Experience in the field with the COVID-19 vaccines and their effectiveness over time has demonstrated that immunity against infection can wane after six months. It should be noted that after two doses, both Pfizer and AstraZeneca provide a sustained reduction in hospitalisation from COVID-19 infection for a period longer than six months.

There are other vaccines that we regularly use that require multiple doses. For example, hepatitis B requires three doses to generate a sustained immunity.

Those with impaired immune systems, such as people undergoing cancer therapies or taking immunosuppressive medications, do require a third dose of COVID-19 vaccine.

Converse to this is that younger people appear to also develop stronger responses to vaccination against COVID-19, and as such may be less likely to need a third dose. This age response could be because the required immunity from the COVID-19 vaccine is a cell-mediated immunity and it is known that this type of immune response wanes with age. It is for this reason that an aged person can have a recurrence of chickenpox manifesting as shingles, even though they had chickenpox as a young child. Their immune systems response to chickenpox wanes with age.

The TGA have just approved a booster shot of Pfizer mRNA vaccine for people aged over 18 to be delivered six months after their second dose.

In the short game of reaching herd immunity ASAP, then yes, people should get double vaccinated and receive a third shot to boost their immune response for neutralising antibodies against the spike of SARS-CoV-2.

But, as variants of concern appear across the globe, we will find new virus variants reaching our shores.

Therefore, if the long game is to induce broad immunity, then using a different vaccine brand (non-mRNA type vaccine) as a mixed booster would be better - as a universal vaccination strategy for broad immune memory can elicit cross protective responses to neutralise virus variants.

We are still progressing vaccination rates across the country with some states lagging far behind their goals. With plenty of vaccine supplies this shouldn’t be a problem but if the idea is to boost immunity in vaccinated people, then is Pfizer the best choice?

Let’s think wisely now before we travel down the road that is too narrow.

Is waning immunity a concern?

The duration of immunity (protection against disease) varies with different diseases and different vaccines. Lifelong immunity is not always provided by either natural infection (getting the disease) or vaccination. The recommended timing of vaccine doses aims to achieve the best immune protection to cover the period in life when vulnerability to the disease is highest. Many vaccines used today are relatively new and data concerning the length of time that they give protection is continually being updated.

The immunity from vaccination wanes over time. Most of the time, the first shot of vaccination builds immunity against the disease, it primes our immune system to ‘learn’ about that particular pathogen (from which the vaccine is based). The subsequent booster shots are vital (in some cases more than 2) in improving the defense (antibody levels) against that particular pathogen. It acts as a ‘reminder’ against the pathogen. The efficacy of a particular vaccine also depends on factors such as age, certain diseases which immunocompromise us, including immunocompromising drugs, and the emergence of a mutated strain [which differs] from the original strain the vaccine was based on.

When you get infected with the coronavirus infection, your body produces strong immune responses to fight the deadly pathogen. Even when you receive your COVID-19 vaccine, it activates your immune system to produce virus-fighting antibodies. All these processes indicate that a person's immune system plays an important role in determining how well protected they are from infections. That said, people who have a compromised immunity may be more prone to SARS-COV-2 infections, even when they're fully vaccinated. This has also opened up discussions around the need for vaccine boosters, also known as the third COVID-19 vaccine dose. Given that fully vaccinated people are also getting infected with the virus, experts have signalled towards the possibility of waning immunity.

We must first understand, vaccination triggers our immunity at two different levels - an early B-cell mediated antibody response, and a delayed T-cell mediated response. As explained before, sustainability of vaccine-related immunity is dependent on how the individual responds to the vaccine, how efficient and effective the T cell response is going to be, whether the virus mutates over time to evade the kind of antibodies produced by the primary vaccination. Keeping all these factors into consideration, vaccination-induced immunity may wane in terms of antibody levels over time as what we are seeing in real-world data from many parts of the world (Israel, UK, etc.) where there is a surge in infections although the majority of people there are vaccinated. 

Vaccine boosters have gained a lot of momentum in recent times. Considering breakthrough infections have become prevalent and new variants continue to emerge every now and then, the demand for a booster dose is increasing. The administration of a booster shot is expected to re-expose a person's immune system to the immunising antigen, the memory of which (following previous doses) could have been lost over a period of time. Most vaccines have a certain time period beyond which another shot is required in order to reactivate the immune system to produce antibodies, providing protection against a particular disease. 

