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AstraZeneca Covid-19 vaccine: How common are blood clots and should I be worried?

European drug regulators have confirmed a link between the Oxford-AstraZeneca Covid-19 shot and rare blood clots, as UK authorities recommended that people under 30 should take alternative vaccines.

Posted: Apr 8, 2021 6:41 PM
Updated: Apr 8, 2021 6:41 PM

Following weeks of speculation about whether Oxford-AstraZeneca's Covid-19 vaccine might be connected to dozens of cases of blood clots reported in the region, European drug regulators have now announced that there is a possible link.

The European Medicines Agency (EMA) stopped short of changing its guidance on how the vaccine is used, maintaining that the overall benefits still outweigh the risks, while some European countries are restricting use of the shot to older age groups only.

On the same day, UK authorities also confirmed the link and emphasized that the benefits of the AstraZeneca shot outweighed its risk overall, but took a different course of action for people under the age of 30, advising that they are given different vaccines, if available.

The regulators have cleared up some questions around the AstraZeneca shot but raised many others. Here are the answers to some of them.

What are the risks of taking the AstraZeneca vaccine?

EMA concluded on Wednesday that the vaccine had caused an unusual combination of blood clots with low counts of blood platelets in dozens of people. It had analyzed 62 cases of cerebral venous sinus thrombosis (CVST), which is clotting in the sinuses that drain blood from the brain, and 24 cases of splanchnic vein thrombosis, or clotting in the abdomen, in people who had received the shot as of March 22. Of those cases -- reported from an area where around 25 million people total had received the AstraZeneca vaccine at that point -- 18 were fatal.

EMA said that these severe blood clotting events were being reported at a rate of about 1 in 100,000. Without age and sex data, it's not known if the risk is higher or lower for particular groups, it said. The overall rate could also change upwards or downwards, as more people are vaccinated and a clearer picture emerges.

In the UK, there were reports of 79 serious blood clots alongside low blood platelet counts. Nineteen of those people died as of March 31, the country's Medicines and Healthcare products Regulatory Agency (MHRA) said. Some of these deaths may have already have been calculated in the earlier EMA analysis.

In terms of the blood clotting events with low blood platelet counts, the overall risk is roughly 1 in 250,000, when considering about 20 million people have received the AstraZeneca vaccine in the UK. Considering the clots appear to be infecting younger people and women in higher numbers, it's possible that the risk could be higher once that data is narrowed down to specific groups.

Are the clots more common in women?

The number of reported blood clots is relatively small, and the data on them is limited, so it's difficult to draw any conclusions about who is most likely to suffer the side effects.

EMA's safety committee chair Sabine Straus said that, so far, most of the cases have occurred in women under age 60, but cautioned the agency did not have enough data based on age and sex to be sure about any particular risk profiles.

The agency can't be sure, for example, that women are not experiencing these clotting events in higher numbers simply because more women are being vaccinated. But it also means it hasn't ruled out the possibility that women are at greater risk.

UK regulators said something similar. Of the 79 cases that were documented, 51 were women and 28 were men. But more women have been vaccinated. Authorities did not present data on Wednesday on what they think the incidence of clotting in women might be.

When EMA says the benefits outweigh the risks, it means that is the case when it looked at the overall picture, pooling everyone together, regardless of age or sex.

Straus conceded, when challenged by journalists on Wednesday, that EMA did not have the data to understand the extent to which benefits might still outweigh risks for particular groups, such as for women or younger age groups.

Women are more predisposed to certain clotting events, such as CVST, than men, so a question for further analysis is whether women in particular are experiencing these clotting events at a higher incidence than usual.

Are younger people more likely to get these clots?

UK health authorities had clearer data on age than sex, saying Wednesday that there was a slightly higher incidence of these blood clotting events in younger adults compared to older ones. And when you consider that Covid-19 is less likely to seriously affect younger people, in some scenarios, the risks actually outweigh the benefits.

That's why Britain took the decision to give people under age 30 alternatives to the AstraZeneca vaccine.

An analysis by the Winton Centre for Risk and Evidence Communication at the University of Cambridge, shared at the UK government's news conference on Wednesday, found that the vaccine was more likely to cause serious harm in 20- to 29-year-olds that it was in preventing them getting so ill they would require intensive care treatment.

Roughly, for every three people the vaccine helps prevent the need for intensive care treatment, four others will experience serious harm from the shot, in a situation where there is low exposure to the virus.

As exposure to the virus increases, as it will when lockdown restrictions loosen, the situation is reversed. People in this age group are twice as likely to benefit from the shot -- in terms of avoiding the need for intensive care -- than they are to be seriously harmed by it. And in a high-exposure situation -- say, if things were back to normal -- these benefits would be more than six times higher than these risks.

