AstraZeneca v Pfizer: Vaccine side effects compared as Australia changes rollout

As Australians under 50 are advised to take the Pfizer rather than the AstraZeneca jab, here’s how the side effects of each compare.

Australians under 50 years old have now been advised to take the Pfizer vaccine over the AstraZeneca jab wherever possible due to a “rare but serious risk” of blood clots.

The nation has joined a growing list of countries worldwide electing to restrict the rollout of the AstraZeneca vaccine. The UK is offering under-30s an alternative, while Canada and France are limiting the jab to over-50s, Germany to over-60s and Iceland to people aged over 70.

However, Prime Minister Scott Morrison stressed that “ultimately, the choice is with individual Australians and their doctor” and that there was “not a prohibition” on the AstraZeneca vaccine, but simply a “preference”.

Professor Paul Kelly said that the recommendation was that the AstraZeneca vaccine should only be given to adults under 50 years of age “where benefit clearly outweighs the risk for that individual’s circumstances”.

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A woman is inoculated with the Oxford/AstraZeneca COVID-19 vaccine in Medellin, Colombia, on April. Picture: Joaquin Sarmiento / AFP

The vaccine is still being recommended for over-50s because the risk of blood clots is lower and the risk of severe illness or death from COVID-19 is higher.

This means that people under 50 with an underlying health condition or frontline job may decide that the benefit does outweigh the risk.

Mr Morrison pointed out that the risk of the blood clots was only about four to six in a million.

This is far lower than the risk of clots among people taking the contraceptive pill (one in 2,000) or travelling on planes (one in 4,656 flights lasting over four hours). In fact, up to 30% of people who have COVID-19 will get thrombocytopenia, making the risk from the virus much higher, Professor Munir Pirmohamed, chair of the UK’s Committee of Human Medicines, told Sky News.

Here’s what you need to know about the expected side effects to help you decide which vaccine to take.

Vaccine side effects

In a survey by AusVaxSafety of 165,209 Australians who have had a vaccine, just over half reported experiencing an “adverse event” afterwards. Just 1.6% reported visiting a doctor or the emergency department.

The most common side effects were fatigue, headaches, injection site pain and muscle or body aches, and they were “generally mild and short-lived”.

Of 57,280 people who had a first dose of AstraZeneca vaccine, 64.6% said they experienced an adverse event, with 23.4% reporting missing work, study or routine duties for a short period — less than a day in most cases — and 1.6% visiting a doctor or emergency department.

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Of 76,743 people who had a first dose of the Pfizer vaccine, just 36.3% reported an adverse event, with just 4.5% missing work, study or duties — again for less than a day in most cases. An even smaller 0.6% of people had to visit a doctor or emergency room.

However, after the second dose of the Pfizer vaccine, 60.2% reported an adverse event with 21.7% missing some daily duties and 1.9% seeing a doctor.

Information from after the second dose of the AstraZeneca vaccine was not available.

The survey participants also reported a slight difference in the nature of the side effects. After the AstraZeneca vaccine first dose, the most common were fatigue (51.5%), headache (44.8%), muscle/body aches (42.3%).

The most common after the Pfizer first dose were injection site pain (29.6%), fatigue (18.8%) and headache (14.2%).

Other symptoms reported included chills, joint aches and pain and fever.

The findings tally with the clinical trials, which found that most reactions were mild to moderate and resolved within a few days of vaccination.

Clinical trials for both vaccines show that adverse reactions are generally milder and less frequent among people aged over 55 for AstraZeneca and 65 for Pfizer.

What about the blood clots?

Some rare cases of very unusual blood clots associated with low levels of blood platelets (thrombocytopenia) have been reported among people who have taken the AstraZeneca vaccine in several countries.

Australia has had just one case — a 44-year-old Melbourne man, who received the jab on March 22 was found to have abdominal clots and low platelet levels last week.

Australia’s Therapeutic Goods Administration (TGA) said it had received one domestic report “of a case of thrombosis involving abdominal blood vessels and thrombocytopenia”, with symptoms appearing seven days after the AstraZeneca vaccine was received.

These rare cases have presented as either a clot in the brain called central venous sinus thrombosis (CVST) or thrombosis in other sites, such as intra-abdominal venous systems, according to the TGA.

It said providers “should be aware of warning signs of a severe condition associated with thrombosis and thrombocytopenia.”

“CVST may present as a new onset persistent headache not settling with analgesia, features of raised intracranial pressure (including acute severe headache, vomiting, confusion), focal neurological deficits, and/or seizures,” it added.

If CVST or another severe thrombotic complication with thrombocytopenia is suspected in a patient who has received the AstraZeneca vaccine, they should be referred an emergency department for further assessment and haematology consultation.

The UK’s medicines regulator said it had found 79 cases of blood clotting associated with low platelet cells in the blood after a first dose of the jab — 51 women and 28 men.

Forty-four of these cases were CVST with thrombocytopenia and 35 were thrombosis in other major veins with thrombocytopenia, with 19 of the cases leading to death, including three under-30s.

The European Medicines Agency on Wednesday confirmed that the blood clotting issue would be listed as a “very rare side effect”.

Potential harms and benefits of the AstraZeneca vaccine at a low COVID exposure risk. Picture: Winton Centre for Risk and Evidence Communication

What should you look out for?

Prof Kelly said the issue had only been seen after the first dose of the AstraZeneca vaccine, “usually within four to 10 days after that vaccine”, which is why anyone who has had a first dose without any serious reaction can continue and have their second.

However, Prof Kelly warned that the clotting “is serious, and it can cause, up to a 25% death rate when it occurs”.

Symptoms to look out for include shortness of breath, chest or persistent abdominal pain, leg swelling, blurred vision, confusion or seizures and unexplained pinprick rash or bruising beyond the injection site.

Anyone with new onset of severe or persistent headache that does not respond to simple painkillers starting four days or more after vaccination should also speak to their doctor.

About 855,000 people across Australia have received the jab as of April 5, despite the Prime Minister predicting that four million Aussies would be vaccinated by the end of March. Mr Morrison had said this was because 3.1 million contracted vaccines that were expected when targets were set out in early January had not turned up.

The country had been relying heavily on the AstraZeneca vaccine, produced locally by CSL, so the new approach could drastically slow down the rollout, with knock-on implications for Australians hoping to resume normal life and travel overseas.

How the latest move will effect on vaccine confidence and uptake remains to be seen.