Data from 149 countries suggests the incidence of COVID-19 has decreased by an average of 13% in association with physical distancing interventions, according to a new study published in the medical journal The BMJ.
It looked at five measures: closing schools, workplace or public transport, restricting mass gatherings or limiting movement (lockdown).
All bar two of the countries had implemented at least three of the five by 30 May this year. All five were in place in 118 countries.
The study team – which included researchers from Oxford and Cambridge universities in the UK and Harvard, Boston, Georgia State and Pennsylvania State universities in the US – used interrupted time-series analyses to compare the change in incidence of COVID-19 before and up to 30 days after implementation.
The study was observational, so limited in its ability to establish cause, and the researchers acknowledge limitations, including their inability to assess compliance with distancing rules or to take account of other measures, such as the use of masks or mobile phone apps.
Nevertheless, they say, it was a large study with a robust analytical approach, and the results were similar after testing a range of alternative approaches to analyses.
On average, physical distancing measures were first implemented nine days after the first reported case. However, some countries took longer, including Thailand (58 days), Australia (51), Canada (46), Sri Lanka and the UK (45), Finland and Malaysia (42), and Cambodia, Sweden, and the US (40).
In combination with school and workplace closures, restriction on mass gatherings appear to be a key component associated with a decrease in COVID-19, the study suggests.
Closure of public transport was not associated with any additional reduction when the other four physical distancing measures were in place, likely as a result of fewer people using public transport.
The reduction in incidence was greater in high income countries (higher GDP per capita), those with an older population (higher proportion aged 65 or over), and those with stronger preparedness for the pandemic (country health security index).
As the pandemic continues to evolve, “it will be crucial to repeat and extend this analysis to assess the impacts of interventions in the longer term, as well as to study combinations and sequencing of the lifting of physical distancing restrictions,” the researchers say.