Side effects from the Oxford and Pfizer vaccines are about the same

10 March 2021
What was claimed

The AstraZeneca Covid-19 vaccine causes more side effects than the Pfizer vaccine.

Our verdict

This misunderstands a summary of the evidence from the MHRA. Clinical trial data suggests that the two vaccines’ side effects are about the same.

This article was published before the MHRA announced a possible link between the AstraZeneca vaccine and a very rare type of blood clot, which can be fatal. It was correct at the time of writing. Our latest fact checks about Covid-19 and the pandemic can be found here.

An article on the Express website gives a very unclear account of the most common side effects from the two Covid-19 vaccines currently being used in the UK.

In the process, the article wrongly implies that side effects with the AstraZeneca/Oxford vaccine are more serious or more common than those with the Pfizer/BioNTech vaccine.

Based on what we know from clinical trials, mild side effects are very common, and the type and frequency of these side effects appears similar with both vaccines.

What does the article say?

The article mixes up two different sources of information about side effects—the clinical trial data, and the Yellow Card reports—which causes confusion throughout.

The Medicines and Healthcare products Regulatory Agency’s (MHRA) weekly Yellow Card summary is cited as the source of information for the article.

This takes data from the Yellow Card scheme, which allows anyone to report a health problem following vaccination. As such, Yellow Card reports do not give a balanced picture of the side effects being experienced in the real world, because not all of them are being reported.

It’s also important to remember that Yellow Card reports are just suspected side effects, not proven ones.

As the MHRA says: “Some events may have happened anyway, regardless of vaccination. This is particularly the case when millions of people are vaccinated, and especially when most vaccines are being given to the most elderly people and people who have underlying illness.” 

We have written about this before.

However, the Express article actually uses data on side effects from the two vaccines’ clinical trials. These trials took place last year, in order to prove that the vaccines were safe and effective. Although they are mentioned in the Yellow Card summary, they are a different source of information, which was collected in a more systematic way.

The vaccines’ clinical trials recorded health information about thousands of people—some vaccinated, some not—in order to compare the two groups and see if being vaccinated was effective at preventing Covid, and whether it led to any other health problems. This is a much more reliable source of information about the actual effects of being vaccinated—including the side effects.

What the Express got wrong

The article says: “In regards to the Oxford-AstraZenenca [sic] vaccine, more than 23,000 participants have been involved in the Yellow Card's latest report.”

This muddles the 23,745 participants who took part in the vaccine’s clinical trials last year with the millions of patients who have so far received the vaccine in the UK—generating 42,917 Yellow Card reports so far.

It goes on to say that the AstraZeneca vaccine “seemingly led to more adverse side effects than the Pfizer jab.” This is inaccurate.

If the Express is referring to side effects listed in the Yellow Card reports, then it is using unreliable data. As the summary itself says: “Yellow card data cannot be used to derive side effect rates or compare the safety profile of COVID-19 vaccinations as many factors can influence [adverse drug reaction] reporting.”

If the Express is referring to the clinical trial data, then it has described its findings in a very misleading way. The AstraZeneca vaccine has “more adverse side effects than the Pfizer jab” only in the sense that the list of “very common” side effects for AstraZeneca is longer, in part because the MHRA’s Information for Healthcare Professionals on the two vaccines defines their side effects in different ways.

Information on the Pfizer trial lists seven symptoms that were “very common”, meaning they occurred in at least one in ten participants—including arthralgia, or joint pain, as mentioned in the Express headline.

Information on the AstraZeneca trial lists 10 symptoms, but records “injection site tenderness” and “injection site pain” as different symptoms, whereas the Pfizer data does not distinguish between the two.

Overall, both trials suggest that most people experience some pain or tenderness in the region of the injection, about half experience headache and/or fatigue, and a significant minority experience muscle pain, fever, chills and/or joint pain. In both trials, the symptoms were usually mild or moderate and resolved within a few days.

It’s also important to remember that many of these symptoms occur quite often for other reasons. In the more detailed data from both the Pfizer and the AstraZeneca trials, people in the control groups (who were given placebos or another vaccine) often experienced symptoms too. Indeed the majority of the control groups in both trials reported a symptom of some kind.

There are currently no signs of serious side effects or any additional safety concerns arising from the Yellow Card reports so far.

Update 16 August 2021

A line was added at the beginning of this article to reflect new information about vaccine safety, which appeared after it was published.

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