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Study estimates impact of physical distancing on reducing spread of COVID-19

Study estimates impact of physical distancing on reducing spread of COVID-19
April 22, 2020

While less effective than the combined approach, quarantine plus workplace measures presented the next best option for reducing SARS-CoV-2 cases, followed by quarantine plus school closure, and then quarantine only. All intervention scenarios were more effective at reducing cases than no intervention.

The study, published in The Lancet Infectious Diseases journal, is the first of its kind to investigate using these options for early intervention in Singapore using simulation. Despite heightened surveillance and isolation of individuals suspected to have COVID-19 and confirmed cases, the risk is ongoing, with the number of cases continuing to increase in Singapore. Schools have not been closed, and workplace distancing is recommended, but it is not national policy [correct as of 23.03.2020].

The study found that the combined approach could prevent a national outbreak at relatively low levels of infectivity (basic reproductivity value (R0) = 1.5), but at higher infectivity scenarios (R0 = 2.0 (considered moderate and likely) and R0 = 2.5 (considered high)), outbreak prevention becomes considerably more challenging because although effective at reducing infections, transmission events still occur.

Dr Alex R Cook, National University of Singapore, said: “Should local containment measures, such as preventing disease spread through contact tracing efforts and, more recently, not permitting short-term visitors, be unsuccessful, the results of this study provide policy makers in Singapore and other countries with evidence to begin the implementation of enhanced outbreak control measures that could mitigate or reduce local transmission rates if deployed effectively and in a timely manner.”

To assess the potential impact of interventions on outbreak size, should local containment fail, authors developed an individual-based influenza epidemic simulation model, which accounted for demography, individual movement, and social contact rates in workplaces, schools, and homes, to estimate the likelihood of human-to-human transmission of SARS-CoV-2. Model parameters included how infectious an individual is over time, the proportion of the population assumed to be asymptomatic (7.5%), the cumulative distribution function for the mean incubation period (with the virus that causes SARS and the virus that causes COVID-19having the same mean incubation period of 5.3 days), and the duration of hospital stay after symptom onset (3.5 days).

Using this model, authors estimated the cumulative number of SARS-CoV-2 infections at 80 days, after detection of 100 cases of community transmission. Three values for the basic reproduction number (R0) were chosen for the infectiousness parameter, including relatively low (R0=1.5), moderate and likely (R0=2.0), and high transmissibility (R0=2.5). The basic reproduction numbers were selected based on analyses of data from people with COVID-19 in Wuhan, China.

In addition to a baseline scenario, which included no interventions, four intervention scenarios were proposed for implementation after failure of local containment: 1) isolation of infected individuals and quarantine of their family members (quarantine); 2) quarantine plus immediate school closure for 2 weeks; 3) quarantine plus immediate workplace distancing, in which 50% of the workforce is encouraged to work from home for 2 weeks; 4) a combination of quarantine, immediate school closure, and workplace distancing. These interventions follow some policy options currently being undertaken (quarantine and some workforce distancing) by the Singaporean Ministry of Health, as standard interventions for respiratory virus control.

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