We may need to vaccinate children as young as 5 to reach herd immunity with Delta, our modelling shows
- Written by: Emma McBryde, Professor of Infectious Disease and Epidemiology, James Cook University
Recently released modelling from the Doherty Institute, which the federal government used to back its roadmap out of the pandemic, misses one critical point — the importance of vaccinating children.
The Doherty modelling instead focuses on vaccinating 70-80% of the adult population as thresholds for easing various restrictions, such as lockdowns. It says vaccinating younger adults, in particular, is important to reach these thresholds.
However, our modelling shows vaccinating children is vital if we are to reach herd immunity, which would allow us to ease restrictions and safely open up.
This would mean potentially vaccinating children as young as 5 years old.
However, we are still waiting to see if this is safe and effective, with trials under way in the United States. So we need a plan that assumes we may never achieve herd immunity.
Here’s what our modelling shows and how it differs from the modelling used to advise the federal government.
Here’s what we did
Our modelling, which we’ve uploaded as a pre-print and has yet to be peer-reviewed, considers different vaccine strategies for Australia to achieve herd immunity. That’s when we can expect no sustained transmission of the virus in the community.
We take into account the Delta variant, which is twice as infectious as the original Wuhan strain of the virus, and has a reproduction number estimated between 5 and 10. In other words, this is when one person infected with Delta is estimated to infect 5-10 others.
Read more: Scientific modelling is steering our response to coronavirus. But what is scientific modelling?
We also consider different contact patterns across various age groups. This is because some age groups are more mobile and have many contacts. If infected, these people are more likely to infect many others, particularly of similar age, which can lead to reservoirs of transmission.
We combine this information with possible vaccine effects. These include the possibility of having the vaccine then becoming infected, having symptoms, and if infected, how serious the illness is and how infectious people are.
This allows us to model what’s likely, given we’re focused on the Delta variant for now, and allows us to assess the impact of strategies across different age groups, types of vaccines and percentage vaccinated.
Our interactive tool also allows rapid response to changing information, such as new variants, or new evidence about vaccine impact.
Delta is more infectious
The Wuhan strain had a basic reproduction number of 2.5. This means, at the start of the pandemic, one person infected with it was expected to infect 2.5 others.
If the Delta variant is twice as infectious, this means its basic reproduction number may be over 5 (at the lower range of international estimates). So this changes the number (and type) of people we need to vaccinate to reach herd immunity considerably.
The simplest form of the herd immunity equation would suggest we needed to fully immunise 60% of the population to achieve herd immunity for the Wuhan strain but as much as 80% for the Delta variant.
If we take into account how different age groups mingle or are in contact with others, the situation is worse.
For the Wuhan strain, children were not as infectious or susceptible to infection and we predict that if we vaccinate 65% of the adults, transmission would not continue among children.
However, with the Delta variant, we predict children will continue to infect other children, even when most adults are vaccinated.
We also know both the AstraZeneca and Pfizer vaccines are less able to protect against the Delta variant, with a reduced efficacy after one dose and slightly reduced efficacy after two doses.
All this makes achieving herd immunity a great challenge.
Read more: When will we reach herd immunity? Here are 3 reasons that's a hard question to answer
We estimate if the reproduction number is 5, then vaccinating 85% of the population, including children down to age 5, will be necessary to achieve herd immunity.
If the reproduction number is as low as 3, then vaccinating children will not be necessary to achieve herd immunity and we will only need to vaccinate 60% of the population.
The Doherty modelling uses an effective reproduction number of 3.6. This explains why its modelling does not see vaccinating children as critical to reaching herd immunity. This is the major difference between our model and theirs.
What happens next?
Of course, new variants may arise pushing Delta aside, and the world post-COVID is unpredictable.
The lesson from Delta is if we don’t vaccinate children, we may need to continue some form of public health action to prevent large-scale circulation of the virus.
This would not require stringent lockdown, but may require ongoing mask use and physical distancing, including in children. The alternative is to reduce the focus on case numbers, expect transmission and focus on protecting the most vulnerable.
Read more: Should we vaccinate children against COVID-19? We asked 5 experts
Do we need to reach herd immunity?
Herd immunity is not the only possible target. Even if we don’t reach full herd immunity, we may achieve “herd protection”. This provides some reduced risk to people who can’t or won’t be vaccinated, and it will make outbreaks smaller and easier to control.
And without full herd immunity, individuals still benefit from vaccination as they are dramatically less likely to die from COVID.
Read more: How well do COVID vaccines work in the real world?
Do we need to change our vaccination strategy?
We predict Australia’s strategy of vaccinating the elderly and vulnerable first is the best strategy for reducing deaths under most circumstances, particularly when there is insufficient vaccine available.
But once the most vulnerable groups have been covered, we should turn our attention to the highest transmitters to achieve herd protection. In Australia, this group is the late teens and young adults.
Whether we next focus on vaccinating children is controversial and many people have voiced their concerns about going down this path. This is because COVID is generally a very mild illness for most children — although long COVID and life-threatening complications can arise.
So we need to balance the risks with benefits. But included in the benefits should be the potential benefit of herd protection and the freedoms that may bring.
Authors: Emma McBryde, Professor of Infectious Disease and Epidemiology, James Cook University
