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AstraZeneca and Oxford University on Wednesday acknowledged a manufacturing error that is raising questions about preliminary results of their experimental COVID-19 vaccine.

A statement describing the error came days after the company and the university described the shots as “highly effective” and made no mention of why some study participants didn’t receive as much vaccine in the first of two shots as expected.

In a surprise, the group of volunteers that got a lower dose seemed to be much better protected than the volunteers who got two full doses. In the low-dose group, AstraZeneca said, the vaccine appeared to be 90% effective. In the group that got two full doses, the vaccine appeared to be 62% effective. Combined, the drugmakers said the vaccine appeared to be 70% effective. But the way in which the results were arrived at and reported by the companies has led to pointed questions from experts.

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South Africa has suspended plans to inoculate its front-line health care workers with the Oxford-AstraZeneca vaccine after a small clinical trial suggested that it isn’t effective in preventing mild to moderate illness from the variant dominant in the country.

South Africa received its first 1 million doses of the AstraZeneca vaccine last week and was expected to begin giving jabs to health care workers in mid-February. The disappointing early results indicate that an inoculation drive using the AstraZeneca vaccine may not be useful.

Preliminary data from a small study suggested that the AstraZeneca vaccine offers only “minimal protection against mild-moderate disease” caused by the variant in South Africa. The variant appears more infectious and is driving a deadly resurgence of the disease in the country, currently accounting for more than 90% of the COVID-19 cases, health minister Zweli Mkhize said Sunday night.

“The AstraZeneca vaccine appeared effective against the original strain, but not against the variant,” Mkhize said. “We have decided to put a temporary hold on the rollout of the vaccine ... more work needs to be done.”

The study, which hasn’t yet been peer-reviewed, involved 2,000 people, most of whom were young and healthy. The volunteers’ average age was 31.

“Protection against moderate-severe disease, hospitalization or death could not be assessed in this study as the target population were at such low risk,” said a statement issued by Oxford University and the University of the Witwatersrand in Johannesburg.

Scientists will be studying whether or not the AstraZeneca vaccine is effective in preventing severe disease and death against the variant, Mkhize said.

Other vaccines have shown reduced efficacy against the variant, but have provided good protection from serious disease and death..

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Denmark, Norway and Iceland announced Thursday they will temporarily suspend the use of the coronavirus vaccine developed by AstraZeneca and the University of Oxford.

The Danish Health Authority said it would temporarily stop using the shot in its vaccination program as a precaution “after reports of severe cases of blood clots in people who have been vaccinated with the COVID-19 vaccine from AstraZeneca.”

“Against this background, the European Medicines Agency has launched an investigation into the AstraZeneca vaccine. One report relates to a death in Denmark. At present, it cannot be concluded whether there is a link between the vaccine and the blood clots,” the health authority said in a statement.

It did not specify how many reports of blood clots there had been, or where they had originated.

Later on Thursday, both Iceland and Norway made similar announcements.

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Scientists in Europe said they had identified a mechanism that could lead the AstraZeneca PLC vaccine to cause potentially deadly blood clots in rare instances as well as a possible treatment for it.

Two teams of medical researchers in Norway and Germany have independently found that the vaccine could trigger an autoimmune reaction causing blood to clot in the brain, which would offer an explanation for isolated incidents across Europe in recent weeks.

Several European countries briefly halted their rollouts of the vaccine this week after more than 30 recipients were diagnosed with the condition known as cerebral venous sinus thrombosis, or CVST. Most of the people affected were women under the age of 55.

The issue affected a tiny portion of those who had received the shot, however, and after investigating, the European drugs regulator ruled that the benefits outweighed the potential risks of the vaccine, and recommended vaccinations resume.

Some countries, such as Germany, France and Italy, resumed vaccinations with AstraZeneca’s shot on Friday, with an added warning that it could be linked to blood clotting. The French healthcare authority, which recorded three cases of CVST connected to the vaccine, advised the government on Friday to only administer the shot to people older than 55.

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European medical regulators on Wednesday concluded that there is a strong link between AstraZeneca’s COVID-19 vaccine and life-threatening conditions involving the unusual combination of blood clots and low levels of blood platelets.

As such, the conditions should be listed as a “very rare side effects” of the vaccine, according to the European Medicines Agency, a regulatory agency of the European Union.

The conclusion was based on the EMA’s in-depth review of 86 blood-clotting events among around 25 million people vaccinated with the AstraZeneca vaccine in Europe and the UK. Of the 86 blood-clotting events, 18 people died. Most—but not all—of the cases occurred in women under the age of 60.

The events reviewed by the EMA included 62 cases of cerebral venous sinus thrombosis (CVST), a rare form of stroke in which a clot prevents blood from draining out of the brain. The remaining 24 cases involved splanchnic vein thrombosis, which involves blood clots in the veins that drain blood from the abdomen.

Beyond the 86 cases examined, the EMA estimated that there’s a reporting rate of one case of the side effect in 100,000 people vaccinated.