Dr Angela Marley (PhD Immunology & Infectious Disease) takes a look at the available data on the AstraZeneca vaccine, and whether suspension is justified.

Recently there have been concerns over the safety of the AstraZeneca vaccine, which has led several European countries including France and Germany to suspend the roll out. The decision was made following reports of blood clots among people given the AstraZeneca vaccination. This has reduced trust in the vaccine. However, does the data support the idea that AstraZeneca’s vaccination cannot be trusted to the extent that it is worth risking Covid-19 infection instead?

This consideration was loud and clear in a quote published by the WHO on the 12th of March 2021 “More than 335 million doses of COVID-19 vaccines have been administered globally so far, and no deaths have been found to have been caused by COVID-19 vaccines. But at least 2.6 million people have been killed by the virus. And more will continue to die the longer it takes to distribute vaccines as rapidly and as equitably as possible.”

What do we know about the safety of the AstraZeneca vaccination?

The European Medicines Agency (EMA) recommended the AstraZeneca Covid -19 vaccine for authorization in the EU on the 29th January 2021. In their report the EMA states they have thoroughly assessed the data on quality, safety and efficacy of the vaccine. The data comes from combined results from 4 clinical trials in the United Kingdom, Brazil and South Africa. These studies involved around 24,000 people from 18 years of age, of which half received the vaccine and half were given a control injection (placebo or other vaccination). The results showed a 59.5% reduction in the number of symptomatic Covid-19 cases in people given the vaccine compared with those in the control group, demonstrating around 60% efficacy of the vaccine.

Adverse effects in participants were reported to be similar in the placebo and the vaccine groups. The most common side effects following vaccine administration were experienced by 1 in 10 people, including mild or moderate tenderness at the site of injection, headache, tiredness, muscle pain, general feeling of being unwell, chills, fever, joint pain or nausea. Less than 1 in 10 people experienced vomiting and diarrhea, and less than 1 in 100 people experienced decreased appetite, dizziness, sweating, abdominal pain or rash. Occasionally allergic reactions occurred following vaccination, and for this reason (as for all vaccines) the AstraZeneca COVID-19 vaccine should be given under close supervision with appropriate medical treatment available.

The EMA report goes on to explain that in line with the EU’s safety monitoring plan for Covid-19 vaccines, the AstraZeneca vaccine will be continued to be closely monitored. This is important because although large numbers of people have received the Covid-19 vaccine in clinical trials, certain side effects may only emerge when millions of people are vaccinated. Indeed, companies are required to provide monthly safety reports, and EU authorities also coordinated independent studies of Covid-19 vaccines to provide more information on the long-term safety and benefit in the general population.

What new safety concerns are there for the AstraZeneca vaccination?

As of 8th March 2021, 37 cases of thromboembolic events had been reported to AstraZeneca among 17 million people who have been vaccinated with the Covid-19 AstraZeneca vaccine in the EU and UK.  For example, in Austria a person was diagnosed with multiple thrombosis (formation of blood clots in the blood vessels) and died 10 days after vaccination, and another was hospitalized with a pulmonary embolism (blockage of arteries in the lungs) but is now recovering. Reports of blood clots have also come from other countries including Norway and Denmark.

In relation to bleeding, the UK reporting system has received 35 cases where people developed low platelets in the period following vaccination with the AstraZeneca Covid-19 vaccine. This is out of 11 million AZ vaccines delivered in the UK. Notably, this finding is similar to the equivalent figure for the Pfizer vaccine, which was at 22 reports.

Let the data do the talking

An analysis done by the EMA and other organizations comparing the incidence rate of thromboembolic events in the general population with what is being reported in people following AstraZeneca vaccination showed that 37 reports for thromboembolic events are actually lower that what you would expect. Indeed the data suggest that the rate of incidence of blood clots and bleeding after vaccination with the Covid-19 AstraZeneca vaccine is lower than the risk of such side effects with other common medication, natural infection with Covid-19 or indeed the annual incidence rates in the general population. In the Astra Zeneca clinical trials described above incidence of thromboembolic events was also lower in the vaccinated group compared with the control group, even though the number of events in total was small.

Here is a summary of current approximate incidence rates of thromboembolic events and bleeding.


© RTL Grafik - see links below for data source

Correlation or causation: what causes side effects following vaccination?

The WHO and EMA have reported that there is no indication of a causative link between the AstraZeneca vaccine and blood clots. Correlation, a perceived link between a side effect and a vaccine, is not the same as causation, a confirmed biological mechanism leading to side effects. On the 18th March the EMA published a report confirming that the benefits of the vaccine in protecting against Covid-19, which in itself increases the risk of clotting problems and may be fatal, continue to outweigh the risk of side-effects.

The EMA also highlights that the AstraZeneca Vaccine is not associated with an increased risk of blood clots and there is no evidence of a problem related to specific batches or manufacturing sites. However, even though there is no causal link the rare incidences of side effects will continue to be monitored and analysed to offer guidance to patients and medical professionals. As with any vaccination, the recommendation remains to talk to your doctor about any concerns you may have on getting vaccinated against Covid-19. Ask questions so that you can do a risk vs. benefits analysis for your specific situation and circumstances.


Angela Marley, PhD Immunology & Infectious Disease