The action taken by some countries to temporarily pause or restrict to certain age groups the use of the Oxford-AstraZeneca vaccine (Vaxzevria) has been based mainly on isolated reports -less than 1 to few cases per million doses administered to date- of cerebral sinus vein or mesenteric vein thrombosis occurring together with thrombocytopenia (lowered platelets) shortly after vaccination. This type of thrombosis can also occur naturally in the absence of vaccination, and it can occur in association with COVID 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 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.
 Clinicaltrials.gov. APhase III Randomized, Double-blind, Placebo-controlled Multicenter Study inAdults to Determine the Safety, Efficacy, and Immunogenicity of AZD1222, aNon-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: March2021.