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Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Jaya Dantas is Professor of International Health in the School of Population Health at Curtin University
In December 2021, AstraZeneca released a media statement that the three-dose course of AstraZeneca's (AZN.L) COVID-19 vaccine is effective against the rapidly-spreading Omicron coronavirus variant, citing data from an Oxford University lab study. The study showed that after a three-dose course of the vaccine, neutralising levels against Omicron were similar to those against the virus's Delta variant after two doses. The results indicated that antibody levels against Omicron after the booster shot were higher than antibodies in people who had been infected with and recovered naturally from COVID-19.
A further media release in Jan 2022, by AstraZeneca reported on a Phase IV trial in a preprint with The Lancet on SSRN. This study undertaken at two sites in Brazil substantially increased antibody levels following a primary vaccine series with CoronaVac (Sinovac Biotech). These data add to the growing body of evidence supporting Vaxzevria as a third dose booster irrespective of the primary vaccination/s that had been administered. The study data show that AZ it can be used as a third dose booster after two initial doses of the same vaccine, or after mRNA or inactivated vaccines, strongly boosts immunity against COVID-19.
In Australia, the TGA has provisionally approved AstraZeneca as a booster although the preference is for Pfizer and Moderna booster. India has already been administering the third dose of AstraZeneca to its population along with the UK and several other countries.
However, as reported by the WHO, we still have significant vaccine equity issues in the world especially in Africa. Availability and access to COVID-19 vaccines offer the best defence in slowing the pandemic, in turn saving lives, and restoring economic activity. Currently, global vaccine production is at 1.5 billion doses per month, thus there is enough supply to equitably distribute vaccines. The vast majority of vaccines have been administered in high- and upper-middle-income countries that are providing boosters and in some cases fourth doses. If vaccine doses had been distributed equitably, the elderly and health workers globally would have received their first and second doses. It is thus vital and important for developed countries to increase their contribution to the COVAX initiative so that 70% of the world can be vaccinated by mid-2022.
Dr Roger Lord is a senior lecturer (Medical Sciences) with the Faculty of Health Sciences at The Australian Catholic University and Visiting Research Fellow with The Prince Charles Hospital (Brisbane)
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.
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 innoculation.
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.
Dr Andy Flies is a Senior Research Fellow at the Menzies Institute for Medical Research, University of Tasmania
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.
Associate Professor Vinod Balasubramaniam is a Molecular Virologist and the Leader of the Infection and Immunity Research Strength from the Jeffrey Cheah School of Medicine & Health Sciences at Monash University in Malaysia
It’s important to increase the vaccine portfolio in the fight against COVID-19, in particular against the current highly contagious Omicron variant, a multi-pronged approach is definitely needed for us to win the war.
While the mRNA vaccines are really potent in inducing a high titre of antibodies, the viral vector vaccines such as the Astra Zeneca vaccine is really efficient in inducing robust, longer-lasting T-cell responses in comparison with others according to many studies.
Diversifying the vaccine portfolio in including Astra Zeneca vaccines as part of the booster dose regimen is important especially when used as a mix-and-match for individuals who experienced adverse reactions with their primary vaccinations with other types of vaccines (e.g., mRNA vaccines).
In addition to that, recent published data have shown that a booster shot of AstraZeneca’s COVID-19 vaccine generated higher antibody responses against coronavirus variants including alpha, beta, gamma, delta and omicron. The increase was observed in people who had previously been vaccinated with either AstraZeneca’s shot or an mRNA vaccine.
Evidence indicates the protection generated by vaccines can diminish over time, perhaps rapidly, and that the Omicron variant is better able to evade immunity from vaccination or previous infection (which still safeguard against serious illness). Booster shots (an additional dose of vaccine following the initial course) play an important role in increasing protection (in terms of effectiveness and antibody protection) against newer variants such as the currently dominant Omicron strain.
Overall, recent data have shown a third dose of Astra Zeneca vaccine after two initial doses of the same vaccine, or after mRNA or inactivated vaccines, strongly boosts immunity against COVID-19.
