How does the AstraZeneca vaccine work?

Adenovirus-based vaccines are not a new concept, in fact, viral vector research began in the 1970s, and has since yielded safe and effective vaccines, such as the Ebola vaccine, the Janssen COVID-19 vaccine, and the AstraZeneca COVID-19 vaccine. There are many adenovirus-based vaccines in development for illnesses such as influenza and RSV. 

So, how does it work?

After the AstraZeneca vaccine is injected into our arm, the vaccine adenovirus, holding the genetic code for the SARS-CoV-2 spike protein, attaches to and enters our own body’s cells. Once inside our cell, it travels to the nucleus to deliver the genetic code. In the modified vaccine adenovirus, the genes that a normal virus would use to replicate have been disabled. It can only make the spike protein.

The genetic code for the COVID-19 spike protein is transformed into messenger RNA (mRNA). This mRNA leaves the nucleus and works with the cell’s protein-making machinery to assemble copies of the COVID-19 spike proteins. 

Parts of the spike proteins are pushed outside the cell and displayed on the cell surface. These protein fragments are then recognised by the body’s immune system defenses, provoking an immune response that targets any virus that displays the same spike proteins. 

The immune system generates antibodies and immune cells that will recognise the COVID-19 spike protein in the future and protect the vaccinated person from serious disease. 

How protective is the AstraZeneca vaccine? 

A vaccine’s protection is measured by its efficacy and effectiveness.

Vaccine efficacy is the measure of a vaccine’s protection in a clinical trial setting. Clinical trials are strictly controlled. They measure the vaccine’s protection within a population compared to a ‘control group’ which receives a placebo, or a different vaccine. In the clinical trial for the AstraZeneca vaccine, the control group was given a meningococcal vaccine instead of the COVID-19 vaccine.

Early data for the AstraZeneca vaccine in Brazil, South Africa, and the UK, showed that two doses of the vaccine, given four weeks apart, provided an efficacy against symptomatic COVID-19 for 6 out of 10 vaccinated people (95% confidence interval [CI] 41.0 – 75.7%). 

Further trials in the US, Peru, and Chile reported a higher efficacy rate, with 7 out of 10 vaccinated participants protected against symptomatic disease (95% CI, 65.3 – 80.5%), with the highest rates of protection for participants aged 65 years or older (estimated efficacy around 8 out of 10 protected against symptomatic disease, 95% CI, 54.2–94.1).

Among the 17,000 participants, there were no severe or critical cases of COVID-19 observed in the vaccinated group, compared with 8 cases out of 8,550 participants who received the placebo. 

Vaccine effectiveness is measured in real-world situations, with a larger and broader population. One way to measure effectiveness is to compare vaccinated people with unvaccinated people to see if they develop COVID-19 symptoms.  

Early data prior to the emergence of the Delta variant, shows that one dose of the AstraZeneca vaccine protected 8 out of 10 people against hospital admission (28–34 days post-vaccination). A higher protective effect was seen against hospitalisation and death due to COVID-19 from 14 days after the first dose of AstraZeneca or the Pfizer vaccine. 

Overall, the AstraZeneca vaccine data shows that there are high levels of protection against symptomatic disease, hospitalisation, and death from COVID-19.

What is the difference between the Pfizer COVID-19 vaccine and the AstraZeneca COVID-19 vaccine?

There are a few key differences between the Pfizer vaccine (mRNA vaccine) and the AstraZeneca vaccine (viral vector vaccine). 

Firstly, the method of delivery is different. While both vaccines are carrying instructions that encode for the COVID-19 spike protein, AstraZeneca uses a harmless virus to deliver its package, whereas the Pfizer vaccine uses a lipid bubble. 

Secondly, the AstraZeneca vaccine converts its genetic code from DNA to mRNA. Once converted, the mRNA helps to create the COVID-19 spike protein and begin the immune system response. The Pfizer vaccine does not need to complete this conversion, it initiates the spike protein production immediately. 

How effective is the AstraZeneca vaccine against the Delta variant?

Data shows that two doses of the AstraZeneca vaccine showed a 67% effectiveness (95% CI 61–72%) against symptomatic disease caused by the Delta variant, in comparison with the Alpha variant (74.5%, CI 68–89%). The UK Health Security Agency found that the effectiveness against hospitalisation with Delta was 71% (CI 51–83%) after one dose and 92% (CI 75–97%) after the second dose. 

Overall, this shows that the AstraZeneca vaccine demonstrates high effectiveness against the Delta variant, especially against hospitalisation. 

How long does protection last?

It remains uncertain how long AstraZeneca will protect against COVID-19. The duration of protection against COVID-19, both after vaccination and after the SARS-CoV-2 infection, is highly dependent on a combination of the following factors and will vary between individuals, countries, and changes in the virus.

Will the AstraZeneca vaccine cause a positive COVID-19 test?

No, the AstraZeneca vaccine will not cause a positive COVID-19 test. The COVID-19 nasal swab detects the COVID-19 virus’ genetic material, not the proteins of the virus, such as the antibodies or spike protein. You must be infected by the virus for the genetic material to be detected inside your nose. The viral vector from this vaccine cannot be detected by a COVID-19 test and nor does it travel to your nose because it cannot replicate and spread beyond your arm muscle.

Last updated: 14 April 2022