The action taken by some countries to temporarily pause or restrict to certain age groups the use of Oxford-AstraZeneca vaccine (Vaxzevria) has been based mainly on isolated reports -less than 1 in 100.000 doses administered to date- of cerebral sinus vein or mesenteric vein thrombosis occurring together with thrombocytopenia (lowered platelets) shortly after vaccination, a syndrome now called Thrombosis with Thrombocytopenia Syndrome (TTS). This type of thrombosis can also occur naturally in the absence of vaccination, and it can occur in association with COVID-19 disease. This rare adverse event has been linked to the vaccine by EMA and investigation continues. However, EMA concludes that the benefit-risk balance of the vaccine remains positive and vaccination should continue normally.
One in five hospitalized patients with COVID-19 develop thrombosis. On the other hand, the results of the recent AstraZeneca US Phase III trial of Vaxzevria[^1] confirms 76% (CI: 68% to 82%) vaccine efficacy against symptomatic COVID-19 and 100% efficacy against severe or critical disease and hospitalization.
Most of the countries have resumed vaccination and continue to use Oxford-AstraZeneca vaccine. Some countries have restricted its use to specific age groups or decided they need more time to continue investigations. These countries may also have access to other COVID-19 vaccines to continue vaccination.
[^1]: Clinicaltrials.gov. A Phase III Randomized, Double-blind, Placebo-controlled Multicenter Study in Adults to Determine the Safety, Efficacy, and Immunogenicity of AZD1222, a Non-replicating ChAdOx1 Vector Vaccine, for the Prevention of COVID-19. [Online] Available at: <https://clinicaltrials.gov/ct2/show/NCT04516746?term=NCT04516746&draw=2&rank=1>. Last accessed: March 2021.