Groups find possible link between AstraZeneca COVID vaccine, blood clots

Blood clot illustration
Blood clot illustration

iLexx / iStock

Vaccine advisors from two regulatory groups in Europe today shared their latest investigation findings into blood clots in some AstraZeneca-Oxford COVID-19 vaccine recipients, with both finding a possible link.

Experts from the European Medicines Agency (EMA) and the United Kingdom's Medicines and Healthcare Products Regulatory Agency (MHRA) both emphasized that the benefits of the vaccine continue to outweigh any risks.

In a related development, the World Health Organization (WHO) vaccine safety group reviewed the latest information from the EMA, MHRA, and other countries and said the link is plausible but not confirmed, with more studies needed.

The investigation into the possible side effects has complicated vaccine rollout in many countries at a time when nations are rushing to immunize their populations against rising COVID-19 levels, much of it driven by more transmissible variants. Several countries have paused vaccination with the AstraZeneca vaccine while experts examine the side effect reports, with some limiting the vaccine's use to adults in older age groups.

EMA finds plausible explanation, asks for more studies

In a statement today, the EMA said its safety committee concluded that the unusual blood clots with low platelets should be listed as very rare side effects. It urged health providers and vaccine recipients to be aware of very rare cases that may occur within 2 weeks of vaccination and seek care immediately if they experience symptoms such as shortness of breath, chest, pain, leg swelling, and persistent abdominal pain.

For their latest analysis, they looked at 62 cases of cerebral venous sinus thrombosis (CVST) and 24 cases of splanchnic vein thrombosis, which included 18 deaths. The reports came from a European Union drug safety database and were among 25 million who had received the vaccine. Most occurred in women within 2 weeks of receiving the first dose.

As of Apr 4, European Union surveillance has received 169 reports of CVST and 53 reports of splanchnic vein thrombosis among 34 million vaccinees.

Though the group didn't determine a cause of the blood clots, it said a plausible explanation is an immune response similar to one sometimes seen in patients treated with heparin. However, they didn't find any specific risk factors. The experts asked for new studies to shed more light on a possible mechanism.

MHRA notes higher incidence in younger groups

Also today, the MHRA said based on its examination of the latest information, evidence of a link between the specific blood clots and the AstraZeneca vaccine is stronger, but more work is needed. Their experts found a slightly higher incidence in younger adults, which led them to recommend that countries take the pattern into account when weighing the vaccine's use.

For their latest assessment, they looked at reports through Mar 31, which included 79 cases, 44 of them CVTS with thrombocytopenia and 35 involving thrombosis of major veins with thrombocytopenia. The majority were in women, but the MHRA notes that more women have been vaccinated against COVID-19. All of the events occurred after the first vaccine dose, and 19 people died. The blood clot cases were among 20.2 million in the United Kingdom who had been vaccinated, for an overall risk of 4 in 1 million.

The MHRA also detailed advice for clinicians, recommending that vaccination of people at higher risk of blood clots be considered only if benefits outweigh potential risk. They also said that pregnant women should discuss the vaccine decision with their health provider, given the known risk of thrombosis with pregnancy. People who have symptoms such as new headache, blurred vision, or shortness of breath 4 or more days after vaccination should seek prompt medical care.

June Raine, MD, MSc, MHRA's chief executive, said in a statement from the group that no effective medicine or vaccine is without risk, and the group will continue to monitor the safety of any vaccine to identify any safety signals and ensure that the benefits continue to outweigh the risks. "It is still vitally important that people come forward for their vaccination when invited to do so," she said.

WHO: Weigh rare events against COVID death risk

In an interim statement, the WHO said its vaccine advisory group met today, and while acknowledging a possible link, it noted that the blood clot numbers in people who have received the vaccine are very low compared with the nearly 200 million people across the world who have received the AstraZeneca vaccine.

It also said the rare events should be weighed against the 2.86 million people who have died from COVID-19.

"WHO is carefully monitoring the rollout of all COVID-19 vaccines and will continue to work closely with countries to manage potential risks, and to use science and data to drive response and recommendations," it said.

More global headlines

  • In the Americas, all four countries in the Southern Cone are experiencing surges, with cases in parts of Bolivia and Colombia doubling over the last week, officials from the Pan American Health Organization (PAHO) said today at a briefing. Intensive care unit (ICU) beds are near capacity in Peru and Ecuador. However, overall virus activity is mixed, with cases slowing in some countries but rising in others, such as Costa Rica, Honduras, and Guatemala.

  • Brazil's daily deaths topped 4,000 for the first time yesterday, and the Fiocruz Institute said in an update that ICU levels have eased a bit in some states but that the country's situation will likely remain critical through April.

  • India today reported a record daily high of more than 115,000 cases, and officials in hard-hit Maharashtra state have asked the government for more vaccine, due to shortages.

  • In Europe, Turkey yesterday reported a new single-day high of 49,584 cases, which come just days ahead of the start of a national lockdown, and Hungary today began easing its restrictions, even though its cases are still surging.

  • In Japan, Osaka prefecture—the country's hot spot—announced a state of emergency today and barred the Olympic torch relay from public roads, and South Korea suspended the use of AstraZeneca vaccine for those younger than 60, pending a review from European officials.

This week's top reads