Protection versus immunity: the great vaccination debate

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Our health authorities have chosen to use this particular vaccine, we now know, because we can access it and because there are no guarantees any vaccine available in the world will prevent transmission – nor that such a thing can even be done for COVID-19.

If there is another outbreak in Australia, wide administration of AstraZeneca’s vaccine will save lives, health authorities say.

Given the uncertainty around stopping transmission, the government has prioritised saving lives with the goal of eventually achieving total immunity.

Since it has been revealed, the government’s strategy has won near-universal backing from major scientific groups.

Professor Raina MacIntyre, head of biosecurity at the Kirby Institute, released a video raising the issue on January 4.

“When you get down to a vaccine that’s 60 per cent effective, you basically need to vaccinate 100 per cent of people [to achieve herd immunity],” she told the ABC two days later. “And that’s not feasible.”

But when the opportunity came to meet the story at the pass on Tuesday – by explaining whether Australia was hoping to use vaccines to stop the virus circulating – Health Minister Greg Hunt chose to largely ignore the issue and launched a personal attack on “commentators”.

The story was ready to explode. On Tuesday, it did.

The Australasian Virology Society and the Australian and New Zealand Society for Immunology both told The Age and The Sydney Morning Herald the government should pause its rollout of AstraZeneca’s vaccine, due to concerns the data we had so far did not clearly show it would stop the virus circulating.

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Word leaked before the story could go to press, leading to an extraordinary internal debate within the AVS. About 6.30pm on Tuesday, the society decided to change positions and pull its objection to the rollout of the vaccine.

“We don’t want to undermine the confidence in the vaccine. And we don’t have the full picture. We need to go with the most effective vaccine so we can have herd immunity. But we just don’t have the full picture at the moment with the AstraZeneca vaccine,” society president Professor Gilda Tachedjian said.

After coming under tremendous pressure, the Society for Immunology would also reverse its position on Wednesday.

But the story was already out.

Herd immunity, explained

Herd immunity describes the scenario many people imagine: everyone has protection against the virus, leaving it with no hosts to jump to, and causing the epidemic to eventually die out.

Is it attainable for COVID-19?

To calculate a virus’ “critical vaccine level” – the level of protection needed in the community to reach herd immunity – scientists look at how infectious a virus is, measured by a value known as R.

R represents the average number of people each infected person passes the virus on to. If that number is over 1, the virus will spread; if it is below 1, the virus will die out.

In most countries, the R value varies between 1.5 and 3.5.

In March last year in Melbourne – as the first wave of the virus spread and before the first lockdown – the R value was between 1.4 and 1.8, a team of modellers estimated in Nature Communications.

To achieve herd immunity, we would need to cut the R value by between 45 and 55 per cent, said Associate Professor James Wood, a University of NSW vaccine modeller and member of the federal government’s Australian Technical Advisory Group on Immunisation.

“That would theoretically be achievable with a vaccine with 70 per cent efficacy against transmission but would require in particular high coverage in the 18-35 year group, which appear to contribute a greater fraction of transmission.”

But there is now a complicating factor that did not exist six months ago: the emergence of B.1.1.7, a variant of the coronavirus that is substantially more transmissible.

That virus has an R that is plausibly 30 to 50 per cent higher, said Wood. That means herd immunity is even harder to reach.

A third wrinkle: it is plausible immune responses from the vaccines will wane over time, and plausible the virus will mutate to evade vaccines.

“If you ask experts in this area, none of them think that the virus is going to be eliminated from Australia long-term,” said Wood.

“What we do expect is that if you’ve been previously vaccinated or infected, then you will experience a much milder disease course and that after a couple of transition years COVID-19 will become just another variety of the common cold.”

Vaccinating the herd

Australia’s current vaccine strategy is to use Pfizer’s mRNA vaccine to vaccinate the most vulnerable starting from mid February; we have enough on order for 5 million people.

At this stage, the rest of the population will be getting a jab made by AstraZeneca; we have 53.8 million doses on order, enough to cover the entire population. Importantly, it is being made here by CSL, guaranteeing supply. A third vaccine on order, under development by Novavax, is yet to report results from phase 3 clinical trials.

Can either of these vaccines provide herd immunity?

Phase 3 clinical trial data published in the New England Journal of Medicine in December estimates Pfizer’s jab provides 95 per cent protection against symptomatic COVID-19.

However, available data does not allow us to know if it stops people passing on the virus, which is key to herd immunity. It may be you can still catch and pass on the virus without showing symptoms.

For AstraZeneca, the data is much cloudier.

Some 4440 people were given the jab in two standard doses. Interim analysis, published in the Lancet, shows it offers 62.1 per cent protection against symptomatic COVID-19.

But a second group of 1367 accidentally received a lower dose. Yet that offered 90 per cent protection. Between the two groups the average was 70.4 per cent.

How do you reconcile those three numbers? That’s not yet clear.

AstraZeneca is doing further trials of the lower dose. The British government has started administering the jab at the standard dose, so we will soon get data on how it behaves in the real world. Australian Chief Medical Officer Paul Kelly has taken to citing the vaccine’s efficacy as a range: between 62 per cent and 90 per cent.

Let’s assume the vaccine is 70 per cent effective. Can we use that to get to herd immunity?

Data suggests AstraZeneca’s vaccine “does cut infectiousness [to] some degree,” wrote Professor Allen Cheng, co-chair of the Australian Technical Advisory Group on Immunisation on Twitter on Wednesday.

“But even if all adults were vaccinated, it probably would not achieve herd immunity.”

Does herd immunity matter?

When the story ran on Wednesday, it sent health authorities into a scramble. Chief Medical Officer Professor Paul Kelly did the circuit of morning shows and a midday news conference. Reporters were briefed on background.

Information that had been jealously guarded suddenly poured out. Herd immunity was not being pursued as a short-term strategy, health officials said.

Primarily, this was because on current data we do not know if any of the vaccines available could confer herd immunity, because we do not know if they will cut or prevent transmission.

That’s as true for AstraZeneca as it is for Pfizer and Moderna.

“We seek herd immunity,” Hunt told The Age. “[But] that outcome will depend on the longevity of protection, the coverage of the population (expected to be high by global standards in Australia), and the impact on transmission, which are yet to be determined for any vaccine.”

Scientists don’t know if the vaccines we have available will stop transmission, but they do know this: Pfizer, Moderna and AstraZeneca’s vaccines will all save lives.

Of the 5807 volunteers in AstraZeneca’s interim analysis who received the vaccine – at any dose – from 21 days after injection not a single one was hospitalised, there were no serious cases of COVID-19, and there were no deaths.

Given the virus has killed nearly 2 million people around the world, the calculus on what to do is pretty simple.

We may even be able to add an extra vaccine – potentially Novavax’s – with more effect on transmission on top in 2022, eventually attaining herd immunity, health officials said. But for now, it is clear what matters.

“In terms of preventing death, it works 100 per cent of the time. In terms of preventing severe illness, it works 100 per cent of the time,” Kelly said.

“Lives will be saved by the AstraZeneca vaccine, I have no doubt about that.”