Explained: Why third Covid-19 booster vaccine is so important

With Aussies warned to prepare for spiralling Covid cases in the weeks ahead, experts have appealed for those who are eligible to get a booster jab. Find out why.

A third booster is now recommended to increase protection, especially against the Omicron variant.

Jane Hansen spoke to Professor Terry Nolan, an epidemiologist, paediatrician and head of the Vaccine and Immunisation Research Group at the Doherty Institute, to answer your burning questions.

1. Why is it now recommended to get a booster shot?

Because immunity against infection and mild illness appears to wane fairly quickly, and because new variants may require a higher level of antibody (which occurs after boosting) to render them harmless.

2. How rapidly does immunity wane?

Vaccine protection is greater against more-severe illness and death than it is against infection and mild illness, (meaning you can still catch Covid).

For Covid, waning of vaccine-induced immunity against infection occurs pretty quickly, starting to decline after only three months or so. However, vaccine protection against severe illness holds up very well (so far, and provided there is no vaccine resistance from new variants).

One dose is good, two doses are better and the third makes a huge difference. Picture: Getty Images

Why did the booster come forward from five to four months? And three months from January 31?

There is very good evidence now that vaccine-induced immunity wanes after about three months. A study from Scotland showed for AstraZeneca-vaccinated people in Scotland from three months there was a progressive decline in protection. The large majority of Australians actually got AstraZeneca, so they are the ones at risk. You should not wait for six months when you know that from three months you have declining immunity. There is also evidence that when you get boosted with an mRNA vaccine (Pfizer or Moderna), it gives you antibody levels that are likely to protect against Omicron. The final thing is that there is mounting evidence Omicron is the same or less as severe, but when you have very large numbers infected, even if a smaller percentage are getting severe disease, it can translate to big numbers in hospital. It’s a slam dunk to boost from as little as three months from the second dose.

3. What’s the science behind the booster shot?

A booster is just that. The body’s immune response is boosted or lifted by repeated doses of a

vaccine, though this does not necessarily occur indefinitely.

How well do they work?

They work very well indeed. A recent study from Israel showed that participants aged 50+ years who received a booster at least five months after a second dose of Pfizer had 90 per cent lower mortality due to Covid-19 than participants who did not receive a booster.

4. What is the likely future of boosters? Is it something we may have to have each year?

We don’t know yet. It is possible that repeat boosters of early vaccines directed at the original

ancestral (or Wuhan) strains, or vaccines targeting specific variants will be required.

A booster can help keep you out of hospital.

5. Are the boosters showing efficacy against Omicron?

Two types of data tell us something about variant resistance to the existing Covid vaccines –

laboratory test-tube data, and population epidemiologic data. Both are helpful but neither is perfect.

(1) Test-tube data:

A paper was published in December as a pre-print (not yet peer reviewed) from scientists at the Kirby Institute in Sydney.

They assessed the impact of Omicron infection on the ability of various things to neutralise

virus in the test tube. They examined (1) serum from vaccinated and/or (2) previously

infected individuals, (3) concentrated human antibodies from Blood Bank plasma donors,

and (4) licensed monoclonal antibody therapies.

They showed greater resistance to neutralisation of Omicron compared to Beta, Gamma and

Delta across serum from both AstraZeneca and Pfizer double-dose vaccinated donors.

There was a 17 to 22-fold reduction in neutralisation titres across all donors who had a

detectable neutralising antibody titre to the Omicron variant.

Of all therapeutic antibodies (treatments) tested, significant neutralisation of the Omicron

variant was found for Sotrovimab (GSK) and Tixagevimab (AstraZeneca), but other

monoclonal antibodies were unable to neutralise Omicron in the test tube.

On the basis of some modelled projections from these test tube data, they predicted that

even with the 20-fold decrease in neutralisation titre, boosting with mRNA vaccines (either

Pfizer or Moderna) would provide significant protection from infection with Omicron

(2) Real-world data of vaccine effectiveness in actual populations affected by Omicron-

Omicron variant:

From a very recent UK Government study, among those who had received two doses of AstraZeneca vaccine, there was no protective effect of vaccination against symptomatic disease with Omicron from 15 weeks after the second dose.

Among those who had received two doses of Pfizer, vaccine effectiveness was 88 per cent (2-9 weeks after dose two), dropping to 48 per cent at 10-14 weeks post dose two and dropping further to between 34 per cent and 37 per cent from 15 weeks post dose two.

Among those who received AstraZeneca as the primary course, from two weeks after a Pfizer booster dose, vaccine effectiveness increased to 71 per cent. Vaccine effectiveness increased to 75 per cent after the booster among those who had received two doses of Pfizer as the primary course.

Delta variant: Vaccine effectiveness drops from 76 per cent 2-9 weeks after dose two, down to 42 per cent at 25+ weeks after dose two with an AstraZeneca primary course. Effectiveness increases to 94 per cent two weeks after a Pfizer booster.

With a Pfizer primary course, effectiveness drops from 88 per cent two to nine weeks after dose two, down to 63 per cent 25+ weeks after dose two, increasing to 93 per cent two weeks after the booster.