A new study has given more details about the "rare but devastating" blood clotting complications associated with the Oxford-AstraZeneca Covid-19 vaccine.
In a peer-reviewed paper published Wednesday in The New England Journal of Medicine, scientists from Oxford University Hospitals NHS Foundation Trust and other institutions analyzed the first 220 cases of the condition reported in the United Kingdom.
The Oxford-AstraZeneca vaccine — now one of the most widely used Covid vaccines in the world — was rolled out in the U.K. in January, making it the first country to administer the shot.
A very small number of people who have been inoculated with the AstraZeneca vaccine have suffered blood clots. The condition, described by health authorities as "extremely rare," is characterized by blood clots accompanied by low platelet levels.
The new study used data from 294 patients who presented to U.K. hospitals between March 22 and June 6. Of those, 170 definite and 50 probable cases of the rare clotting — which scientists referred to as vaccine-induced immune thrombocytopenia and thrombosis — were identified.
All of those patients had been given their first dose of the Oxford-AstraZeneca shot, and went to the hospital with symptoms between five and 48 days after their vaccination. The median time between patients receiving their vaccination and going to the hospital was 14 days, the findings showed.
The overall mortality rate for VITT in the study was 22%.
Researchers also found that 41% of patients presenting with VITT had no diagnosed underlying health issues. Of those who reported a past or current illness, the study found that no conditions or medications were prevalent that "would be unexpected in the general population."
"Against the backdrop of a successful vaccination program in the United Kingdom, VITT has emerged as a rare but devastating complication," the study's authors said in their report. "We have found that it often affects young, otherwise healthy vaccine recipients and that it is associated with a high mortality."
"In our cohort, 85% of the patients were younger than 60 years of age, despite the predominance of (Oxford/AstraZeneca) vaccination in older adults," scientists noted.
As a precaution, the U.K. has been offering people under the age of 40 an alternative to the Oxford-AstraZeneca vaccine since May.
People identified as having suffered with VITT were aged between 18 and 79, with the median age being 48, the study showed.
Up to and including July 28, an estimated 24.8 million first doses of Oxford-AstraZeneca's Covid vaccine had been administered in the U.K., with an estimated 23.6 million second doses received.
As of July 28, government figures show that for every million first or unknown doses of the Oxford-AstraZeneca shot, 14.9 people developed rare blood clotting with low platelet levels. That fell to 1.8 cases per million following a second dose of the vaccine.
The overall case fatality rate for that period was 18%, the government data showed, with 73 deaths. Six of those occurred after the second dose.
At the end of last month, AstraZeneca published a study which found the rate of VITT after the first dose of its vaccine was 8.1 per million, which decreased to 2.3 per million after a second dose.
Up until July 28, 411 suspected cases of VITT had been reported in the U.K., according to official data.
In a statement on Thursday, AstraZeneca said the research published in the New England Journal of Medicine was drawn from "a small sample size."
"Recent real-world evidence drawn from millions of individuals shows that AstraZeneca's vaccine has a comparable safety profile with other vaccines and that incidences of thrombosis with thrombocytopenia are extremely rare and treatable," said a spokesperson.
The spokesperson added that there was "a far greater risk" of rare blood clotting events through contracting Covid-19.
"Vaccines remain the most effective protection against Covid-19 and are the best route out of this pandemic," they said.
Both the U.K. and the EU's medicines regulators have identified possible links between the Oxford-AstraZeneca vaccine and rare blood clots.
In April, the company said it would comply with regulatory requests in the U.K. and Europe to update its Covid vaccine labels. However, it stressed the WHO had said "a causal relationship is considered plausible but is not confirmed."
The U.K.'s Joint Committee on Vaccination and Immunisation has consistently said that the benefits of the Oxford-AstraZeneca vaccine continue to outweigh the risks for the vast majority of people.
Multiple health authorities, including the WHO, the European Medicines Agency and the International Society on Thrombosis and Hemostasis have agreed that the benefits of administering the vaccine outweigh the risks.
Correction: This article was updated to correct the name of the institution that provided a majority of support for the research. It was Oxford University Hospitals NHS Foundation Trust.