Should I get my second AstraZeneca dose? Yes, it almost doubles your protection against Delta


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Meru Sheel, Australian National University; Cyra Patel, Australian National University, and David Tscharke, Australian National UniversityThe weekend’s news of COVID-19 outbreaks and various lockdowns around Australia reminds us there’s no room for complacency. We need to accelerate Australia’s COVID-19 vaccine roll-out and ensure people are fully vaccinated as soon as possible.

Just over six million Australians (30% of those eligible) have received their first dose of a COVID-19 vaccine. As of June 17, 3.8 million Australians had one dose of AstraZeneca.

Despite the benefits of vaccination, some people are concerned about the small but real risk of clotting after receiving their AstraZeneca vaccine. Some have even cancelled their booking for their second dose.

But until you’re fully vaccinated – with two doses of AstraZeneca or two doses of Pfizer, at the recommended time intervals – you’re not optimally protected. After your second AstraZeneca dose, your protection against the Delta variant almost doubles, from 33% to 60%.

Why are 2 doses are better than 1?

When you get the AstraZeneca vaccine, or an mRNA vaccine such as Pfizer, it directs your body to make the SARS-CoV-2 spike protein at the injection site.

This prompts an immune response, which recognises and remembers this spike protein.
But the vaccines don’t make the spike protein for very long, nor do they spread it. This restricts the size of the immune response.

Limiting each dose and delivering it twice generally leads to better and longer-lasting immune responses than a single dose.

Just like your own memory, which improves by repeated viewing or listening with a break in between, our immune memory generally improves with repeated exposure to something it needs to protect us against.


The Conversation (adapted from Vaccine Immunology, Plotkin’s Vaccines [Seventh Edition] 2018), CC BY-ND

When should you get your second dose?

The recommended interval between doses of AstraZeneca is 12 weeks, while the minimum is four weeks after the first dose.

This is based on clinical trial data which showed around 73% “efficacy” after the first dose. This means in experimental studies, the first dose of the vaccine reduced the risk of getting COVID19 caused by the original strain by 73%.

When people received their second dose after 12 weeks or more, the efficacy reached 81%.




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In these trials, a longer duration between the first and second doses – 12 weeks, rather than eight or less – resulted in higher levels of antibodies which can stop the virus.

These antibodies will inevitably fall over time, but starting at a higher level after the last dose means it will take longer for them to fall to levels where protection is compromised.

How well does AstraZeneca protect against variants?

When the Alpha variant is dominant, the AstraZeneca vaccine provides 55-70% protection after one dose and 65-90% after two doses. Alpha is the variant associated with the Queensland flight crew outbreak.

For the Delta variant, which is circulating in Sydney, one dose provides 33% protection against symptomatic COVID-19. So after one dose, you’re 33% less likely to get sick with the Delta strain than someone who is unvaccinated.

The level of protection increased to 60% in those who received two doses of the AstraZeneca vaccine.

This compares to 88% for Pfizer.



Both vaccines offer greater protection against severe disease. Two doses of AstraZeneca reduces your chance of needing to be hospitalised with COVID-19 by 92%, while for Pfizer it’s 96%, compared with someone who wasn’t vaccianted.




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Overall, contact tracing data shows one dose of either AstraZeneca or Pfizer vaccine can prevent disease from spreading to members in the same household by around 50%.

Today the New South Wales health minister revealed that of the 30 people at a Hoxton Park house party, 24 returned a positive COVID test. While you can still get COVID-19 when you’re vaccinated (generally a milder form), in this case, the six who have so far tested negative had all been vaccinated.

What about blood clots following second dose?

Thrombosis with thrombocytopenia syndrome (TTS) is a rare clotting condition that can occur after AstraZeneca vaccination because of an abnormal immune response.

The risk of TTS is smaller following the second dose of AstraZeneca than the first dose.

Of the nearly 16 million people who received the second dose of AstraZeneca in the UK, 23 developed TTS, a rate of 1.5 per million people vaccinated. This compares with 14.2 per million for a first dose.




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The risk of death from TTS is further reduced. If you received your first dose of AZ vaccine without developing TTS, you are even less likely to get it with the second dose.

That’s why health authorities recommend people who safely received their first dose of AstraZeneca vaccine have their second dose and protect themselves optimally.

Can I get the Pfizer vaccine for my second dose instead?

Some countries, including Canada and some in Europe, have approved mixing and matching different COVID-19 vaccine brands.

This is based on data on antibody responses, which showed a stronger immune response when people get a different type of COVID-19 vaccine for their second dose.

But while this is encouraging, we don’t have clear evidence that mixing vaccines will protect against COVID-19.

Mixing vaccine brands also increased common side effects including reactions such as headaches, fever, body aches and tiredness.

