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.

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.

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.

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.