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Modelling the nation’s COVID-19 response

Successful contact tracing systems for COVID-19 rely on effective quarantine and isolation

Test, trace, and isolate is a strategy used widely in the fight against COVID-19. On its own, contact tracing is unlikely to contain a COVID-19 outbreak, but it may reduce transmission enough to allow population-wide social distancing measures, including lockdowns, to be relaxed. Our researchers have applied their wildlife disease modelling knowledge to this aspect of the COVID crisis.

Recognising the need for robust ways to measure the effectiveness of contact tracing in reducing the spread of COVID-19, a team of researchers, including Manaaki Whenua’s Dr Rachelle Binny and Dr Audrey Lustig, developed a model to investigate the importance of contact tracing, quarantine, and isolation in reducing transmission.

“We used an age-structured branching process model for COVID-19 transmission in Aotearoa New Zealand, in the presence of contact tracing and case isolation,” says mathematical modeller Dr Binny.

“Our results show that a high-quality, rapid contact tracing system, combined with strong support for people in quarantine or isolation, can be highly effective in reducing the spread of COVID-19. In the best cases we found that contact tracing can reduce the effective reproduction number – the average number of people who are infected by a single infected person – by up to 60%.  However, this reduction in transmission relies heavily on having effective quarantine and isolation for cases and traced contacts,” she explains.

“If case isolation or quarantine are imperfect, or some contacts aren’t traced or are traced more slowly, then the reduction is only around 40%, meaning that stronger social distancing measures would be needed to control an outbreak.”

Predicting the elimination of evolving COVID-19 variants

A second research paper involving Dr Binny, ‘Predicting elimination of evolving virus variants’, models the recent emergence of multiple SARS-CoV-2 variants and the risk these pose to global efforts to control the COVID-19 pandemic. In the study, researchers created a simple model of disease spread, which includes the evolution of new variants and varying vaccine effectiveness for these new strains.

They found that viruses that mutate into multiple new variants need fast vaccine delivery in order to be contained. The researchers concluded that rapid vaccine updates to target new strains are more effective than slow updates, and that containing spread through non-pharmaceutical interventions is vital while these vaccines are delivered.

The study also suggested that a continuous vaccination roll-out programme, where updated vaccines are given to unvaccinated individuals, rather than revaccinating high-priority individuals, may slightly increase the probability of elimination. However, the researchers note that this prediction warrants further investigation using population-structured models to assess the risk this would pose to vulnerable individuals such as frontline workers or those at higher risk of severe disease.

“When enough people in a population are vaccinated, new variants are less likely to arise and outbreaks of existing variants can be more easily controlled with contact tracing and lower alert levels,” explains Dr Binny. “If new variants emerge that are resistant to current vaccines, then our model suggests that the faster the vaccines can be updated to target these new variants, the better our chances of elimination. Yet, even when New Zealand completes its vaccine roll-out, there will still be a risk of new vaccine-resistant variants emerging in outbreaks abroad. This could undo a lot of our hard work, so it’s also important that New Zealand work with other countries to achieve high vaccination coverage globally.”

Ongoing COVID-19 response research highlights

  • Our modellers contributed to two technical reports describing the mathematical modelling that was used in the days following the detection of the Delta Variant in August to provide situational awareness and inform the Government’s high-level outbreak response.
  • A new paper ‘Early intervention is the key to success in COVID-19 control,’ led by Dr Binny, modelling the importance of the timing of interventions for containing New Zealand’s March/April 2020 outbreak, concludes that the early introduction of stay-at-home orders was crucial in reducing the number of cases and deaths and enabling elimination throughout New Zealand.
  • A modelling paper involving Manaaki Whenua modellers concludes that New Zealand may no longer need strict nationwide lockdowns to control COVID-19 if more than 90% of the eligible population are fully vaccinated. Even while vaccine levels are below this high threshold, vaccination still significantly reduces the numbers of infections, hospitalisations, and deaths during an outbreak, and greatly improves the chances of eliminating an outbreak quickly, before it can grow very large.

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