Covid-19 vaccination – is herd immunity achievable?

12th January 2021, Dr Chee L Khoo

Covid-19 vaccination

So, here we are. It’s almost 12 months since the beginning of the Covid-19 pandemic. A number of vaccines will become available within weeks. GPs can’t be the sole provider of the vaccines but we will a major part of the vaccination roll out program. We haven’t heard much from NSW Health on how the program will be rolled out, which will be the first subgroups we will be vaccinating nor will the vaccine ordering system be better than last year’s influenza vaccination program. We need to be advised well ahead of roll out to put in place some planning as we have never had national mass vaccination since the small pox. It will affect the running of all our practices.

What will the vaccines do and not do?

So far, Pfizer and AstraZeneca have published their “full” results. On the surface, the Pfizer vaccine reported a better “efficacy” rate than the AZ vaccine but at this stage, we cannot say that one vaccine is better than the other. Although a subgroup (the UK cohort) in the AZ trial did swabbed asymptomatic participants which suggest that the AZ vaccine may protect against infection by Covid-19, neither companies are claiming protection against catching the disease. As mentioned in the December article, it appears that both vaccines are claiming reduction in severe disease and deaths but not disease prevention.

Perhaps, the vaccine will prevent infection in some and reduce disease severity in others. If they don’t actually prevent infection, then vaccinated patients will still be transmitting the virus and new cases will continue to grow. Compulsory vaccination before you get on the plane? Compulsory vaccination for all visitors to aged care facilities? How would vaccination help reduce transmission in those scenarios?

Eliminating, eradication, suppressing or controlling?

Remember at the beginning of the pandemic where there was talk about eradicating, eliminating or suppressing the virus. I am not sure that those people calling for suppression or eradication knows what they are actually referring to.

Disease eradication is the global reduction of infection to zero cases, whereas disease elimination is the absence of sustained endemic community transmission in a country or other geographical region (1). Suppression, perhaps, means some degree of disease control to reduce the spread of the disease. An example of disease eradication is small pox where we have successfully eradicated small from the world. Some countries have achieved the elimination of polio and measles, but eradication remains elusive, especially with global resurgence of measles in 2018–19.

For Covid-19 which has pre-symptomatic and asymptomatic transmission, to achieve elimination requires a number of things: an efficacious vaccine, regular large scale testing of the population (including testing of asymptomatic carriers), good contact tracing and an R number (R0) of < 1 (see below).

Thus far, we have pretty much all the ingredients to achieve elimination (eventually) except for the vaccines. Will the AZ and Pfizer vaccines be efficacious enough to get us there?

The R number (R0)

R0 refers to the effective reproductive number of the virus (2). It reflects the virus capacity to spread and indicate the average number of people that one infected person can pass the virus to. The R0 is not fixed. It develops over time. It is affected by how a population behaves (social isolation or not), and any immunity already possessed thanks to infection or vaccination. Location is also important – a densely populated city is likely to have a higher R0 than a sparsely peopled rural area. With social distancing and other measures, the generally accepted R0 for Covid-19 is about 2.7. The UK (and possibly South African) strain is said to be more transmissible at between 3-4. The strategy to attain control is to get the R0 below 1.

Herd immunity?

Herd immunity is attained when the entire population is protected either from natural infection or by mass vaccination. The number of susceptible individuals is too small for infection to spread. Attempting to achieve herd immunity by letting increasing numbers of people to get the infection means many will die even though the mortality rate with Covid-19 is low. Sweden, Brazil, US and UK found out the wrong way, didn’t they? I guess the only other option is via mass vaccination.

The level of herd immunity depends on the R number of the virus and the efficacy of the vaccine. The higher the R number, the higher the herd immunity required to control spread of the disease. The higher the efficacy of the vaccine, the lower the vaccine coverage need to be to achieve herd immunity. For example, a vaccine with an efficacy of 90% will require vaccine coverage of at least 66%. A vaccine with an efficacy of 60% will require vaccine coverage of 100% which is not achievable.

The roll out program – is there a strategy in place?

If we only vaccinate at risk population (older patients and patients with significant co-morbidities), we could reduce the mortality but not the number of cases. Younger individuals have a lower mortality rate but have a higher transmissible rate due to their less-than-ideal social habits. Vaccinating this group of individuals could reduce case numbers but may not reduce overall mortality rate.

