Several countries have already altered their advice after previous research indicated that -- in a tiny number of cases -- thrombosis with thrombocytopenia syndrome (TTS) can be a possible side effect of COVID vaccines that use an adenovirus vector, or "engineered" virus, such as those from AstraZeneca and Johnson & Johnson.

Thrombocytopenia produces potentially life-threatening blood clots with low levels of blood platelets -- the small cell fragments in our blood that prevent bleeding.

The new study, published in the journal BMJ, was the first to compare thrombocytopenia rates between adenovirus and mRNA vaccines -- such as Pfizer -- across multiple countries.

The international research team analyzed health data from more than 10 million adults in France, Germany, the Netherlands, Spain, the United Kingdom and the United States who received at least one vaccine dose between December 2020 and mid-2021.

In Germany and the UK, they matched the data of 1.3 million people who had a first dose of AstraZeneca to 2.1 million who took Pfizer.

There were a total of 862 "thrombocytopenia events" recorded in the 28 days after a first dose of AstraZeneca, compared to 520 for Pfizer, the study said.

That meant AstraZeneca's vaccine had a 30-percent higher risk of thrombocytopenia than Pfizer. 

However when it came to a second dose, there was no additional risk between any of the vaccines.

The study was observational, meaning it could not show cause and effect. But the researchers said that additional analysis found the results to be consistent.

While very rare, these risks "should be considered when planning further immunization campaigns and future vaccine development," the study said.

'Safe and effective'

Sarah Pitt, a microbiologist at the UK's Brighton University who was not involved in the research, said the "well-designed" study "adds to the data showing that all vaccines are safe and effective."

The "extremely rare" cases of thrombocytopenia occurred after just 0.04 percent of vaccine doses in Germany and the UK, she told AFP.

Most countries had already been avoiding using adenovirus COVID vaccines for the older and young age groups who are more at risk, she added.

European countries have largely pivoted towards mRNA vaccines. AstraZeneca's jab was never authorized in the US, which limited the use of the Janssen (Johnson & Johnson) COVID-19 jab to certain individuals earlier this year.

However, both AstraZeneca and Johnson & Johnson's vaccines play a pivotal role in the Covax global vaccine distribution scheme, which provides doses to lower-income countries.