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Why have pediatric trials of the AstraZeneca vaccine been stopped?

A UK-based trial on children aged from six to 17 has been stopped due to concerns over causal links between the drug and clotting disorders.

A medical worker holds a vial of AstraZeneca COVID-19 vaccine at a vaccination centre in Ronquieres, Belgium April 6, 2021.

A trial of the Oxford-AstraZeneca covid-19 vaccine in children has been halted while the UK’s drug regulator, The Medicines and Healthcare products Regulatory Agency (MHRA), investigates potential links between the vaccine and thrombosis in adults. The trial, on 300 volunteers between the ages of six and 17 put forward with parental approval, started in February with the aim of determining whether the AstraZeneca vaccine produces a strong immune response in children in that age group.

The pausing of the UK-based trial comes amid global concern that the AstraZeneca vaccine can cause rare blood clotting in adults. A European Medicines Agency (EMA) official, apparently speaking in an official capacity, said there appeared to be a link between the vaccine and cases of clotting in adults. EMA investigations into a potential causal link between the vaccine and thrombosis is ongoing, although the regulator has consistently stated “the benefits of the AstraZeneca vaccine in preventing covid-19, with its associated risk of hospitalization and death, outweigh the risks of side effects.”

Confidence in AstraZeneca vaccine waning

Nevertheless, 18 countries including Sweden, Denmark, Norway, Italy, France, Germany, Spain, Portugal and the Netherlands briefly halted distribution of the AstraZeneca vaccine in March after cases of severe coagulation disorder and strokes affecting mostly younger recipients were reported in EU countries including Austria and Denmark.

The halting of the European rollout of AstraZeneca vaccines also had worldwide repercussions, with Canada, Thailand, Mexico and the Philippines among countries to suspend or limit the drug’s administration. While most of those countries have now resumed the rollout, confidence in the vaccine program has been shaken by the possible link with clotting disorders.

According to a recent BBC report, 30 cases of clotting had been reported among AstraZeneca recipients in the UK as of 2 April, of which 22 were reports of cerebral venous sinus thrombosis, or clotting on the brain. The MHRA said the number was “very small” given that these came from a total of 18.1 million vaccines given in the UK up to 24 March.

"No concerns over the safety of AstraZeneca trial"

Speaking about the Oxford-AstraZeneca trial in children, Professor Andrew Pollard of Oxford University said while there are no concerns over the safety of the trial itself, it was decided it would be prudent to wait for more information before resuming.

“"While there are no safety concerns in the pediatric clinical trial, we await additional information from the MHRA on its review of rare cases of thrombosis/thrombocytopenia that have been reported in adults, before giving any further vaccinations in the trial."

The EMA, whose safety committee "has not yet reached a conclusion and is continuing the analysis," and the MHRA are both expected to release their conclusions on the matter in the coming days. The MHRA, for its part, has also reiterated that the benefits of receiving the covid-19 vaccine far outweigh the risks.


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