Presymptomatic COVID-19 transmission a major driver of outbreaks

4 minute read

Silent transmission from asymptomatic and presymptomatic individuals could be the major driver of COVID-19 outbreaks, research suggests.

Welcome to Tuesday’s The Medical Republic‘s COVID Catch-Up.

From now on, we’re condensing the day’s COVID-19 news into one convenient post. Got any tips, comments or feedback? Email me at

6 July

  • A modelling study has suggested that silent transmission from asymptomatic and presymptomatic individuals could be the major driver of COVID-19 outbreaks. Writing in PNAS, researchers noted that previous research suggests COVID-19 could be most infectious during the presymptomatic phase, and that asymptomatic infections account for around 18%-31% of all infections.
    They estimated that presymptomatic individuals could account for nearly half of transmission, and asymptomatic individuals for up to nearly 7% of transmission. “Consequently, even immediate isolation of all symptomatic cases is insufficient to achieve control,” they wrote.
  • Pregnant women with higher BMIs and lower lymphocyte counts who contract COVID are more likely to be hospitalised in intensive care, a retrospective cohort study has found.
    The study, published in Emerging Infectious Diseases, reviewed medical records of 100 pregnant women diagnosed with COVID-19, 52 of whom were hospitalised and 10 were treated in intensive care. Women with high BMI (median of 30) were significantly more likely to end up in intensive care than those with lower BMIs, as were those who had lower lymphocyte counts at diagnosis. Only one neonate tested positive for SARS-CoV-2.
  • Looking for COVID-19 information in other languages? Both the Federal Department of Health and Department of Home Affairs have multilingual COVID-19 resources on their website, including posters for clinical practices, and SBS has a range of information and articles in languages other than English.
  • The Cardiac Society of Australia and New Zealand has issued a position statement on how to evaluate and manage suspected acute coronary syndromes during the COVID-19 pandemic, when health services may be experiencing unprecedented demand.
    The guidance covers risk stratification, and strategies for low and intermediate risk patients, such as accelerated discharge for low risk individuals.
  • Obesity may be a risk factor for respiratory failure in COVID-19, but being underweight is associated with the highest mortality, new data suggests. A retrospective study, published in the Annals of Internal Medicine, of 1687 adults hospitalized with COVID-19 in New York City found the risk of dying from COVID-19 was actually highest in underweight individuals, lowest in overweight, but then increased with increasing obesity; making a so-called ‘J-shaped curve’ of association. However when they looked at the risk of respiratory failure, there was a more linear relationship with increasing BMI , with the lowest risk seen in normal weight individuals.
  • COVID-19 has claimed another two lives, both in Victoria; one man in his 60s and one in his 90s, as Dan Andrews has just announced metropolitan Melbourne and Mitchell Shire are locked down for six weeks from midnight tomorrow night. The announcement returns those two regions to Stage 3 restrictions, meaning stay-at-home orders are once again in place.
    The state also recorded the highest daily number of new infections since the pandemic began, with 124 new cases reported on Monday. Five of the 31 people in hospital are in intensive care. New cases have been reported in two abattoirs, as well as Alfred Hospital, Royal Melbourne Hospital and Camberwell Grammar School.
    A list of pop-up and drive-through testing sites can be found here, and this site also includes information on wait time at various locations.
    In New South Wales, one of seven new cases was in a returned traveller who was released from quarantine and had tested negative during quarantine, but developed symptoms when he returned home to Newcastle.
    The AMA has called for states to pause the lifting of COVID-19 restrictions until the outbreaks in Victoria are brought under control, with AMA President Dr Tony Bartone warning that these outbreaks are a reminder that “the battle against COVID is far from over”.
    Here are the latest confirmed COVID-19 infection figures around Australia, to 9pm Monday:
    National – 8586, with 106 deaths and 7420 recovered
    ACT – 108
    NSW – 3429
    NT – 30
    QLD – 1067
    SA – 443
    TAS – 228
    VIC – 2660
    WA – 621


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