Should I have my AstraZeneca booster shot at 8 weeks or 12?
Prime Minister Scott Morrison appeared to have made a captains call yesterday by encouraging people in New South Wales outbreak areas to have their second AstraZeneca shot closer to eight weeks after the initial shot rather than wait for the generally recommended 12 weeks. We would be encouraging the eight-to-12-week second dose be done at the earlier part of that period  That is consistent with medical advice  and given the risks to people from the outbreak in that area we believe it is important they get that second dose of AstraZeneca as soon as possible.  The official health advice from ATAGI, the Australian Technical Advisory Group on Immunisation, remains most people have their booster shot at 12 weeks for optimal COVID protection, but under certain circumstances that can go down to four weeks. Those circumstances include imminent travel or if theres a risk of COVID-19 exposure. ATAGIs concern, and that of some other vaccine experts, is if you have your booster shot earlier than 12 weeks, your body wont develop enough immunity to reliably protect you from serious disease. Here is what we know so far. The evidence underpinning the recommended 12 week gap between the first and booster AstraZeneca shots comes from a study published in The Lancet. The study found leaving less than six weeks between the initial shot and the booster gave 55.1 per cent efficacy (protection from symptomatic disease). Leaving 6-8 weeks between shots increased efficacy to 59.9 per cent, and waiting 9-11 weeks, efficacy was 63.7 per cent. However, if the gap was 12 weeks or longer efficacy jumped to 81.3 per cent. So to get the best protection from the AstraZeneca vaccine, you need at least 12 weeks between your first and booster shots. Now we find ourselves with an active outbreak of the highly transmissible Delta variant of SARS-CoV-2 in Sydney. The Lancet paper didnt include data on the Delta variant as it wasnt widely circulating at the time, but this is fast becoming the dominant variant globally. Yet we do know two doses of the AstraZeneca vaccine protects against serious COVID-19 after infection with the Delta variant, whereas one dose doesnt. Mr Morrisons call for some people to have their AstraZeneca booster from around eight weeks hasnt come completely out of the blue. Its an approach the UK has been using to get ahead of the infectious Delta variant, the same variant circulating in NSW. We know leaving less time between AstraZeneca shots generally reduces vaccine efficacy. This is where things get a bit tricky if we actually want to put a figure on precisely how much vaccine efficacy reduces. A study published in Nature reported a single dose of AstraZeneca vaccine induced essentially no Delta virus-neutralising antibodies. However, two doses induced a neutralising antibody response in 95 per cent of people, albeit at a significantly lower level than with the Alpha variant (which originated in the UK). Still, neutralising antibodies against Delta were there in the vast majority of people after two shots, antibodies that could mean the difference between a mild illness and hospitalisation with severe disease. There are some limitations with this study. First, it did not directly assess vaccine efficacy (you need to conduct a clinical trial for that). Second, it used a range of intervals between first and booster shots, so we cannot definitively say the precise protection from the Delta strain at eight weeks versus 12 weeks. However, assessing the capacity of vaccinated peoples antibodies to neutralise viruses in the lab is a good indicator of the quality of vaccine-induced protection  and this study really highlighted the need for a booster shot for protection against the Delta variant. So with infection numbers in Sydney looking more ominous by the day, coupled with the knowledge one vaccine dose is all but useless against the Delta virus, it is clear getting two doses into the arms of as many people as possible as quickly as possible is the strategy. Two doses, even at eight weeks apart, while not providing the highest possible level of protection, will still protect many from severe disease. A drop in immunity is not the only thing to consider when weighing up the pros and cons of having your AstraZeneca booster shot early. Weve just heard more Pfizer shots are on their way, sooner than expected. If second shots of Pfizer are made available to people who have already had two shots of AstraZeneca (and this is a big if), this could be a game-changer. In this case  and remember this mix-and-match approach has not been officially sanctioned  it might not matter too much if an early booster dose of the AstraZeneca vaccine gives you sub-optimal immunity. The Pfizer booster would lift your immunity instead. However, it remains to be seen whether such a major policy shift would happen in time to protect people currently in lockdown in NSW. Take-home message  The Delta variant is highly transmissible. So weeks matter, and with Australia still heavily reliant on the AstraZeneca vaccine, for now it does make sense to reduce the time between the first and second jab. This is clearly preferable to remaining unprotected for an extra month, particularly if you are at higher risk of infection and/or severe disease. Nathan Bartlett, Associate Professor, School of Biomedical Sciences and Pharmacy, University of Newcastle  This article is republished from The Conversation under a Creative Commons licence. Read the original article.