How do the Pfizer and Oxford AstraZeneca COVID-19 vaccines compare? And how do they affect different people?

March 8, 2021

This article has not been updated recently

More than 700,000 ZOE COVID Symptom Study contributors have logged their jabs in the app, together with daily health reports about any after effects they might have experienced. 

All this data is helping us to understand the effects of COVID-19 vaccines and how well they are working across the UK.

Initially, we only had data from the Pfizer vaccine, but now we have enough data from both the Pfizer and Oxford AstraZeneca vaccines to compare their performance. 

In our latest expert webinar, Professor Tim Spector, ZOE COVID Symptom Study lead, Dr Anna Goodman, Infectious Diseases Consultant at Guy's and St Thomas' Hospital in London, and Professor Ellie Barnes, an expert in immunology from Oxford University, share our latest data about how the different vaccines are affecting different groups of people and why immunity varies for different groups. 

How do the Pfizer and Oxford AstraZeneca vaccines compare in terms of performance and side effects?

Both vaccines work in very similar ways to build immune protection against SARS-CoV-2 - the coronavirus that causes COVID-19.

“The vaccine contains some code that gets your body to produce something called an antigen, which is a tiny bit of the COVID virus spike protein. Then if you see the spike protein again in the form of the real COVID virus, your body recognises it, and you have some immunity,” Anna explains.

“Both of these two vaccines, the Pfizer vaccine and the Oxford AstraZeneca vaccine, induce really high levels of antibodies and T cells, and both of those components may play a role in protecting you from a subsequent infection,” says Ellie. 

Our initial data shows that both vaccines reduce the risk of infection to a similar extent seen in large scale clinical trials. 

“We see around a 70% reduction in mild disease after the vaccine, which is what we would expect. It’s really good news,” says Tim.

We also found that for the first vaccine dose, around three in ten people who had the Oxford AstraZeneca vaccine reported whole body (systemic) after-effects, such as tiredness, headache or chills compared with around one in ten people who had the Pfizer jab. 

And we previously reported that people tended to feel worse after their second dose of the Pfizer vaccine, or after their first dose if they had previously had COVID-19.

“We saw in the trials that people get more symptoms with the first AstraZeneca dose and second Pfizer dose,” explains Anna. “But, reassuringly, the data shows that in the real world, the rates of side effects were much lower than we expected from any of the trials.”

Why do different people have different responses to the vaccine?

We are all individuals with our own unique biology, so it makes sense that vaccine performance and after-effects can vary from person to person. 

For example, it is well known that male and female immune systems function differently, and women often have better immune responses to the flu vaccine than men. But many other factors influence your immune function, including your diet and lifestyle, as well as your underlying genetics and any other health conditions. 

As a result, different people will have varying levels of immune response after the vaccination and can experience a range of after effects. 

“Some people will make more antibodies and have increased T-cell responses after vaccination compared with others. But at the moment, we can’t associate measured immune responses with protection. Even if you have a relatively low measure of antibodies after vaccination, you may still be fully protected,” says Ellie. 

However, the team warned against using commercially available antibody tests to measure your immunity following the vaccine. These tests usually measure antibodies to other parts of the virus that aren’t used in the vaccine and can only tell you if you’ve been exposed to the real virus, not whether you have had an immune response to the vaccine. 

“Just because you've had a negative antibody test doesn't mean that your vaccine hasn't worked,” says Anna. 

We found that females, younger people, and people who had a previous COVID infection were more likely to report after-effects from either vaccine. But having side effects does not mean you are more or less protected than someone with no side effects.  

“The side effects that you get are related to your innate immune response, which is your immune system's first line of defence to either an infection or to a vaccine. That is quite a separate thing from the kind of T-cells and antibodies that protect you from subsequent infection later on, which takes much longer to evolve,” says Ellie. 

How will people with a weakened immune system respond to the vaccine?

“There are hundreds of thousands of people in the UK that are immune suppressed,” says Ellie. 

This includes people with inherent problems with their immune systems, chronic diseases like renal disease, liver disease or cancer, and people who take immune-suppressive therapies to control conditions like rheumatoid disease or inflammatory bowel disease. 

“It's a very big issue, which has been ignored so far in the phase three trials for all the vaccines,” says Ellie.

Although the vaccines are considered safe for people with weakened immune systems, we don’t know much about how effective they are in this group.

This is an important question that Ellie and her colleagues are addressing through a new research study called Octave, which is currently recruiting up to 5,000 people across the UK with health conditions that lower their immunity, including cancer, inflammatory arthritis and kidney and liver disease

You can play your part in contributing to vital research about vaccine safety and efficacy by downloading the ZOE COVID Symptom Study app, logging your jab, and taking just a minute every day to log your health.

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