COVID-19 and blood clotting risks

An image spreading on social media refers to hospital patients.

With concerns over rare blood clots, Denmark suspends use of the Oxford-AstraZeneca vaccine. An image comparing the ‘risk of blood clots’ spreads:

We can translate sources for each number:

Oxford-AstraZeneca (Vaxzevria) vaccine

The source is either unstated or incorrect. The estimate is from the UK’s Medicines and Health products Regulatory Authority:

By 31 March 20.2 million doses of the COVID-19 Vaccine AstraZeneca had been given in the UK meaning the overall risk of these blood clots is approximately 4 people in a million who receive the vaccine.

The European Medicines Agency used a figure from the Paul-Ehrlich-Institut in Germany. Their estimated rate was about one in 100,000 vaccinations. Disparities between national estimates come from coverage, case definition, periods, and population differences.

In general, blood clots can range from mild to life-threatening. The incidence refers to unusual blood clotting with low platelets. Another name is vaccine-induced immune thrombotic thrombocytopenia (VITT). There is building evidence of the importance of age and these rare and serious blood clots. Estimates can also update with more cases and vaccinations.

Researchers smoothed the observed rates by age. (Image: Winton Centre)

Combined hormonal contraceptive pills

This estimate comes from the European Medicines Agency, in a 2013 review:

The risk of blood clots in the veins varies between CHCs, depending on the type of progestogen (a hormone) they contain, and ranges from 5 to 12 cases of blood clots per 10,000 women who use them for a year[.]

It refers to the incidence of blood clotting in the veins, in a year. That incidence differs depending on the kind of hormone. The underlying rate for women who do not take these pills is about two per 10,000 women each year.


This figure is from a 2013 meta-analysis of blood clotting in the veins amongst smokers:

From eight population-based studies that reported information on person-years in smokers and nonsmokers, we could calculate absolute annual rates of VTE cases from the general population: 176.3 cases per 100,000 person-years in smokers and 152.0 cases in nonsmokers, corresponding to an absolute risk increase of 24.3 (95% CI 15.4–26.7) cases per 100,000 person-years.

The estimate carries uncertainty. This is the incidence amongst smokers each year. It is not the absolute increase in risk from smoking.

COVID-19 infection

Heart Research Institute UK suggests COVID-19 patients in intensive care have more clots:

These rates are much higher than we would usually see in patients requiring admission to ICU for reasons other than COVID-19.

The figure comes from a recent meta-analysis on COVID-19 patients in hospital:

The estimated overall incidence rates of PE and DVT were 16.5% (95% CI: 11.6, 22.9) and 14.8% (95% CI: 8.5, 24.5), respectively.

The 16.5% figure is for pulmonary embolism among COVID-19 patients — not cases. There is uncertainty around this number: the interval estimate is 11.6% — 22.9%. That may be an overestimate, as this meta-analysis could miss studies with no such events.

In England, the ratio of confirmed cases to COVID-19 hospitalisations is about one in 10. Testing will not detect all infections. Some infections are yet to reach a hospital room. This ratio is likely to overstate the rate of infected people reaching hospitals.

The image refers to estimated incidences of different types of blood clots. Despite the rubric, blood clotting differs in nature and severity.

Incidence rates can be comparable. The hazards of each clotting event are not. In error, the image refers to COVID-19 ‘cases’ — the figure is for clotting in hospital patients.

This blog looks at the use of statistics in Britain and beyond. It is written by RSS Statistical Ambassador and Chartered Statistician @anthonybmasters.

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