Fact Check-Vaccine trials were not rushed; possible link to extremely rare blood clots is unlikely to be detected in trials 

A potential link between blood clots and the Oxford/AstraZeneca COVID-19 vaccine is not evidence the jab was “rushed” through clinical trials, experts have told Reuters. The vaccine was tested under all the usual conditions; however, blood clots are hard to detect as an adverse reaction due to the extreme rarity of the condition.

More than 10,000 Facebook users have shared a video in which a pathologist claimed blood clots found in recipients of the AstraZeneca vaccine “would have been discovered in the normal course if they had extended the testing period, which they chose not to do” (here). He added that the British government was “reckless” to introduce a jab that he believed was “rushed to market quite inappropriately” and has “certainly not been shown to be safe”.

However, it is not accurate to say the vaccine was rushed to market, while experts have told Reuters that extremely rare side-effects would likely be picked up only during mass roll-outs.

“The safety and efficacy of COVID-19 vaccines currently in use have been rigorously tested, and the trials did not skip any steps”, Dr Namandje Bumpus from the Johns Hopkins University School of Medicine (here) told Reuters in an email.

AstraZeneca’s Phase 3 clinical trial of more than 23,000 participants reported 175 severe adverse events, over half from the control group that did not receive the vaccine. Just three events were considered possibly related to either the control or real jab, and blood clots were not mentioned in the analysis published in December 2020 (here).

Reuters also presented the pathologist’s claim to experts at Meedan Health Desk (here), who wrote in an explainer: “Even with diverse and large study groups, it is possible that researchers may not uncover some side effects, reactions, or serious adverse events during clinical trials. Events that only occur in a few people out of a million or more can be very difficult to detect.” (here)

The scientists added: “Many millions of people are being vaccinated with the new COVID-19 vaccines, it is not surprising that some rare events, like allergic reactions and blood clots, are being reported. Researchers now need to work to determine if the events are related to the vaccines, and why.”

The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has reported an average 4.9 cases of blood clots for every million doses of the AstraZeneca vaccine; however, a definitive link between the two has not been confirmed.

“On the basis of this ongoing scientific review, MHRA has concluded that the evidence of a link with COVID-19 Vaccine AstraZeneca is stronger, but more work is still needed,” the agency said in its most recent report (here). It continues to say the benefits of the jab far outweigh the risks for the vast majority of people but cautions the risks may be more “finely balanced” in younger people, who will receive an alternative jab where possible.


False. Approved vaccines for COVID-19 have undergone the usual trial procedures. Extremely rare potential adverse reactions occurring in a handful of people per million doses would only be detected in a mass rollout.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here .