Some covid-19 adenovirus vaccines including AstraZenca and Johnson & Johnson are found to have potential link to a rare albeit serious side effect resulting in blood clots with low platelets, particularly in younger patients.

Despite stating that the benefits outweigh the risk, many drug agencies / governments recommend for younger age groups to have a different vaccine (mRNA).

However in other countries this is not possible, for example there are no mRNA vaccines. Assuming that one has access to blood tests (e.g. using private healthcare) and wants to get tested in order to reduce the risk:

  1. should a blood count be done before taking the vaccine? how long before the first dose?
  2. how long after each dose should a blood count be done?



AstraZeneca blood clots link: https://www.webmd.com/vaccines/covid-19-vaccine/news/20210422/scientists-find-how-astrazeneca-vaccine-causes-clots

Johnson & Johnson potential blood clots (NBC article quoting the CDC): https://www.nbcnews.com/health/health-news/johnson-johnson-vaccine-linked-28-cases-blood-clots-cdc-reports-n1267128

NHS (UK government health system) providing an alternative to under 40s: https://www.nhs.uk/conditions/coronavirus-covid-19/coronavirus-vaccination/coronavirus-vaccine/

  • What is it you expect blood counts to reveal? If there's a blood test available that predicts this possible side effect, I've never heard of it.
    – Carey Gregory
    Commented Jun 4, 2021 at 20:22
  • References added. I expect blood counts to determine the level of platelets, finding low platelets before it is too late (e.g. thrombocytopenia). Thanks. Commented Jun 4, 2021 at 22:07
  • I haven't seen anything to suggest that preexisting low platelets is a known risk for vaccine-related thrombocytopenia, and so I can't imagine there are any guidelines for this sort of testing, which is also unlikely to be possible or economical at the scale that vaccinations are required given the rarity of the event.
    – Bryan Krause
    Commented Jun 4, 2021 at 23:22
  • "Thrombotic risk is not a function of the platelet count, and thrombocytopenia does not protect the patient from thrombosis." -- https://pubmed.ncbi.nlm.nih.gov/20008204/
    – Carey Gregory
    Commented Jun 5, 2021 at 4:22

1 Answer 1


Vaccine-induced immune thrombotic thrombocytopenia (VITT): the incidence is unknown but it appears to be exceedingly rare. Risk factors for VITT are also unknown, although female sex and younger age (<55-60 yrs) were proposed as possible risk factors on initial reports. A blood test before taking these vaccines will not be informative.

When VITT is suspected and the patient has received one of adenoviral-vectored vaccines (Astra Zeneca or Janssen/Johnson and Johnson) within the previous 5 to 30 days, appropriate tests including but not limited to blood counts are indicated and should be evaluated by a physician.


Thrombotic Thrombocytopenia after ChAdOx1 nCov-19 Vaccination. N Engl J Med. 2021; https://pubmed.ncbi.nlm.nih.gov/33835769/

Thrombosis and Thrombocytopenia after ChAdOx1 nCoV-19 Vaccination. N Engl J Med. 2021; https://pubmed.ncbi.nlm.nih.gov/33835768/

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