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.

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.

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%.

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

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.

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.

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.

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

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.