Pre-clinical (animal) trials were conducted in rhesus macaques, mice and ferrets. These trials showed that there were no adverse safety events or evidence of immune enhanced inflammatory disease and robust neutralising antibody and cellular immune responses were triggered. In animals, the vaccine appeared to reduce disease severity, but did not prevent infection or transmission (nasal shedding of the virus still occurred).

Phase 1/2 clinical trials showed antibody response against the SARS-CoV-2 spike protein peaked by day 28 (and remained elevated to day 56) in participants who only received 1 dose of vaccine. Responses following natural exposure to COVID-19 infection were compared with vaccine responses and were in similar ranges.

Phase 2/3 studies showed a clear impact on specific antibody response following a booster dose at day 56. This response was seen across all age groups. There was no increase in the anti-vector antibody responses.

Safety profile

Side effects were more common on the day following vaccination, and were mild to moderate in severity. Pain and tenderness at the injection site (83%), fatigue (68%) and headache (70%) were the most common symptoms in participants receiving Vaxzevria (AstraZeneca). Fever occurred in 18% of trial participants and flu-like symptoms of malaise (61%) and muscle ache (60%) were also common.

Reactions were less common in older adults (aged over 55 years) and following the second dose.

Vaccine Efficacy

Phase 3 US trials showed a vaccine efficacy of 79% at preventing symptomatic COVID-19 and 100% efficacy at preventing severe disease and hospitalisation. Efficacy in participants aged 65 years and over was 80%.

Pooled data from phase 2/3 studies in the UK and Brazil found an efficacy of 66.7% (57.4-74.0) for preventing symptomatic COVID-19 following 2 doses of Vaxzevria and 76% following one dose. Protection did not wane during the initial 3 month period following first vaccination with antibody levels maintained. There were no hospital admissions for COVID-19 in the vaccinated group from 21 days following first vaccination.

A longer prime boost interval (>12 weeks) produced higher efficacy (81.3%) than a short interval (55.1% at <6 weeks).

Post-licensure surveillance


An independent expert review of 7 reported cases in Australia of suspected anaphylaxis following administration of the COVID-19 AstraZeneca vaccine has concluded that there is no increased risk of anaphylaxis associated with the vaccine above the expected rate for any other vaccine. Anaphylaxis is a very rare side effect that can occur with any vaccine.

Thrombosis with thrombocytopenia syndrome

Thrombosis with thrombocytopenia syndrome (TTS), is a rare and new syndrome which has been reported in people who have received an adenoviral vector COVID-19 vaccine (eg. Vaxzevria (AstraZeneca) and Johnson & Johnson/Janssen). The syndrome is distinct from other clotting conditions as it is characterised by thrombosis formation (blood clots) combined with thrombocytopenia (low platelet levels).  Symptoms occur 4-30 days following vaccination. Early recognition can lead to effective treatment.

Capillary leak syndrome

Cases of capillary leak syndrome have been reported following vaccination with Vaxzevria (AstraZeneca). The syndrome results in fluid leaking from capillaries (small blood vessels) into surrounding tissue and can lead to severe organ damage or death if left untreated. A causal link between capillary leak syndrome and the vaccine has not been established, however as a precautionary measure it is recommended that individuals with a history of capillary leak syndrome receive an alternate brand of COVID-19 vaccine.

Guillain-Barre Syndrome (GBS)

Following intensive post-licensure surveillance and investigations by the TGA and other international vaccine regulators, there is growing evidence of a possible link between GBS and Vaxzevria (AstraZeneca). In response to this GBS has been listed as a precaution for those receiving Vaxzevria.

Immune thrombocytopenia (ITP)

An association between Vaxzevria (AstraZeneca) and immune thrombocytopenia (ITP) has been reported. Rates of ITP following vaccination have been reported more frequently than the expected background rates of ITP. A link between COVID-19 vaccines and ITP is currently being investigated.


Authors: MVEC Education Team

Reviewed by: Rachael McGuire, (MVEC Education Nurse Coordinator) and Francesca Machingaifa (MVEC Education Nurse Coordinator)

Date: September 15, 2022

Materials in this section are updated as new information and vaccines become available. The Melbourne Vaccine Education Centre (MVEC) staff regularly reviews materials for accuracy.

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