When I posted about Australia’s slow vaccination progress we were ranked dead last among OECD countries in the full vaccination “race“. We have now nosed past South Korea into second last place. Sydney’s COVID outbreak and lockdown is certainly a factor, but whatever the reason, the increased rate of vaccination we’re seeing is good news.

The Sydney outbreak is also leading to growing numbers of COVID victims being admitted to intensive care units (ICUs) and, sadly, to a number of deaths. So far, while a small number of the patients in ICU have had one vaccination shot, none have been fully vaccinated. But based on international experience, it’s only a matter of time before that will change. We should also expect to see some deaths of fully vaccinated people. I am sure that the minute that happens there will be enormous media attention, but will it actually say anything about the effectiveness of the vaccine?

In fact, it won’t. It will simply be a statistical consequence of an increasingly vaccinated population. It’s sobering to recall that in the race to develop vaccines last year, 50% efficacy in trials was set as the hurdle for regulatory approval. As it turned out, the vaccines all did far better with trial efficacies around 80-90%. These are extremely good vaccines, but they are still not 100% effective at preventing disease, hospitalisation or even death. As more and more people are vaccinated, we should therefore expect to see a higher proportion of fully vaccinated patients in ICU or dying (see chart below). Of course, many people will still see this outcome as an indictment of the vaccines. Friend of the Mule, Dan, gave me this ironic analogy for this confusion: “Now that nearly everyone in Australia wears seatbelts, it turns out that nearly 100% of people who die in road accidents were wearing their seatbelts. Proving that seatbelts are not effective anyway in preventing road fatalities!”.

Focusing on the proportion of people in ICU or dying who are vaccinated misses the important point here: as vaccination rates increase, the total number of people in hospital or dying will be dropping. John Burn-Murdoch of the Financial Times illustrates this point nicely with a clever infographic.

NEW: people worry when they hear "40% of hospitalisations are fully vaxxed", but this chart shows that's actually good news.

The more people you vaccinate, the higher their share of hospitalisations, but the *total* number in hospital is a fraction of what it would otherwise be pic.twitter.com/rPVbvl8zTW

— John Burn-Murdoch (@jburnmurdoch) July 24, 2021

How much of a reduction on ICU admissions should we expect to see as a result of increased rates of vaccination? It will depend on how effective the vaccines are at preventing severe disease. While current estimates of the effectiveness of the vaccines are around 80% (with some variation across the vaccines), there is some evidence that the effectiveness of preventing severe disease is over 90%. The impact of vaccines on hospitalisation will also depend on how much COVID is circulating in the community: when the risk of contracting COVID is low, even unvaccinated people are unlikely to end up in hospital.

The chart below shows the expected effect of vaccination on rates of admission to ICU for four different assumed effectiveness rates of the vaccine and for three different scenarios for the prevalence of COVID in the community. These scenarios are taken from the Australian Technical Advisory Group on Immunisation (ATAGI) paper “Weighing up the potential benefits against risk of harm from COVID-19 Vaccine AstraZeneca“. The “low” risk scenario corresponds to an infection rate similar to the first COVID wave in Australia (29 infections per 100,000 over 16 weeks), “medium” to an infection rate similar to the Victorian second wave (275 per 100,000 over 16 weeks) and “high” is similar to Europe in January 2021 (3,544 per 100,000 over 16 weeks).

When the risk of contracting COVID is high, higher vaccination rates will dramatically reduce hospitalisation. This is particularly evident in the UK experience, where COVID peaked well above the “high” scenario in this chart in January 2021 and then again in July. In the intervening six months Britain has had one of the more successful vaccine roll-outs and, as a result, hospitalisation and death rates in July were a fraction of the January experience.

I hope Sydney’s lockdown will be successful in containing the current outbreak, but if it’s not, the faster we roll out the vaccine, the fewer people will end up in ICU and the fewer people will die.

Possibly Related Posts (automatically generated):

• Careful with that thing – you could kill someone (27 July 2021)
• Vaccination of the Nation (5 July 2021)
• What’s Going On In Sweden? (2 April 2020)
• Swine Flu League Table (15 June 2009)