We could also vaccinate contacts of infected patients identified by contact tracing (ring vaccination). Naturally, the larger proportion of contacts traced and the faster they are vaccinated, the more impact there is in reducing the spread.

Obviously, the best strategy is for mass vaccination. Australia has secured 10 million doses of the Pfizer vaccine and 53.8 million of the AstraZeneca vaccine. Both vaccines are working their way through the TGA and the first patients will probably be vaccinated in February 2021. Don’t forget each patient will be getting two shots and 10 million doses means 5 million patients. We have secured 50 million doses of Novovax but they have yet to publish their data.

The impact of mass vaccination depends on vaccine efficacy and the speed vaccination can be achieved. Professor Raina MacIntyre did deterministic mathematical model of epidemic response with limited supply (age-targeted or ring vaccination) and mass vaccination for NSW recently (3,4). According to the modelling, vaccinating 50,000 people per day will take 177 days to vaccinate the entire population of NSW with one dose. Distributing at least 60,000 doses per day is required to achieve control.

To achieve mass vaccination quickly is no mean feat, though. Slower rates of vaccination will result in us living with COVID-19 longer and with higher cases and deaths. However, unlike many other parts of the world, Australia has successfully control of Covid-19 pandemic through a combination of closing our international borders, effective lock downs, free and openly encouraged testing, population buy-in, financial support and luck. Despite a number of outbreaks over the last few months, we have managed to keep new cases nationally to 10-15 per day (except during the outbreaks) with minimal mortality.

It’s funny how the term urgent is actually very relative. In our world, urgent means if we don’t attend to the patient with asystole immediately, the patient dies. A request for clinical notes is deemed urgent by the solicitors because they faxed you a request two days ago. GPs are going to be part of this urgent roll out program together with pharmacists, hospital clinics and God knows who. As the pandemic is still raging along, we better roll the vaccines out urgently, shouldn’t we? Well, whose urgency is it?

How will the roll out affect primary care?

All this aim to mass vaccinate the entire population very quickly is all well and good but if GPs are part of that roll out program, how will that impact upon our practice? We are not talking about the financial impact on the normal work-flow at the practice. This vaccination is an add-on to our normal load of patients we look after every day. This can be an additional 20-30 consultations per GP per day for some weeks to vaccinate everybody.

What must primary care do to prepare?

You will remember last year’s influenza vaccination roll out debacle that saw the whole process totally disorganised, fragmented, chaotic and frustrating in primary care. The Covid-19 vaccination will be even more complicated as both vaccines involve two jabs per patient over a 3-4 week period. Further, the Pfizer vaccine, once delivered, needs to be used up within 5 days. Let’s hope NSW Health is using the feedback from us from last year constructively.

We need to think about how the vaccines are ordered, when they arrive and do you have the capacity in your vaccine fridge to accommodate all the additional vaccines. If vaccination is rolled out all-at-once (God forbid), imagine the chaos that will ensue at the practice. The phones will be ringing hot. Patients need to be observed for 15-30 mins after their jabs and where will they be waiting? We will need a vaccine register to recall the patients for their second vaccine. All this while we see our normal general practice patients. It all sounds like a recipe for chaos unless we start planning now.

So, is herd immunity achievable? We will only find out if we try. Achieving herd immunity will take some time. Even when we achieve herd immunity and perhaps, “elimination”, the virus will still be lurking around in the community. There will be outbreaks from time to time. Thus, social distancing, hand hygiene, facemasks and targeted lock down will continue much of this year if not longer.


  1. Heywood AE, Macintyre CR. Elimination of COVID-19: what would it look like and is it possible? Lancet Infect Dis. 2020 Sep;20(9):1005-1007. doi: 10.1016/S1473-3099(20)30633-2. Epub 2020 Aug 6. PMID: 32771079.
  2. Basic reproduction number – Wikipedia Accessed 11th January 2021.
  3. Modelling of COVID-19 vaccination strategies and herd immunity, in scenarios of limited and full vaccine supply in NSW, Australia. C Raina MacIntyre, Valentina Costantino, Mallory Trent. doi:
  4. Principles of vaccines programs for the control of COVID-19 – YouTube presented by Professor Raina MacIntyre. Accessed 11th January 2021.