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Why We Should All Have A COVID Vaccine


Author's note: this was written on 15 January and more up to date information is expected soon.

There are concerns that the Astra-Zeneca (AZ) vaccine may not be able to prevent local transmission of coronavirus. But it will save many lives, and we all should have it as soon as we can, and here's why. 

Australia will have 53 million doses of this Melbourne-produced AZ vaccine, enough for 26 million people.

We only have a deal for ten million doses of the Pfizer vaccine (for five million people) and this may be the better vaccine.

AZ’s vaccine is reported to have 70% efficacy, but Pfizer’s has 95% efficacy, at preventing COVID-19 disease. This means there was a 70% reduction in disease in the AZ study, when comparing the vaccinated group with those who were injected with a placebo.

Part of the confusion comes from different success figures for different doses of AZ’s vaccine, but they average to 70%. Both vaccines have a good safety profile.

But the reporting really lets us down by failing to emphasise that these figures tell us how well the vaccines prevent illness with symptoms, not how well they prevent all infections, with or without symptoms.

Many people infected with this coronavirus have asymptomatic infections (no symptoms at all), so if we want a vaccine to prevent COVID spreading through the community we need to know how well it prevents any infection, not just those causing symptoms.

Studies of both vaccines are continuing, and the Pfizer study has not reported how well that vaccine prevents asymptomatic infection, but interim data from the AZ study show that it is less effective at preventing infection than disease, with efficacy of only 56%. (Efficacy of preventing any positive test from table 2 in their paper - likely to change as the study continues.)

How effective must the vaccine be to prevent transmission within our population? (Warning there’s maths coming, but grit your teeth and pretend to smile.) That depends on how infectious the virus is, which is measured by R0, the average number of people infected by each case and it’s between 2 and 3 for the virus causing COVID, and likely higher with the new infectious variant.

If R0 is higher, more of the population must be immune to prevent ongoing spread of infection, and the non-immune fraction of the population must be less than the inverse of R0. And the immune fraction must be at least 1 - 1/R0.

Let’s say 95% of the population receive the AZ vaccine, then 95% of 56% is 53%, meaning just over half the population would be immune. That would stop the spread if the R0 was only 2, but not if it was higher, which it probably is without social distancing, masks etc. Sorry about the maths, but well done for not crying.

The above numbers also don’t tell the entire story. The AZ vaccine is good at preventing serious disease. There were no hospitalisations in those who received two vaccine doses, compared to ten in the control group, one of whom died (this group received a placebo injection). It will protect the hospital system and the elderly.

And it will help to reduce transmission; if every second person is immune, the virus will spread more slowly and will be easier to control, using contact tracing combined with other measures, such as wearing masks on trams.

Also it may reduce transmission more than the numbers tell us, if for example, vaccinated people who get infected are less infectious.

We will learn much more as the studies progress, including how well the Pfizer vaccine prevents infection. Australia also has other vaccine deals in the pipeline. About five million Australians will receive the Pfizer vaccine and it appears this will go to workers in quarantine, health and aged care and to the very elderly, all of which makes sense, as it’s probably more effective.

Questions have been raised about whether the AZ vaccine will work as well in people from different races or those suffering other serious illnesses, and is it good enough to give to First Nations people who have a lower life expectancy.

Ideally we should get more of the Pfizer vaccine, but with over 500 deaths per day in Italy, and over 1500 in the UK, it doesn’t seem right that Australia should be anywhere near the front of the queue.

But don’t let the political commentary around whether the Morrison government ordered the right vaccines deter you from having the AZ vaccine. Both you and our community are safer if you have it and the cheap political scoring on Twitter could do more harm than good. 

Just how well the AZ vaccine reduces transmission, will depend on how many of us have it. We need as close to 100% of the population as possible to have it, to lift that immune proportion as high as possible.

So I urge everyone to line up for the two shots as soon we get our turn.

Sign up for my newsletter here. 

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Dr Tim Read
Greens MP for Brunswick
15 January 2021
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