An abundance of clots?

clot n. 1. a soft thick lump or mass: a clot of blood. 2. Brit. informal. a stupid person; fool.

Collins English Dictionary (1986)

As David Spiegelhalter (2021) points out, blood clots happen all the time: "at least 100 every week" in a population of 5 million. Set against this benchmark, reports of 30 clotting events among millions of vaccinated people are therefore unremarkable. Nevertheless, irresponsible media have widely reported this non-news. As a result, a number of EU nations have paused their use of the AstraZeneca vaccine against the virus that causes Covid-19. This decision was described by some as acting out of "an abundance of caution".

However, pausing part of the vaccination campaign, even for a week, will cost lives. Vaccination is not just a public-health measure, it is also a race the coronavirus behind Covid-19 will not stop spreading during the pause. People will continue to get infected, some will be hospitalised and some will die. Fewer vaccinations this week means more hospitalisations and deaths next month.

There is also a secondary effect that could have even bigger consequences, namely increased vaccine hesitancy. This term doesn't just cover the usual anti-scientific conspiracy theorists and cranks, but also ordinary people who are newly unnerved by the media reports. On a personal note, on Sunday I had to convince my father-in-law to take whatever vaccine is offered at his appointment tomorrow. In his late 70s and with a high-school level of education, he is not equipped to cut through the media cacophony on his own. There will be tens of millions of people like him.

One reason to be angry at the media (with honourable exceptions) is that there is unambiguous good news that has been overshadowed: those receiving the maligned AstraZeneca vaccine experienced a 94% reduction in hospitalisations from Covid-19 in Scotland compared to the unvaccinated. This latter statistic from Vasileiou et al (2021) comes with a confidence interval, data sources, a statement of funding, declarations of interest and approvals from both an ethics committee and a privacy panel. In short, some responsible, methodic scientific research.

And what of those reports about blood clots? While there isn't evidence linking thrombosis to vaccination, there is evidence linking it to Covid-19 infection (Aktaa et al, 2021). While the pandemic rages, thrombosis risk is actually higher for the unvaccinated. Pausing vaccinations is therefore not a cautious decision, nor even a neutral one it is damaging and reckless. The real clots of concern here have nothing to do with blood.


Aktaa, S., Wu, J., Nadarajah, R., Rashid, M., de Belder, M., Deanfield, J., Mamas, M. A. and Galea, C. P. (2021) Incidence and mortality due to thromboembolic events during the COVID-19 pandemic: Multi-sourced population-based health records cohort study, Thromb Res., 202: 1723, doi: 10.1016/j.thromres.2021.03.006.

Collins English Dictionary (1986), second edition, ISBN 0 00 433134-6.

Spiegelhalter, D. (2021) There's no proof the Oxford vaccine causes blood clots. So why are people worried?, The Guardian, 16th March 2021.

Vasileiou, E., Simpson, C. R., Robertson, C., Shi, T., Kerr, S., Agrawal, U., Akbari, A., Bedston, S., Beggs, J., Bradley, D., Chuter, A., de Lusignan, S., Docherty, A., Ford, D., Hobbs, R., Joy, M., Katikireddi, S. V., Marple, J., McCowan, C., McGagh, D., McMenamin, J., Moore, E., Murray, J. L. K., Pan, J., Ritchie, L., Shah, S., A., Stock, S., Torabi, F., Tsang, R. S. M., Wood, R., Woolhouse, M. and Sheikh, A. (2021) Effectiveness of First Dose of COVID-19 Vaccines Against Hospital Admissions in Scotland: National Prospective Cohort Study of 5.4 Million People, pre-print.


The author gladly received his AstraZeneca vaccine on March 4th 2021. And he won't hesitate to take his booster when it is offered.



Stuart McDonald
(Mar 16, 2021)
Excellent blog Stephen.

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Stephen Richards
Stephen Richards is the Managing Director of Longevitas