April 23 (UPI) -- The World Health Organization's vaccine advisory group updated its guidance for the use of the AstraZeneca vaccine to include a rare blood clotting syndrome as a precaution.
The WHO announced the update in a press release Thursday, stating the decision was made by its Strategic Advisory Group of Experts on Immunization based on new data that continues to emerge through ongoing clinical trials and monitoring of the vaccine's rollout worldwide.
The update states thrombosis with thrombocytopenia syndrome, a very rare condition of blood clotting combined with low platelet counts, has been reported between four and 20 days after administering the vaccine jointly developed by AstraZeneca and Oxford University.
It said a "casual relationship" between the vaccine and the syndrome is considered possible and further investigation is ongoing with most cases having been reported in Britain and the European Union among younger people.
"However, benefit-risk assessments may differ from country to country, and countries should consider their epidemiological situation, individual and population-level risks, availability of other vaccines and alternate options for risk," the updated advisory said, adding the benefit-risk ratio is greatest among older demographics as they are at higher risk of negative outcomes if they contract the virus.
Cases of the rare blood clotting syndrome have been connected with both the AstraZeneca two-shot vaccine and Johnson & Johnson's one-shot jab.
On Tuesday, the European Union's medical regulator called for warnings of blood clotting to come with the Johnson & Johnson shot.
The United States, the European Union and South Africa had temporally halted administrating the Johnson & Johnson vaccine over the issue, with the EU restarting its campaign after updating its guidance on Tuesday and South Africa shortly after saying it will also resume use of the drug.
The U.S. Centers Disease Control and Prevention's advisory committee on immunization practices is to meet Friday when it is expected to decide if the halt should be lifted.
Denmark last week ordered officials to stop administering the AstraZeneca shot due to its possible link to the blood clotting syndrome.
More than a dozen European nations had halted administering the drug in the middle of last month with most restarting their inoculation campaigns days later when the European Medicines Agency said the shot's benefits far outweigh its risks.