The potential blood clotting side effects appear to become less likely in older age groups, contrasting with the increasing dangers of Covid-19 among older people, according to the UK's experts.

Their data suggested that for those in their 60s, 0.2 people per 100,000 would suffer serious harms due to the vaccine -- about 2 people in a million.

But for every one of those cases, there would have been 70 people in their 60s admitted to intensive care units (ICUs) last month, when the risk of Covid-19 was relatively low.

During the peak of the UK's second wave, that number balloons to about 640 ICU admissions for every person in their 60s to suffer vaccine complications, according to this data.

As lockdowns ease, the threat of Covid-related complications is likely to rise given increased transmission -- while the threat of dangerous vaccine side effects should remain at the same low level.

What if I'm already more at risk of blood clots?

Blood clots are common, and a significant portion of the population is already more predisposed to develop them as a result of underlying conditions, genetics, lifestyle or other medication they're taking.

For example, studies estimate that anywhere between 3% and 15% of people in Europe have Factor V Leiden, a genetic condition passed through generations that increases the risk of blood clots. In the United States, between 5% and 8% of people have one of several genetic risk factors, according to the Centers for Disease Control and Prevention.

But the blood clots that authorities are seeing as unusual are specific severe clotting around the brain and abdomen, and are accompanied by low counts of blood platelets. This is an unusual combination because platelets typically help the body's blood clot in a good way, to stop bleeding, for example. Usually high platelet counts and severe clotting go together.

In the UK, regulators said that as a precaution, people "of any age who are at higher risk of blood clots because of their medical condition" should be given the vaccine "only if benefits from the protection from Covid-19 infection outweighs potential risks."

It's already advised that people should talk to their doctor or pharmacist before getting an AstraZeneca vaccine if they have had problems with bleeding or bruising, or if they are taking blood thinning medication.

If I've had a first dose, should I get a second?

Unless you've experienced blood clotting after your first AstraZeneca dose, UK authorities say you should still get your second, while EMA is still advising everyone to take the shot, with the exception of people with conditions who have been advised against it.

"Anyone who did not have these side effects should come forward for their second dose when invited," the MHRA said.

Most cases of blood clotting occurred within two weeks of a person getting their first dose, EMA said, though there is naturally less evidence about second doses because fewer have been administered. In the UK, to date, no reports have followed a second dose. But again, far fewer second doses have been administered.

Still, both agencies say the benefits of the vaccine far outweigh the risks -- so the advice remains to go ahead with AstraZeneca vaccination appointments unless told otherwise.

If you have concerns, it's worth speaking to your medical provider about them.

EMA also warns that there isn't enough data yet on switching vaccines for your second dose. Some ongoing trials are looking into the effect of taking two different vaccines.

How do I identify a blood clot?

Regulators have advised that patients be informed of the possible side effects of the AstraZeneca vaccine, as they would be for any medications or vaccinations that carry potential risks.

It's important to remember that mild to moderate side effects are relatively common in the hours and days after taking any of the approved Covid-19 vaccines.

For AstraZeneca, these can include tenderness at the site of the injection, fatigue, headaches, nausea, chills, or a general unwell feeling, says the EMA. One or more of those are very common; they affect more than one in 10 people, the UK's MHRA advises.

It's less common for side effects to last more than a few days, and EMA advises patients seek urgent medical attention if, in the weeks after being vaccinated, they experience symptoms like shortness of breath, chest pain, leg swelling, persistent pain in the stomach, neurological symptoms, such as severe headaches or blurred vision, or tiny blood spots under the skin. They could be signs of a blood clot, or another adverse reaction.

Is the vaccine still safe?

Reports of blood clots have added to the information countries have about the AstraZeneca vaccine, but they haven't changed the overriding conclusion that the shot is saving more lives than it is putting at risk.

As mentioned, in the UK, the rate of people who have developed these unusual blood clots with low platelet counts is roughly 1 in 250,000, or 4 in a million.

"The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of the vaccine in preventing Covid-19 outweigh the risks of side effects," EMA concluded on Wednesday.

Again, this conclusion takes a big-picture look at what's happening, and UK data shows that age is a factor worth considering. Younger people may also want to consider their role in transmitting the virus to older people, who have a higher risk of becoming severely ill.

Real-world data shows that the vaccine is reducing hospitalizations and deaths from Covid-19. A Public Health England analysis estimates that 6,100 deaths were prevented in people age 70 and older in England up to the end of February, after the first few weeks of the UK's vaccination program.

And a single dose of the shot reduced the risk of hospitalization from Covid-19 by more than 80% in people older than 80, and gave up to 73% protection against symptomatic disease, the same agency found last month.

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