AstraZeneca vs Pfizer COVID Vaccine

AstraZeneca and Pfizer were two of the pharmaceutical companies behind the COVID-19 vaccines used most widely in the UK. Both vaccines proved highly effective at preventing severe illness, but they were built using different technology and have different characteristics worth understanding.

	AstraZeneca (Oxford-AstraZeneca)	Pfizer (Pfizer-BioNTech)
Technology	Viral vector (modified chimpanzee adenovirus)	mRNA
Developed with	University of Oxford	BioNTech
Original dosing	Two-dose course	Two-dose course
Storage	Normal fridge temperatures	Originally required ultra-cold storage
Protection against severe illness	High	High, slightly higher efficacy against symptomatic infection in early trials
Notable rare risk identified	Very rare blood clotting condition	Very rare myocarditis, mostly in younger men
Approved for use in the UK	Yes	Yes

How the AstraZeneca vaccine works

The Oxford-AstraZeneca vaccine, developed in partnership with the University of Oxford, uses what's known as a viral vector approach. It takes a harmless, weakened version of a chimpanzee adenovirus (a common cold virus in chimpanzees, which cannot cause illness in humans) and modifies it to carry the genetic instructions for the spike protein found on the surface of SARS-CoV-2. Once injected, your cells briefly produce this spike protein, prompting your immune system to learn how to recognise and fight the real virus if you're exposed to it later.

This viral vector technology was not entirely new when COVID-19 vaccines were developed. It builds on research previously used for vaccines against diseases such as Ebola.

How the Pfizer vaccine works

Pfizer's vaccine, developed with the German biotechnology company BioNTech, uses messenger RNA (mRNA) technology. Rather than using a viral vector, it delivers a small piece of genetic code that instructs your own cells to temporarily produce the spike protein directly. Your immune system then responds to this protein in the same way, building up recognition and defences against the real virus.

mRNA vaccine technology had been in development for years before the pandemic, but COVID-19 marked the first time it was used at this scale in an approved vaccine.

Comparing efficacy

Clinical trials showed both vaccines were highly effective at preventing severe illness, hospitalisation and death from COVID-19, which was always the most important measure of success. Early trial data suggested the Pfizer vaccine had a slightly higher efficacy rate at preventing any symptomatic infection compared with AstraZeneca, though both showed strong, comparable protection against the outcomes that matter most: serious illness and death. Real-world effectiveness can also shift over time as new variants emerge, which is one reason booster doses have continued to be offered to eligible groups.

Comparing side effects

Both vaccines commonly caused short-term side effects such as a sore arm, tiredness, headache and mild fever, typically resolving within a day or two. A small number of more specific risks were identified for each as monitoring continued after rollout: a very rare blood clotting condition was linked to the AstraZeneca vaccine in a small number of recipients, while a rare risk of myocarditis, inflammation of the heart muscle, was linked to mRNA vaccines including Pfizer's, mostly in younger men. Both risks are rare and health authorities adjusted recommendations for specific age groups as this evidence emerged.

Dosing and logistics

Both vaccines were originally given as a two-dose course, with the second dose administered some weeks after the first. One practical difference was storage: mRNA vaccines like Pfizer's originally required ultra-cold storage, which made distribution more logistically demanding, particularly in parts of the world with less advanced cold-chain infrastructure. AstraZeneca's vaccine could be stored at normal fridge temperatures, making it easier to distribute more widely, including in lower-income countries.

Where things stand now

Vaccine recommendations and availability continue to evolve as the virus and the evidence change. Current UK vaccination programmes are generally based on updated formulations targeting more recent variants, offered to eligible groups such as older adults, healthcare workers and people with underlying health conditions. If you want to know which vaccine is currently being offered and whether you're eligible, the NHS website and your GP surgery are the most reliable, up-to-date sources.

Other vaccines used around the world

AstraZeneca and Pfizer weren't the only vaccines developed against COVID-19. Moderna produced another mRNA vaccine, similar in design to Pfizer's and also widely used in the UK and elsewhere. Novavax developed a more traditional protein subunit vaccine, offering an alternative for people who preferred not to have an mRNA or viral vector vaccine. Other vaccines, including Sinovac and Sinopharm from China and Sputnik V from Russia, were widely used in different parts of the world, though they weren't approved for use in the UK.

Booster strategy

Immunity from any vaccine can wane over time and the virus itself has continued to evolve into new variants. This is why booster doses have become a regular part of COVID-19 vaccination strategy, similar to the annual flu vaccine. In the UK, booster programmes have generally targeted older adults, healthcare workers and people with health conditions that put them at higher risk, typically offered ahead of the winter months when respiratory illness tends to peak. Booster formulations have also been periodically updated to better match circulating variants.

Frequently asked questions

Can you choose which vaccine you receive?

This depends on local supply and current guidance. In some cases people have been able to express a preference, particularly if they had a reaction to a previous dose, but availability generally determines which vaccine is offered at any given clinic.

Is it safe to mix AstraZeneca and Pfizer doses?

Research into mixed vaccine schedules has generally found them to be safe and effective and mixed schedules were used in a number of countries, including the UK, particularly where supply constraints made it necessary.

Do these vaccines still protect against newer variants?

Protection against infection can reduce against newer variants, but vaccination, including with updated booster formulations, has continued to provide strong protection against severe illness and hospitalisation.

How to check which vaccine you received

If you're unsure which vaccine you received, there are a few ways to check. In the UK, the NHS App shows your COVID-19 vaccination record, including the specific vaccine and batch number for each dose you received. Your GP surgery can also provide this information if you don't have access to the app. If you received your vaccine somewhere other than the UK, that country's equivalent health records system, or the vaccination card or certificate you were given at the time, should have the same details.

Knowing which vaccine you received can be useful in a few situations, for example if you're asked about it before receiving a booster, if you're travelling to a country with specific vaccine requirements, or simply out of personal interest in understanding your own vaccination history.

Protein-based alternatives

Alongside viral vector vaccines like AstraZeneca's and mRNA vaccines like Pfizer's, protein subunit vaccines such as Novavax offered a third approach. These work by delivering a lab-grown copy of the spike protein itself, rather than genetic instructions for your cells to make it, using a technology more similar to some long-established vaccines for other diseases. This made Novavax an option some people felt more comfortable with, given its closer resemblance to more traditional vaccine technology and it was approved as an alternative in the UK and a number of other countries.

Why different countries approved different vaccines

Not every vaccine was approved everywhere. Regulatory decisions depend on the specific evidence submitted to each country's regulator, manufacturing and supply agreements and each country's own assessment of risk and benefit for its population. This is part of why AstraZeneca's vaccine, for example, saw continued strong use in some countries while being paused or restricted for certain age groups in others, following the same underlying safety data but reaching somewhat different conclusions about how to apply it.