to the identification and isolation of persons with symptomatic COVID-19, effective control of spread requires reducing the risk of transmission from people with whom there are no symptoms and do not know they are infected.
These findings suggest that small measures such as wearing masks, hand hygiene, social distancing and strategic testing of people who are not ill will be fundamental to slowing the spread of COVID-19 until safe and effective vaccines are available and widely used.
The London General Practice commends all measures which help prevent the spread of this disastrous disease.
Dr Paul Ettlinger
BM, DRCOG, MRCGP, FRIPH, DOccMed