The TGA has approved the use of the AstraZeneca COVID-19 vaccine as a booster for individuals 18 years or older where this is approved by a physician.
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I am uncertain why this advice came from the TGA rather than ATAGI or whether this decision relates in any way to supply of the recommended Pfizer or Moderna vaccines for the booster vaccination.

The current rationale for booster vaccination appears to be based entirely around the antibody response which wanes rapidly following inoculation.
This response is currently not measured in individuals following vaccination to determine if antibodies are produced and if the concentration following a booster is increasing.

The process is further complicated by the emergence of COVID-19 variants.
All the COVID-19 vaccines approved for use in Australia were all constructed based on the spike protein from the original Wuhan strain of COVID-19.

Mutations in this target domain mean that antibody generated following vaccination may not effectively recognise other variants and compromise the protective response.
This is particularly true for Omicron variant which contains 15 different mutations to the original strain.

This raises a concern as to whether booster vaccinations are required every few months to maintain protection and if variations are needed to the vaccine construct to maintain the effectiveness of the antibody response generated.
No significant consideration has yet been made of the T-cell response which is more robust and is maintained for a longer period of time.

In the interim, booster vaccination is important to maintain a significant protective response to prevent serious illness, hospitalisation and death however future booster vaccination will need to consider more closely the cellular immune response so that this process does not need to occur too frequently.

Mix-and-match vaccine approaches, also known as heterologous prime-boost regimens, have generally proved effective for protecting people from developing severe COVID-19 disease.

A mix-and-match strategy of the AstraZeneca followed by the mRNA (Pfizer or Moderna) vaccines have been more thoroughly tested than an mRNA vaccine followed by the AstraZeneca vaccine. However, the AstraZeneca booster has also proven effective at blocking severe COVID-19 disease.

From a basic immunology perspective, swapping vaccines should help to focus the immune response on the SARS-CoV-2 spike protein encoded in the vaccines, rather than the other components of the vaccine, thus providing the intended boost in protection against the coronavirus. The AstraZeneca booster will be most useful for people that had strong side effects to the mRNA vaccines or have histories of myocarditis and pericarditis.

Finally, much of the global population is still waiting for their initial vaccines; sharing vaccines with developing nations is important for reducing the emergence of new variants.