It’s possible that as more information becomes available from ongoing studies using AstraZeneca, Pfizer, Moderna and Novavax vaccines, the advice of mixing vaccines may eventually change.

But until more real-world data on vaccine safety and effectiveness are available, the advice in Australia is for people to receive two doses of the same vaccine.

The goal is full vaccination

The best way out of this pandemic is to ensure everyone is vaccinated as quickly as possible. Full protection kicks in about two weeks after completing vaccination.

While one dose offers some protection, it’s not as high as two doses. And we don’t know how long the protection from one dose will last.

Right now, a second dose of AstraZeneca vaccine is ready and waiting for everyone who had their first dose 12 weeks ago. So if you’re eligible, get both doses of the vaccine brand available to you, rather than settling for second-best and incomplete protection.




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The Conversation


Meru Sheel, Epidemiologist | Senior Research Fellow, Australian National University; Cyra Patel, PhD candidate, Australian National University, and David Tscharke, Professor in Immunology and Infectious Diseases, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

What if I can’t get in for my second Pfizer dose and the gap is longer than 3 weeks?


Jessica Hill/AP/AAP

Nathan Bartlett, University of NewcastleBookings for the first dose of the Pfizer vaccine have been halted in Victoria this week, amid shortages of the vaccine.

Some Victorians who’ve had their first Pfizer dose already will need to wait six weeks to get their second.

Some people are wondering if it’s OK to get their second Pfizer shot beyond the recommended three week gap between their first and second dose.

And yesterday, the federal government recommended the Pfizer vaccine as the preferred vaccine for people under 60. Previously, it was only recommended for people below 50. This will place even more pressure on our currently limited supply of Pfizer vaccine, and could lead to wait times being longer than three weeks for some.




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The good news is, you can wait up to 12 weeks between your first and second dose of the Pfizer vaccine. In fact, some preliminary evidence suggests you might get even stronger immunity with a longer wait time.

The only downside is you’re at risk from the virus the longer you wait for your second dose.

So the improved immunity conferred from waiting longer must be weighed against the risk of contracting COVID in the meantime.

You can wait longer than three weeks

The Australian Technical Advisory Group on Immunisation (ATAGI) recommends a minimum of three weeks between the first and second Pfizer dose. However it says this gap can be extended to up to six weeks.

The minimum time to establish immune memory following first exposure to a new vaccine is roughly three weeks. This is the minimum time, but waiting longer between the first and second jab is absolutely fine in terms of efficacy.

This makes sense based on what immunology experts understand about our immune response to vaccines.

By about two weeks after vaccination, adaptive immunity has kicked in. This involves immune cells called T and B cells working together to produce antibodies that target the SARS-CoV-2 spike protein, and are able to block infection.

At this stage, some of these become “memory” immune cells, and by about the third week they have established immune memory. This means these virus-recognising cells are on hand to rapidly respond if we are exposed again.

If that exposure is via a second immunisation, this will boost the immune response to the vaccine and increase immune memory, which in turn enhances protection against the virus.




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The secondary immune response is faster and bigger because you have a pool of memory immune cells primed and ready to jump into action. The memory response is also faster, so by two weeks after the second jab, protection has significantly increased.

You’re not fully protected against COVID until about seven to 14 days after the second Pfizer dose.

Waiting longer might be even better

Many vaccines confer improved protection with longer gaps between doses, and preliminary data suggests this seems to be the case with Pfizer too.

One pre-print study, yet to be peer reviewed, suggests waiting 11-12 weeks for the second Pfizer dose actually produces an even more potent antibody response in people over 80.

The levels of antibodies in people who waited 12 weeks for their second dose were 3.5 times higher than those whose gap was three weeks.

What are the risks of waiting?

We must remember the level of protection isn’t the only consideration. The time it takes to get there is also important. Delaying the second dose increases the time it takes for you to achieve a high level of immunity, and therefore increases your susceptibility to infection, and risk of COVID.

One dose does provide some protection from severe COVID, but not enough, which means you can still become infected and transmit the virus to others. Preliminary data suggests one Pfizer dose provides only 33% protection against the Delta variant, while two doses confers 88% protection.

However, this risk must be weighed against the risk of contracting COVID in Australia currently. Community outbreaks are relatively contained, so the risk in between doses is not as high as it is during periods of rampant transmission.

In saying that, as we’ve seen from Victoria’s recent lockdown and new cases in Sydney this week, COVID transmission is still smouldering in Australia and we must not let our guard down yet. In this context it’s important everyone who can get vaccinated does, and as soon as possible.The Conversation

Nathan Bartlett, Associate Professor, School of Biomedical Sciences and Pharmacy, University of Newcastle

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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