Five reasons NSW Covid case numbers have stayed low since reopening

When New South Wales exited lockdown in October, the premier, Dominic Perrottet, warned that with extra freedoms would likely come extra cases and hospitalisations.

Modelling predicted up to 1,900 daily cases during the state’s first easing and a second, larger peak around Christmas. The Burnet Institute forecast hospitalisations would peak between 2,286 and 4,016 in Sydney by the end of September.

Instead daily cases have continued to drop after “freedom day” on 11 October when 446 cases were reported and 769 people were being treated for Covid in hospital.

Almost a month later, NSW’s 14-day reference rate is below one, hospitalisations have dropped by more than two-thirds, and just 222 new Covid cases were reported on Tuesday.

Health experts name five key reasons why NSW has managed to keep case numbers so low while reopening its economy to the world.

Gregory Dore, an infectious disease physician and epidemiologist at the Kirby Institute, said the main reason case numbers were much lower than predicted was the effectiveness of the vaccine at containing outbreaks had been underestimated.

Doherty Institute modelling that informed the national plan predicted even high levels of vaccination would not be enough to contain Covid, with 300 to 1,000 cases a day expected with “medium seeding” occurring.

“Models underestimated the impact of the vaccination on transmission, and the rate of the vaccine take-up in those first few months, particularly targeted vaccination spreading to broader geographical areas,” Dore said. “There was a phenomenal uptake.”

A vaccination blitz in hard-hit hotspots in NSW pushed some LGAs in south-west and western Sydney from having the lowest first-dose rate in the city to the fastest vaccination pace nationwide. On 1 August, just 33% of south-west Sydney had received a vaccination dose. Just two months later, the figure was 91.8%.

University of Sydney researchers have coined the term “protection in real-time” to explain how the rapid pace of vaccine uptake may have offset the impact of waning vaccine immunity.

Optimal immunity after vaccination comes around a fortnight after receiving a second dose. But protection against severe disease may only require a lower level of immune response, and clinical trials have found the Pfizer vaccine provides partial protection just two weeks after the first vaccination dose.

With a large proportion of recently vaccinated people circulating in the population, immunity levels were high, as was protection against severe infection.

Dore said the NSW government’s decision to keep a short interval period between Pfizer doses also allowed NSW Health to vaccinate quickly.

“That optimised the impact in preventing infection,” he said. “Within the first few months you get the best bang for your buck and reduce infectiousness if breakthrough cases occur.”

Dore said as well as underestimating the effectiveness of the vaccine, modelling may also have overestimated the severity of the Delta strain.

“I think we overestimated how badly it affects your risk of developing serious disease, and overestimated the duration of stay in hospitals. The spectrum of cases in hospital were less severe than they may have originally envisaged.”

Doherty modelling predicted the Pfizer vaccine was 93% effective at reducing overall transmission of the Delta variant, and AstraZeneca was 86%. Later data from overseas has suggested Pfizer and AstraZeneca are between 94% and 96% effective at reducing hospitalisations.

“Compared to the original strain … we thought Delta would make it much more likely you’d end up in hospital, putting vaccination aside,” Dore said.

“It’s variable data, and it’s not straightforward. But the virulence and severity of the strain was thought to be much greater, and that can have a marked impact … once you shorten the duration of the stay (in hospital) that has a cumulative effect.”

The University of NSW epidemiologist Dr Abrar Chughtai said NSW Health had managed to keep on top of outbreaks by keeping key health measures in place after restrictions eased.

Chughtai said continuing to mandate masks in public places had prevented the spread of the virus. Testing rates, contact tracing and isolation requirements were also maintained despite caps lifting for venues.

“While many models predicted very high case numbers, NSW Health is still doing aggressive testing, tracing and isolating,” he said.

The University of Sydney School of Public Health professor Alexandra Martiniuk cited the state government’s “gradual reopening, with vaccination being a requirement for entry in most indoor venues and social distancing measures” as key to keeping cases low.

She said NSW had also maintained “excellent” testing rates, with low test positivity compared with other countries with similar vaccination levels. About 0.3% of tests were coming back positive in NSW, while tests had continued to exceed 50,000 a day.

Martiniuk said warm spring weather was likely encouraging people to socialise outdoors more than they may in other countries, but contact tracing was also key to the low positive rate.

“Eighty-three per cent of results are available within one day of sample collection, and 93% of positive cases interviewed by NSW Health within 24 hours of notification,” she said.

NSW has one of the highest vaccination rates in the world, with first doses exceeding 93% and second doses passing 90% on Tuesday.

The “phenomenal” speed at which the nation ramped up the rollout is counter to countries like the US and the UK, which stalled after approaching the 50% mark.

The Doherty Institute’s final report, released this month, said ongoing public health measures including testing, tracing, isolating and quarantine combined with public health and social measures were “critical interventions” to achieve low case numbers.

But the report also acknowledged the pace of the vaccination rollout had “exceeded expectations”, particularly in NSW, the Australian Capital Territory and Victoria which had high community transmission, placing the nation on track to reach 90% targets “much faster” than original simulations predicted.

Martiniuk said the high vaccination rate in NSW, coupled with the stages at which it had emerged from lockdown, had managed to keep cases under control.

“NSW reopening has occurred fairly, and was done gradually, whereas the countries we often compare to reopened much sooner and more restrictions were taken away all at once, like England in July,” Martiniuk said.

Singapore (where case numbers are rising) has a higher proportion of fully vaccinated population (82%), but the majority of Singapore’s vaccines were given earlier.

“Singapore also had more vaccine hesitancy in their older populations, so greater case severity and hospitalisations.”

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