People who have experienced Vaccine-induced Thrombotic Thrombocytopenia (VITT) after AstraZeneca vaccine can safely receive a second vaccination with an mRNA vaccine, findings from a European study suggest.

A team, led by haematologist Professor Andreas Greinacher from University Medicine Greifswald in Germany undertook an observational study in which 65 patients who had VITT after receiving adenovirus vector vaccine (ChAdOx1 nCoV-19 )were followed for 25 weeks.

The researchers looked for changes in reactivity of platelet-activating anti-platelet factor 4 (PF4) IgG antibodies, via an anti-PF4/heparin IgG enzyme immunoassay (EIA) and a functional test for PF4-dependent, platelet-activating antibodies, as well as new thrombotic complications.

The results, published in the journal Blood, showed that the functional assay become negative in 74% of patients, with the median time to a negative test result 15.5 weeks (range 5-28 weeks).

Of 53 patients who were followed-up for 20 weeks or more after onset of VITT, 62% showed a negative platelet-activation assay within 20 weeks, 21% within 20-30 weeks, and 17% showed platelet-activating antibodies until the last available sample.

“We therefore recommend to follow VITT patients for at least 20 weeks after acute VITT to recognise those with a persisting prothrombotic state,” the authors noted.

Also, anti-PF4 IgG antibody levels as measured by OD in the anti-PF4/heparin IgG reduced in most patients (from median 3.12 to 1.52), but sero-reversion to a negative result was seen in just 22% patients.

Five patients (8%) exhibited persistent platelet-activating antibodies and high EIA ODs for more than 11 weeks. All received therapeutic dose anticoagulation post VITT occurred; two experienced recurrent episodes of thrombocytopenia while the other three remained asymptomatic.

Reassuringly, none of the 29 VITT patients given a second vaccination dose with an mRNA COVID-19 vaccine developed new thromboses or a relevant increase in anti-PF4/heparin IgG EIA OD.

The data showed that in 20 out of 22 patients monitored after vaccination platelet counts remained stable. Two showed a decrease of 25%-27%, but both showed decreasing anti-PF4 IgG EIA OD values and no recurrence of platelet-activating antibodies.

Also, in 15 patients a negative platelet activation assay was recorded before the second vaccination shot, while in 10 patients it remained positive.

“Our observations indicate that mRNA vaccines are very well tolerated regardless of whether PF4-dependent platelet activating antibodies are still circulating or not,” the authors concluded.

“In addition, this is very strong in-vivo evidence that the mRNA vaccines do not contain or induce the cofactor required for anti-PF4 antibody mediated prothrombotic activation of platelets.”

“This important study provides further reassurance of the safety of giving an mRNA vaccine to patients who have experienced VITT after ChAdOx, and supports our observations in the UK where VITT patients have received subsequent COVID-19 mRNA vaccines without adverse effect,” Dr Sue Pavord, a Consultant Haematologist at Oxford University Hospitals, told the limbic.

“Vaccination against COVID-19 infection remains important for individuals and whole populations,” she stressed.