About COVID-19 Vaccine AstraZeneca

COVID-19 Vaccine AstraZeneca is a two-dose viral vector vaccine which protects against COVID-19 disease caused by the SARS-CoV-2 virus.

How the vaccine works

COVID-19 Vaccine AstraZeneca is a viral vector vaccine. The technology used is like that in the COVID-19 Vaccine Janssen. There is no live SARS-CoV-2 virus in the vaccine, so it is impossible to get COVID-19 from this vaccine. It is composed of two key elements: 1) a modified chimpanzee adenovirus (ChAdOx1) viral vector, which can enter human cells but cannot replicate or cause illness; and 2) the code to make SARS-CoV-2 spike protein. The key difference between this vaccine and the Janssen vaccine is that the AstraZeneca vaccine uses a chimpanzee adenovirus and the Janssen vaccine uses a human adenovirus. The aim is to use adenoviruses which we have no prior immunity to, so our bodies will not block the adenovirus before it can enter our cells and do its work. These vaccines are classified as non-live vaccines.

After entering our cells, the modified adenovirus in the vaccine instructs our body how to produce copies of the COVID-19 virus (SARS-CoV-2) spike proteins, which in turn activate the immune system against the COVID-19 virus. 

If there is later exposure to the real COVID-19 virus, our immune system can act quickly to block the COVID-19 virus from entering our cells to protect us from illness. To find out more see this New York Times article.

Vaccine effectiveness

The effectiveness data for COVID-19 Vaccine AstraZeneca, from both the clinical trials and now from extensive international use, are excellent. 

Phase 3 trial data showed an overall vaccine efficacy against symptomatic COVID-19 of 55 to 81%. Promising real-world data has suggested AstraZeneca’s COVID-19 vaccine has an effectiveness against hospital admission of at least 80% after a single dose (2, 3). There is also preprint data from UK healthcare workers showing vaccination with either Pfizer or AstraZeneca COVID-19 vaccines substantially reduces COVID-19 cases in household contacts. These findings suggest that vaccination may not only protect against disease but plays a role in reducing viral transmission (4). Data on vaccine effectiveness against the Delta variant is accumulating. There were only modest differences in vaccine effectiveness against the Alpha and Delta variants when given two vaccine doses. A more pronounced difference was seen between the first and second doses, following receipt of only one dose, highlighting the importance of receiving the complete course (5). 

Vaccine safety

There is robust safety data on COVID-19 Vaccine AstraZeneca from clinical trials and extensive real-world use. Common adverse side effects seen following vaccination include mild pain and tenderness in the arm at the site of the injection, fatigue, headache, sore muscles and fever. Reactions are generally mild and self-resolving within 1-2 days.

A very rare but serious side effect of blood clots with low levels of platelets (thrombosis with thrombocytopenia syndrome or TTS) has been seen following vaccination with the COVID-19 viral vector vaccines, mostly in those aged under 50 years and after the first dose. The incidence of TTS remains very rare (1 to <15 cases per million doses, depending on age) with the benefits of vaccination outweighing this small risk. The ages at which these vaccines are given vary in different countries taking in to account the risks associated with COVID-19 illness. COVID-19 disease itself is associated with an increased risk of blood clots (more than one in five cases have evidence of blood clots).

More information is available in this video explaining TTS in more detail.

Although this vaccine has been approved for use in New Zealand for individuals aged from 18 years, its availability is yet to be decided. If you are returning to New Zealand having received one dose of the COVID-19 Vaccine AstraZeneca, to be fully vaccinated, it is recommended that you receive one further dose of the available Comirnaty (Pfizer) COVID-19 vaccine, when you are eligible. If you received two doses of the AstraZeneca vaccine, you are considered fully immunised and do not require any further doses at this stage. 


1.    Voysey M, Clemens SA, Madhi SA, Weckx LY, Folegatti PM, Aley PK, Angus B, Baillie VL, Barnabas SL, Bhorat QE, Bibi S. Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK. The Lancet. 2021 Jan 9;397(10269):99-111.
2.    Bernal JL, Andrews N, Gower C, Robertson C, Stowe J, Tessier E, Simmons R, Cottrell S, Roberts R, O’Doherty M, Brown K. Effectiveness of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines on covid-19 related symptoms, hospital admissions, and mortality in older adults in England: test negative case-control study. bmj. 2021 May 13;373.
3.    Vasileiou E, Simpson CR, Shi T, Kerr S, Agrawal U, Akbari A, Bedston S, Beggs J, Bradley D, Chuter A, De Lusignan S. Interim findings from first-dose mass COVID-19 vaccination roll-out and COVID-19 hospital admissions in Scotland: a national prospective cohort study. The Lancet. 2021 May 1;397(10285):1646-57.
4.    Shah AS, Gribben C, Bishop J, Hanlon P, Caldwell D, Wood R, Reid M, McMenamin J, Goldberg D, Stockton D, Hutchinson S. Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households. MedRxiv. 2021 Jan 1.
5.    Lopez Bernal J, Andrews N, Gower C, et al. Effectiveness of Covid-19 vaccines against the B.1.617.2 (delta) variant. N Engl J Med. DOI: 10.1056/NEJMoa2108891.
6.    Public Health England. Information for healthcare professionals on blood clotting following COVID-19 vaccination. [updated 8 June 2021; accessed 26 July 2021] https://www.gov.uk/government/publications/covid-19-vaccination-blood-c…

Last updated: 30 July 2021