Last Monday, Sydney came out of a lockdown of more than 100 days after reaching the milestone of having 70% of the population over the age of 16 fully vaccinated.
Modeling predicted that New South Wales would “open up” with around 1,900 daily cases when this goal was reached.
However, the state only registered 496 new local cases that day. And the current seven-day average for NSW is 488 cases, with the number rising downwards.
In addition, other modeling suggested that COVID-19 admissions would peak between 2,200 and 4,000 in Sydney by the end of September.
On September 21, the highest COVID hospital occupancy for the entire NSW was 1,268 patients. There are currently 711 COVID patients admitted to the hospital in NSW as of October 14th.
We suggest that there are two main factors that can address these discrepancies.
Vaccine efficacy underestimated
First, vaccine predictions have typically used estimates of efficacy against the Delta variant based on the UK Scientific Advisory Group for Emergencies (SAGE) roadmap published in June. This indicated a hospitalization efficiency of 87% for Pfizer and 86% for AstraZeneca.
Recent data in many countries, however, have shown efficacy against severe infection and hospitalization is somewhat greater. Another UK study suggested 95% hospitalization protection for both Pfizer and AstraZeneca. And a study from the Netherlands found 96% and 94% protection against hospitalization for Pfizer and AstraZeneca, respectively.
This difference may be the reason for the difference between the actual NSW hospital admissions and the predicted ones based on the actual rollout of the vaccine.
Real time protection
The second reason for the current NSW situation may be a term we have termed “real-time protection”.
The rapid pace of vaccine uptake during NSW’s Delta wave ensured that there was a large proportion of recent vaccines in the population.
This may offset the effect of decreased vaccine immunity.
Optimal immunity after vaccination occurs approximately two weeks after I received the second dose. However, a partial protective effect of vaccination with Pfizer was evident in clinical trials as early as 12 days after the first dose.
In addition, protection against severe infection may require only a lower level of immune response after vaccination.
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How has it gone abroad?
The protection in real-time concept can be used to explain the impact of vaccination in other countries, which can provide a “real” perspective on the future of the pandemic in Australia.
Denmark reached 25% vaccination of the total population before the arrival of the Delta variant. During the Delta wave, there were reduced hospitalizations and deaths compared to previous waves and a dissociation between the number of cases and deaths.
NSW’s achievement of reaching the 70% threshold last week actually equates to around 56% of the total population of NSW. At the peak of its Delta wave in July, Denmark reached 50% vaccination coverage of the entire population.
The restrictions at this time in Denmark required proof of vaccination, previous infection or a recent negative COVID test to get into certain indoor settings, such as restaurants and cinemas.
With a population size similar to Greater Sydney, the coming months in Denmark can serve as an important comparison of how the pandemic can unfold in Australia.
Similarly, in Singapore, vaccination rates are high, around 80% of the total population, and the rate of spread of the vaccine is very similar to Denmark.
Singapore has seen a recent increase in cases since the easing of restrictions, with case numbers at their highest. However, 98% of these cases are mild or asymptomatic. This suggests that vaccines have a major impact on reducing the severity of COVID, but a less pronounced ability to completely discontinue disease transmission.
Another example of the impact of real-time protection is the situation in Israel. Israel is often used as a benchmark for vaccine effectiveness. Its vaccine program involved a rapid rollout of mRNA vaccines, predominantly Pfizers. Preliminary studies in the country showed that the vaccine had high efficacy against symptomatic COVID-19 and hospitalization.
However, Delta arrived in Israel for a large increase in COVID-19 cases with associated increases in hospitalizations and deaths.
While this may provide some insight into the impact of Delta in Australia, there are important differences.
Why did hospitalizations occur in Israel? And what are the lessons for Australia?
Israel saw a large portion of the eligible population quickly vaccinated. About 50% of the total population was fully vaccinated by mid-March. But after this, there was a marked slowdown in recording.
Thus, a combination of declining immunity and a large unvaccinated population may have exposed Israel to the Delta.
While the Pfizer vaccine shows excellent efficacy against severe COVID-19, recent evidence from Israel suggests some declining protection against serious disease over time, leading to the introduction of the country’s booster program in July. A third dose was originally offered for over 60s before being extended to anyone aged 12 and over.
In Australia, the widespread rollout of booster shots in the near future would be premature. The priority now is to get all eligible people fully vaccinated and consider boosters for target groups.
The federal government announced last week that booster shots would be available to Australians who are “highly immunocompromised” from this week.
Governments should also consider a “mix and match” approach to booster shots. This strategy is being pursued in the UK, based on evidence that combining different vaccines can lead to stronger immunity.