Herd immunity? Not with AstraZeneca in the frame. Part 2

Feb 26, 2021

Because the failed immunity rate for the AstraZeneca vaccine is more than seven times that of the Pfizer vaccine, if the Australian rollout takes place as planned, about 5.5 million people (22% of the population) could still be at risk of getting ill, while some of the remaining 20 million could still become infectious but be asymptomatic.

Covid vaccines and the unknown unknowns

What we know about Covid-19 is that there is much we don’t know – about the virus itself and the vaccines on offer. All this uncertainty is driving the state and federal governments to be very cautious.

We can expect Australia to continue with international border controls and quarantining of all arrivals in some form. We note the discussion about building facilities around the country and suggested refinements of the CBD hotel systems. Other countries, despite vaccination programs, are using similar or other methods to control entry.

Regrettably, uncertainty might also be driving the federal government’s rush to vaccinate (most of) the whole population, no matter what.

Even with all the vaccines available, a certain proportion of the population will not be immune from Covid despite being vaccinated. Hence, problems will continue until we use high efficacy vaccines and the world gets to the ‘herd immunity’ stage, where the virus dies out and has been eliminated, if not eradicated.

Here we mention the main areas of uncertainty.

Mutations: The virus is changing and some mutations might be even more infectious and more deadly and the current vaccines might not be effective against them. Will we have to vaccinate everyone again because of a significant mutation?

Long term effects of vaccines: The current vaccines have been developed at record pace and are not fully tested in the community in regard to longer-term negative effects.

Efficacy v effectiveness: Efficacy is the ‘success’ rate in the trial stage, with testing on a restricted range of subjects. Effectiveness is the ‘success’ rate when the vaccine is finally administered to the population, that is to the real world. These rates might differ. We might eventually find that a vaccine with a high published efficacy rate is significantly less effective in the real world and/or for some groups, such as the elderly.

Differing efficacy rates: The Pfizer vaccine has a published efficacy rate of 95%; the AstraZeneca vaccine has an efficacy rate quoted as low as 62%. Thus with Pfizer we would expect that of those who would normally have become ill without vaccination, there would be 95% fewer; with AstraZeneca we would perhaps expect only 62% fewer.

Differing protection rates: Of those who have Pfizer, it seems only 5% will not have immunity; with AstraZeneca perhaps up to 38% will not have immunity. The failed immunity rate for AstraZeneca is more than seven times the failed immunity rate of Pfizer.

Symptomatic v infected: This article provides some explanation of what the Covid vaccination testing involves. What the Pfizer vaccine measured was the number of people who became ill (symptomatic) after having had the vaccine, but not the number of people who became infected after having the vaccine. Thus, it seems that swab testing was not incorporated, which means that while those in the 5% were clearly infected, some in the 95% group might in fact still have caught the virus, not shown any symptoms and still have been infectious to others.

The vaccinated can still become infectious: We saw above that it seems the Pfizer is much better at reducing illness in the community. However, a vaccine with a high efficacy score might still produce many asymptomatic but infectious individuals who go undetected and who could potentially spread the disease. Thus a ‘vaccination certificate’ does not mean that someone is not infectious.

Severity of symptoms: The efficacy rate might not be a useful indicator of how ill those who are vaccinated but still get the disease could become, though there is hope that the vaccines will significantly reduce the severity of symptoms and the need for hospitalisation.

How long they are effective for: Will the vaccines be effective forever? For some years with boosters needed? For only six months? Will yearly injections be required, like the flu?

Multiple infections: Can people who have had Covid catch it again and become infectious, and possibly be asymptomatic?

Long-Covid: Many who have acquired the disease, from mild to severe cases, have reported long-term debilitating effects. Will the vaccines be effective in stopping long-Covid?

Existing cases: Are the vaccines of any use to those who have the disease now or who acquire it before being vaccinated?

Conflicting opinions: Disagreement amongst health experts, scientists and politicians add to uncertainty and concerns.

Australia might not reach herd immunity with our current strategy

Herd immunity relates to a situation where the degree of immunity (from past infections combined with vaccinations) is so high that the disease will not pass on or take off in any significant way. With herd immunity, there is still the risk that an infectious person could enter the community and infect some of those not immune, but the spread will not go far and will be short lived in terms of number of cases.

Australia’s current situation is the complete opposite of herd immunity. Yes, under current pre-vaccination policies the chance of anyone catching Covid is effectively zero and any outbreak does not go far or last long. However, open our international borders, stop quarantining and contact tracing and we might soon look like Bolivia, with “bodies piling up at homes and on the streets”.

This article explains why Australia won’t reach herd immunity with the current plan and why information about our two key vaccines “should prompt an urgent rethink of our vaccination strategy”. In short, the AstraZeneca vaccine with its 62% efficacy is inadequate to offer herd immunity.

The pointers appear to be against using any AstraZeneca, and that the strategy should be changed to using the Pfizer on the high risk groups, protecting the rest of the community via quarantining and delaying the rollout for the rest of the population until we have enough Pfizer doses or until we have access to one with a comparable efficacy rate (such as the Novavax vaccine).

(There is some discussion about the AstraZeneca efficacy rate. It is up to decision makers as to what level they choose to work with; to be conservative we have used a lower figure here.)

Once the rollout is completed as planned, those who have been given the AstraZeneca vaccine, based on the efficacy figures, will be running a greater risk of catching the disease than those with the Pfizer vaccine, and will still need to be protected.

Thus, assume the 20 million Pfizer doses are given to the 10 million most deserving and vulnerable Australians. Everything else being equal, they will have a risk factor of 5% of developing the disease compared to not being vaccinated. Assume the other 15.5 million Australians are given the AstraZeneca vaccine. Their equivalent risk factor could be as high as 38% (if the efficacy rate is as low as 62%).

That combination of two vaccines will be an improvement on having no vaccine, but not sufficient improvement. On the basis of the efficacy rates, it seems we can estimate that 500,000 of the 10 million who had the Pfizer vaccine would not have immunity and still be at some risk of catching the disease and becoming ill; and that more than 5 million of those with the AstraZeneca would not have immunity and, likewise, would still be at some risk.

We can thus conclude that if the Australian rollout takes place as planned, about 5.5 million or 22% could still be at risk of getting ill from the coronavirus, while some of the remaining 20 million could still become infectious but be asymptomatic. The raw risk factor is not altered for those not vaccinated so these numbers will be somewhat higher given that a (large) number will not have a vaccine.

These rates would seem too high for us to open our borders to the rest of the world. Hence the need to question the use of the AstraZeneca vaccine. If fully vaccinated by Pfizer, the estimated number of those who would not have immunity and be at risk would be closer to 1.2 million.

The government and health officials might decide that herd immunity is not necessary and that, like the flu, we will live with the virus, hoping outbreaks are not widespread or catastrophic. Without worldwide immunity, however, there is the risk of dangerous mutants appearing.

In Part 3 we explain why a ‘vaccination certificate’ is no guarantee of a person’s infectious status but unfortunately could still become a method of discrimination. And we look at the vaccine take-up rate in light of concerns about the effectiveness of AstraZeneca.

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