It’s already feeling like a long, exhausting winter.
But even in this dark hour, there is a ray of sunshine bringing some really good news – Covid-19 vaccines are already doing their job, and we’re all better off for it. We may not see the effect of our immunisations very clearly yet – it is, unfortunately, quite hard to see the cases of disease that never happened – but they are definitely there even though we are far from the lofty heights of the herd immunity threshold.
This sounds confusing, because the message has often felt like it’s herd immunity or bust, but it turns out that this is mostly just a miscommunication about what herd immunity is. Rather than thinking about herd immunity as a fixed threshold above which there is only joy and below which are the darkest reaches of Hades, think of it as a spectrum where any proportion of people being immune to a disease reduces the spread of that infection by a meaningful amount.
You can actually think about this in terms of the numbers. Let’s take the basic reproductive number (R) of Covid-19 that’s commonly used as a best estimate, 2.5, meaning that on average an infected person will spread the disease to another 2.5 individuals. Assuming that our vaccine is 70% effective against infection (a bit lower than the efficacy seen in scientific studies of AstraZeneca), we can think about how protected we are against this original Covid-19 strain already.
We know that about six million vaccine doses have been given in Australia so far. Using data from some of the states, it seems that about five million of these are first doses and one million second, so roughly 20% of all Australians have received at least one dose of a vaccine against Covid-19 (nearly 30% of adults).
Now, many of those doses have been given quite recently, and it takes a while for vaccines to get to their full effect. Moreover, our immunisations are most effective after the second dose, which helps to prevent infection. However, this gives us a useful schema for thinking about how protected we already are by our vaccinations.
With 20% of the population immunised using a 70% effective vaccine, we’d expect that the R of Covid-19 would be reduced by about 14%, from 2.5 down to 2.15. This may sound small, but it’s in line with the benefits that studies have identified from interventions such as banning public events and closing schools. Given the exponential nature of initial outbreaks, that sort of reduction could easily be the difference between an out-of-control epidemic and something that we can quickly get a lid on.
Even if you start thinking about more transmissible variants, this level of vaccine protection is still pretty impressive. Assuming we’re talking about a variant that is double as transmissible as our initial strain, we’d be looking at a reduction from an R of 5 down to 4.3, which is quite a big reduction.
But the real payoff comes when very large numbers of people are vaccinated. Once you have 50% of the population immunised, we’d expect to see the reproductive number fall by more than a third. With 70% vaccinated, it’s more like 50% reduced. Getting to these very high numbers of vaccinations means the spread of disease falls far enough that we could probably control it entirely with just our fantastic contact-tracing teams and not need to rely on other measures at all.
Ultimately, the point is that herd immunity is more complex than a single number, and vaccines are pretty amazing. Even with only 30% of our adults having received at least one Covid-19 vaccine dose, we’re already reaping the benefits of a population that is less susceptible to the disease than it was before. Once we get up to the really high rates, we might be able to relax a lot more about the future that the pandemic holds.
Of course, there are many unknowns here: we may need to combine vaccine regimens for best effect, or have boosters in the years to come. We may not feel comfortable relaxing even once it’s possible, because the pandemic has taken a heavy toll on our collective mental health.
But it is very reassuring to see Australian vaccination numbers climb regardless. The pandemic is not over yet – there are still horrifying tolls being seen every day in lower-income nations across the world – but with vaccine rates climbing there is hope for our future here in Australia.
And on a personal note, I should be getting my second AstraZeneca jab sometime next week.