8 Sept 2020

Lessons from China: Predicting second - and subsequent - waves, and when restrictions can ease

Research from Monash University on the early stages of the COVID-19 pandemic in China gives important clues into predicting the size and potential causes of second, and subsequent waves and also when restrictions can be lifted once infection rates start to decline.

The data reveal that the first 30 cases may mark a critical threshold for the transition from a slow to a fast-growing phase of the COVID-19 epidemic. Importantly, after a strict city-wide quarantine measure is applied, maintaining a high uptake of mask-wearing and reduced social contacts are essential to enable the epidemic to follow a smooth decline to elimination. However, if mask usage is 50% or less, there are likely to be more outbreaks.

The data are particularly relevant as Victoria faces a potential extension of Stage 4 restrictions if the current infection rate stays in the double and sometimes triple figures.

The reports have just been published in International Journal of Infectious Diseases, the Innovation and PLOS Medicine, led by Lei Zhang, who is an adjunct Associate Professor from Melbourne Sexual Health Centre in Monash University’s Central Clinical School and School of Public Health and Preventive Medicine, and a Professor from China-Australia Joint Research Center for Infectious Diseases in Xi’an Jiaotong University. These studies were a collaborative effort of the universities.

The first report looked at the early characteristics of the COVID-19 epidemic – with a view to finding indicators that could inform public health officials of the urgency and size of control measures that need to be put in place.

The researchers collected publicly available data from 436 Chinese cities that reported on cases of COVID-19 from 16 January to 15 March (when the pandemic in China had largely been resolved). Of these,  45 cities reported more than 100 confirmed cases over the two months and it was these cities that became the focus of the study. Apart from the pandemic epicentre in Wuhan, there were 15 neighbouring cities in Hubei and 29 cities in other Chinese provinces.

The report found that as few as 30 cases “could be a critical threshold for switching from a relatively slow growing phase to a fast-growing phase,” the authors said.

Importantly, the average time for the number of confirmed cases to increase from 30 to 100 was 6.6 (5.3–7.9) days, and the average case-fatality rate in the first 100 confirmed cases was 0.8% (0.2–1.4%). According to A/Prof Zhang, the subsequent epidemic size per million population was significantly associated with both of these indicators. “We predicted a ranking of epidemic size in the cities based on these two indicators and found it highly correlated with the actual classification of epidemic size,” he said.

“The first 30 cases appear to be an essential indicator for the initiation of a fast-growing phase of COVID-19. It is possible that the detection of 30 cases may represent a time when the epidemic shifts from one associated primarily with imported cases to one primarily driven by local transmission.
"Once it reaches this critical mass, local transmissions start to dominate, and a large number of domestic transmissions begin to surface.”

The second report looked at Hubei province, home to 13 cities including Wuhan, the epicentre of the pandemic, between 23 January and 8 April when there was a metro-wide quarantine in place that was stricter than the Stage 4 restrictions in Melbourne.

The researchers found that – once quarantine was in place – infections continued to grow via household transmission. “Household contacts are often without preventive measures and prone to super-spreading,” the authors said, while acknowledging that prevention of household transmission is difficult especially when an infected person is in the latent, asymptomatic period. However, transmission can be reduced by regular household cleaning with disinfectant.

The third report evaluated the reopening of Wuhan city and identified the key criteria to prevent a second major outbreak. The study indicated that the prevention of a second epidemic is viable after the metropolitan-wide quarantine is lifted, but requires sustaining high facial mask usage and a low public contact rate.  Low facial mask usage (<50%) combined with a ‘back-to-normal’ public contact (100% pre-quarantine level) would always lead to a significant second outbreak in most reopening scenarios.

Media: 08 Sep 2020 Herald Sun, " "We're asking for the 'impossible'" by Alanah Frost and Hannah Moore

  • Zhang L, Tao Y, Wang J, et al. Early characteristics of the COVID-19 outbreak predict the subsequent epidemic scope. Int J Infect Dis. 2020;97:219-224. doi:10.1016/j.ijid.2020.05.122
  • Zhang L, Tao Y, Shen M, Fairley CK, Guo Y. Can self-imposed prevention measures mitigate the COVID-19 epidemic?. PLoS Med. 2020;17(7):e1003240. Published 2020 Jul 21. doi:10.1371/journal.pmed.1003240
  • Zhang L, Tao Y, Wang J, et al. Early characteristics of the COVID-19 outbreak predict the subsequent epidemic scope. Int J Infect Dis. 2020;97:219-224. doi:10.1016/j.ijid.2020.05.122

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