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ACT latest jurisdiction to reduce AZ dosing interval


Matt Woodley


6/09/2021 4:43:11 PM

The COVID outbreak in Canberra means people can get a second dose of AstraZeneca after 4–8 weeks. What does this mean for efficacy?

Tray full of AstraZeneca doses.
AstraZeneca is said to offer high levels of protection regardless of whether the second dosage is given at six, eight or 12 weeks. (Image: AAP)

At the time of publication at 5 pm on 6 September, the Australian Capital Territory had 222 active COVID cases, including 11 that had been registered in the previous 24 hours.
 
Prior to the outbreak, which began when five new cases were registered on 12 August, the ACT had not recorded a new COVID case since April.
 
But with the territory now averaging 18 new cases each day over the past week, Chief Minister Andrew Barr said the ACT’s Chief Health Officer Dr Kerryn Coleman had recommended the new advice, which is in line with guidance issued by the Australian Technical Advisory Group on Immunisation (ATAGI) in July.
 
‘The best vaccine is the one that you can get today,’ Chief Minister Barr said. ‘We want to see more Canberrans fully vaccinated faster to give them protection during this outbreak.’
 
Associate Professor Nathan Bartlett, Head of Viral Immunology and Respiratory Disease at the University of Newcastle, told The Guardian real-world data shows AstraZeneca offers ‘very high levels of protection against hospitalisation and severe disease from Delta’ regardless of whether the second dosage is given at six or eight weeks.
 
‘It’s certainly clear that it doesn’t really matter when you had those two doses in terms of protecting against severe disease, just as long as you’ve had two,’ he said.
 
Professor of Immunology at Victoria University, Vasso Apostolopoulos, also said the dosing interval ‘really doesn’t matter’.
 
‘People … shouldn’t worry that they’re not going to get as good an immune response,’ she said.

‘There’s an urgent need for them to get vaccinated right now, as soon as possible, because we need to control this.’
 
Anyone with a second-dose appointment already booked at the ACT Government’s Calvary Mass Vaccination Clinic will receive a message letting them know they are able to have their second dose sooner, while people have also been told to speak to their GP about bringing their appointment forward.
 
Victoria and New South Wales, the other two parts of Australia attempting to manage significant outbreaks, have already recommended that AstraZeneca dosage intervals be reduced from 12 weeks to between 4–8 weeks.
 
According to ATAGI, a single dose of AstraZeneca reduces the risk of symptomatic infection by around 30% and hospitalisation by 71%, while two doses reduce the risk of symptomatic infection by 67% and the risk of hospitalisation by 92%.
 
And while a 12-week interval has been shown to provide superior efficacy compared to a shortened window, the benefits of full protection sooner are said to outweigh the risks.
 
‘[In] earlier trials … the protective efficacy against symptomatic COVID-19 was 55% when the two doses were given four weeks apart, compared to 81% when given 12 weeks apart,’ ATAGI’s advice reads.
 
‘[But] shortening the interval from 12 weeks to no less than four weeks between doses is acceptable and may be appropriate in certain circumstances, for example, imminent travel or anticipated risk of COVID-19 exposure.’
 
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Dr Belinda Elizabeth White   7/09/2021 6:59:10 AM

I’m lucky I don’t work in/ near an outbreak so it is an obvious decision to wait the usual 12 weeks to gain maximum efficacy. Has there been modelling of if we can control an outbreak if people only get 55% efficacy? I’d like to see the proof that the 4 week window has the same control on Delta spread and severe disease than a 12 week window. It feels like the government is encouraging the shorter window just so they can be proud of a higher number double vaccinated quicker, without thinking about the long term ramifications of severely knocking down efficacy of vaccinations


Dr David Lap Yan Lee   7/09/2021 7:52:21 AM

Our government has now bought so much vaccine and I’m sure we will be getting a booster by winter next year likely with the 2022 fluvax.