The need for booster doses should be based on the facts below:

Epidemiology and burden of disease:  

• Epidemiology of breakthrough cases, by disease severity, age, co-morbidity and risk groups, exposure, type of vaccine and time since vaccination, and in the context of variants of concern (VoCs).

Vaccine-specific data: 

• Efficacy, effectiveness, duration of protection of vaccines in the context of circulating VoCs from observational studies and if possible randomised controlled trials.
• Supplementary evidence from immunological studies assessing binding and neutralising antibodies over time, as well as biomarkers of cellular and durable humoral immunity when possible.

Assessing the performance of booster doses: 

• For most emergency use listed COVID-19 vaccines, small-scale clinical studies have been conducted demonstrating a strong ability to boost the immune response following currently recommended primary series.
• While preliminary data on effectiveness of booster vaccination have been obtained for one product, additional data on efficacy, effectiveness, and duration of protection of original and variant-adapted vaccine booster doses in the context of SARS-CoV-2 wild-type and VoCs would be helpful.
• Safety and reactogenicity of booster vaccination, including heterologous boosting (using a different vaccine than earlier doses), needs to be studied at a larger scale.
• Optimal timing of the booster dose, consideration of homologous versus heterologous boosters, possibility for dose-sparing for booster doses, booster needs in previously infected individuals, specification and prioritisation of high-risk populations, programmatic feasibility and sustainability, community perception and demand, as well as equity considerations.

Introducing booster doses should be firmly evidence-driven and targeted to the population groups in greatest need. The rationale for implementing booster doses should be guided by evidence on waning vaccine effectiveness, in particular a decline in protection against severe disease in the general population and in high-risk populations, or due to a circulating Variants of Concern, (VoC). To date, the evidence remains limited and still inconclusive on any widespread need for booster doses following a primary vaccination series, especially in healthy younger age groups without any comorbidities.

Professor John Skerritt, head of the TGA, believes that approval will be given for 5-11 year old children to get the Pfizer vaccine by the end of November.

This is welcome news and an amazingly fast approval for the TGA.  

The children will receive two doses of 10 micrograms, 21 days apart. This is a third of the dose given to people 12 and over.

Vaccination will both reduce the chance of them getting infected with the virus and reduce the chance of them infecting others if they do get infected.

Children at this age rarely get sick even if they are infected unless they have underlying health conditions.

Ideally, we would just be able to withdraw smaller doses from existing vials. However, Pfizer are likely to use a new formulation, and batches will have to be ordered from Pfizer, with the potential for holdups.

It is very timely that the TGA has approved booster shots of the Pfizer COVID-19 vaccine.

There is clear evidence that immunity wanes over time.

This is no different to other vaccines for common viruses such as influenza.  

We already have approval from the TGA, and also recommendations from ATAGI, about booster shots for the immunocompromised  - for which I am one.

However, we are now going to be ahead of the curve in providing booster shots for the whole community in order to suppress the spread of the SARS-COV2 virus.

Vaccination not only suppresses the spread, it also provides fewer chances for new variants to occur.

Evidence from around the world clearly shows that immunity against COVID-19 wanes over time, and that this translates into reduced protection from the disease. 

In parallel, studies show that boosting with another dose of vaccine is successful in increasing these immune levels, and so brings protection back up to very high levels.

The mRNA vaccines, of which Pfizer is one, are very effective at boosting immune levels, so it is very positive that this has been approved for Australians.

A meeting organised by the World Health Organization (WHO) on Monday night allowed the global COVID vaccine research and development community the chance to closely review all the safety and efficacy data around a third booster dose of vaccine.  

The data for mRNA vaccines, such as Pfizer and Moderna, showed even lower rates of the rare adverse events of treatable myocarditis and pericarditis than seen for the second dose of these vaccines.  

There was strong protective benefit from breakthrough infections after administering a third dose to previously vaccinated people who were showing reduced immunity six months after their second dose.

The boosted immunity reached even higher levels than originally attained after the second dose and had much better protection against the new variants of concern, such as the Delta strain.  

The increased personal protection also translated to reductions in transmissions in the community, providing a more general benefit.