By Douglas Busvine and John Miller
BERLIN/ZURICH (Reuters) - Instances of a very rare clotting condition in women aged under 60 who received AstraZeneca's COVID-19 vaccine were 20 times higher than would normally be expected, Christian Bogdan, a member of Germany's vaccine committee, said on Wednesday.
His comments came as the European Medicines Agency (EMA) and Britain's medical regulator acknowledged a possible link between the AstraZeneca vaccine to rare blood clots with low blood platelet counts. The EMA conducted an in-depth review of 86 cases, including 18 fatalities.
Most were in women, but with just 169 total cases reported to the EMA after 34 million doses had been given, they appear to be extremely rare. In comparison, four women out of 10,000 would get a blood clot from taking oral contraception.
Bogdan did not specify how many cases of blood clots with low blood platelet counts would be expected in a normal population, but said their higher prevalence in one population group over a defined timeframe represented a "very clear risk signal".
"We looked at how many cases occur in society as a background incidence, and compared that with cases observed between four and 16 days after vaccination," he told an online briefing.
"When you take that into account, then you come to an observed-to-expected ratio...of 20 (times higher) in women between 20 and 59 years old," he said.
The German committee recommended last week that people under 60 who had received one AstraZeneca shot should get a second dose of another vaccine.
Health experts also told the briefing, organised by the Science Media Center, that more research was needed to discover what may predispose people to clots with low blood platelet counts, and as well as the precise mechanism that leads to them.
One explanation, cited by the German vaccine committee members on Wednesday, suggests AstraZeneca's vaccine is associated with production of an unusual antibody that activates blood platelets and causes the rare clots; other investigators are investigating a link to birth control pills.
Andreas Greinacher, a scientist from Germany's Greifswald University whose team has linked the antibody to the rare clots, said his work indicates that neither birth control nor having a clotting factor mutation, called Factor V Leiden, play any role.
"Many individuals are afraid who have one of the common prothombotic underlying conditions like taking hormonal contraceptives or having Factor V Leiden, but all those factors do not modulate the risk for these antibodies," Greinacher told Reuters. "It's not a game changer, at all."
(Reporting by John Miller and Douglas Busvine; editing by David Evans and Philippa Fletcher)