Coronavirus: Kiwi-led research compares AstraZeneca COVID-19 vaccine risks with Pfizer's

A joint New Zealand and UK study into the risks posed by two COVID-19 vaccines is more good news for our vaccine rollout, scientists say.

They looked at health records of more than 2.5 million Scots who got either the AstraZeneca or Pfizer-BioNTech vaccine, and found while there was a "very small increase" in the risk of a minor and easily treatable blood condition for those getting the AstraZeneca jab, no such risks were identified in the group who got the Pfizer. 

Victoria University of Wellington epidemiologist and professor of population health Colin Simpson, who co-led the research, said it was good news for New Zealand's ongoing vaccine rollout, which is using the Pfizer vaccine exclusively. 

"We wanted to investigate the possible link between COVID-19 vaccines and low platelet count, blood-clotting and bleeding events. This was to provide urgently needed information on vaccine safety," he told Newshub.

"Very encouragingly, we didn't identify any increased risk in those receiving the Pfizer-BioNTech vaccine, but what we did find was a very small increase in the risk of idiopathic thrombocytopenic purpura - IPT, a bit of a mouthful - in those with the Oxford/AstraZeneca vaccine." 

Patients with ITP have low counts of blood platelets, which help stop bleeding when vessels are damaged. Having too few of them increases the risk of bleeding, and in rare cases, blood clots. 

Just eleven cases were found per million doses administered - similar to the risk posed by the Hepatitis B, MMR and flu vaccines. None were serious. 

"ITP is not so rare as central venous sinus thrombosis [brain clotting], but is also much less likely to cause fatal outcomes," said Peter McIntyre, medical advisor at New Zealand's Immunisation Advisory Centre.

While all vaccines go through large trials before getting approved, extremely rare events can be hard to pick up. No link to an increased risk of deadly blood clots was found in the study, the scientists saying even 5 million people might be too few to detect any increased risk, it's that rare.

"There's a much higher risk of these outcomes if you develop COVID-19," said Dr Simpson, urging countries not to ditch the AstraZeneca jab, which is easier to transport and cheap to make. "Although an association with the AstraZeneca vaccine was found, we have to stress that causality is yet to be demonstrated." 

The Scottish rollout has largely focused on the elderly. The few people who developed ITP tended to be older - around 69 years old - and had underlying health conditions, such as coronary heart disease, diabetes or chronic kidney disease.

Dr Simpson says they'll be keeping a close eye on the data as more young Scots get vaccinated. 

Dr McIntyre said the research showed the Pfizer vaccine was "coming up roses again". He said Scotland, with a population about the same as New Zealand, was a good place to study - the research couldn't be done here because we don't have COVID-19. 

About half-a-million doses have been administered in New Zealand to date. New Zealand opted for the Pfizer jab after early results overseas proved its efficacy in the real world, without headline-grabbing reports of potentially fatal side-effects. 

The research was published in journal Nature Medicine