Scientists are exploring several possibilities that might explain at least 18 reports of extremely rare blood clots in the brain that occurred in individuals in the days and weeks after receiving the AstraZeneca Covid-19 vaccine, Reuters reported.

European investigators have put forward one theory that the vaccine triggers an unusual antibody in some rare cases; others are trying to understand whether the cases are linked with birth control pills.

But many scientists say there is no definitive evidence and it is not clear whether or why AstraZeneca's vaccine would cause an issue not shared by other vaccines that target a similar part of the coronavirus.

Most of the rare blood clots have been seen in women and most cases have been reported in Europe. Two cases have been reported in India.