Herd immunity muddies the waters

Burnet Institute

16 January, 2021

Social distancing is going to be needed for some time yet

Burnet’s Director and CEO, Professor Brendan Crabb AC told The Australian’s health editor Natasha Robinson that the ­debate over herd immunity in recent days is because of Australia’s outstanding success in managing COVID-19 and the ­incredible global achievement of producing successful COVID-19 vaccines inside of a year from the first outbreak.

Read the entire article by Natasha Robinson in The Australian.

With case numbers exceedingly low, and the nation poised to roll out the first COVID-19 immunisations in an extraordinary achievement for science, the fact that vaccines had suddenly become a bad news story seems in many ways absurd.

But in the blink of an eye, the government’s vaccine strategy was under attack, with scientific institutes locked in debate over the relative merits of the AstraZeneca vaccine and doctors campaigning for more Pfizer.

The goal of the government’s vaccine strategy was suddenly a live issue. It quickly became clear that the attainment of herd ­immunity was not the immediate aim. Herd immunity is a state whereby enough people are vaccinated that resistance to a disease builds up in the community, a virus cannot find new hosts, and an epidemic dies out.

The government made clear this week that its primary goal was protecting the population from disease, with herd immunity hopefully developing over time.

“The AstraZeneca vaccine is very effective against severe illness,” said Chief Medical Officer Paul Kelly. “It will save lives. By using this vaccine, we’ll be able to protect a large proportion of the population in Australia.”

Many experts were frustrated that the focus had suddenly turned to herd immunity. AstraZeneca’s vaccine — with 70 per cent average efficacy but only 62 per cent among those who got two doses — appears unlikely to result in herd immunity in the population.

“I think that discussion was fundamentally flawed with people selecting small pieces of information and propagating them in the wrong context,” says Paul Griffin, an infectious diseases physician and microbiologist. “I think it’s a potentially harmful and irresponsible discussion that threatens to undermine people’s confidence in these vaccines.”

Professor Griffin says that it is impossible to say that any COVID-19 vaccine will confer herd immunity. “We don’t know how much any of these vaccines will block transmission,” he says. “And if a vaccine has no impact on transmission, then the number to achieve herd immunity is very high, if at all attainable.

“What we know about these vaccines and AstraZeneca, in particular, is it’s exceptionally good at stopping severe disease. If we have a vaccine that stops people getting really sick, that’s an enormous benefit.”

(Professor) Brendan Crabb, Director of the Burnet Institute, says the ­debate over herd immunity is paradoxically enabled because of Australia’s outstanding success in managing COVID-19 and the ­incredible global achievement of producing successful COVID-19 vaccines at lightning speed.

“What we’re really talking about with this debate, is how do we handle the increasingly good news?” Professor Crabb says.

“Let’s have discussion by all means, but the public should be very clear that globally this is a very good news story. And on the Australian front, the government has barely put a foot wrong,” Professor Brendan Crabb said.

The government’s vaccine strategy was developed at a time when data on what vaccines would prove to be effective was scarce. Its advisers had to rely on early animal studies and make educated guesses about which vaccine platforms were the most promising. It also had in mind cold-chain storage requirements and whether we could make any vaccine onshore.

“Normally you wouldn’t even think of making investments until you’d seen all of the phase 3, and possibly even post-release data,” Professor Crabb says. “They had to make all of these ­decisions with the Wisdom of Solomon approach. And they made good bets.”

Australia has bought 10 million doses of the Pfizer vaccine, 53.8 million Astra Zeneca vaccine doses and 51 million doses of ­Novavax’s candidate.

“Did someone start out in February last year and say if we don’t have a vaccine for herd immunity we’re not going ahead? No way,” says Professor Crabb. “The bar would have been quite low. And we’ve revised things as the stories got better and better.”

Federal Health Minister Greg Hunt now says the government aims to “ensure that we have whole-of-population protection at the same time as pursuing herd immunity”.

It seems clear, meanwhile, that any notion that the vaccine would be a silver bullet is misguided. And Professor Crabb says anyone making predictions about herd immunity would simply be guessing. “In the end, it’s going to be a relationship between whatever that real world effectiveness of the vaccine is and how many people get immunised,” he says.

“At the moment Australia can eliminate COVID-19 completely from the community without a vaccine. We don’t want what it takes to do that to continue into our normal life, but for a time we’ll be coexisting with a vaccine and restrictions and social distancing and all of those measures.

“And it will be about finding out what hopefully is the least ­restrictive measures we can possibly have, but we’re still going to need them and that’s going to have to be monitored and titrated very carefully until we reach a point where we feel we don’t need them anymore because we’ve got herd immunity.’’

Find out more about Burnet’s COVID-19 research on our Know-C19 Hub.

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