WHO Committee Reviews Rare Blood Clots Linked to AstraZeneca COVID-19 Vaccines

The World Health Organization's Global Advisory Committee on Vaccine Safety (GACVS) has reviewed rare blood clotting disorders, known as Thrombosis with Thrombocytopenia Syndrome (TTS), occurring in relation to AstraZeneca's COVID-19 vaccines, Vaxzevria and Covishield. GACVS stated that based on the latest available data, the risk of TTS associated with these vaccines appears to be very low.

Data from the UK suggest the risk is approximately four cases per million vaccine doses, while estimates in the European Union are around one per 100,000 doses. GACVS emphasized that benefit-risk analyses should consider local epidemiology, including COVID-19 incidence and mortality, targeted age groups, and the availability of alternative vaccines.

The biological mechanism causing TTS is still under investigation, but a platform-specific mechanism related to adenovirus-vectored vaccines cannot be ruled out. Ongoing reviews of TTS cases and related research should include all vaccines using adenoviral vector platforms. TTS has not been linked to mRNA vaccines. Investigations into TTS following the Johnson & Johnson vaccine are also underway.

GACVS supports further research to understand risk factors, such as age and sex, as preliminary data suggest an increased risk in younger adults and women, though further analysis is required to confirm these findings. The committee recommends additional epidemiological, clinical, and mechanistic studies to fully comprehend TTS.

Clinicians are advised to be vigilant for symptoms of TTS, such as new, severe, persistent headaches or other significant symptoms like abdominal pain and shortness of breath, occurring between 4 to 20 days after adenovirus-vectored vaccination. In cases of suspected TTS, thrombosis and platelet levels should be investigated, and heparin administration should be avoided due to potential adverse effects.