The AstraZeneca Covid-19 vaccine could possibly cause rare blood clots in some people but further studies are required to confirm the link, said experts with the World Health Organization (WHO). The opinion by the subcommittee of WHO’s Global Advisory Committee on Vaccine Safety (GACVS) follows a review of latest information from the European Medicines Agency (EMA), the United Kingdom’s regulatory body MHRA, and other countries. The EMA concluded that unusual blood clots with low blood platelets should be listed as very rare side effects of the AstraZeneca vaccine, while the MHRA said evidence of a link “is stronger but more work is still needed.” The UK has announced that it will offer an alternative shot to people under 30. “Based on current information, a causal relationship between the vaccine and the occurrence of blood clots with low platelets is considered plausible but is not confirmed. Specialised studies are needed to fully understand the potential relationship between vaccination and possible risk factors”, the WHO subcommittee said in an interim statement. The AstraZeneca vaccine constitutes a large part of the portfolio of the COVAX global solidarity initiative, which has already shipped more than 36 million doses worldwide to more than 86 countries. Though concerning, cases of blood clotting are very rare as low numbers have been reported among the nearly 200 million people worldwide who have received the vaccine, according to the statement. “Rare adverse events following immunizations should be assessed against the risk of deaths from Covid-19 disease and the potential of the vaccines to prevent infections and reduce deaths due to diseases”, the subcommittee said. The statement underlined the importance of vaccines against a disease which has killed some 2.6 million people worldwide, and that like all medicines, can have side effects. “The administration of vaccines is based on a risk versus benefit analysis”, the subcommittee said. Meanwhile, WHO is carefully monitoring the rollout of all Covid-19 vaccines, and will continue to work with countries to manage potential risks.