TORONTO, May 11 (Reuters) - The Canadian province of Ontario will stop offering first doses of AstraZeneca's COVID-19 vaccine because of evidence that the risk of rare blood clots is somewhat higher than previously estimated, provincial officials said on Tuesday.

"This decision was made out of an abundance of caution due to an observed increase in the rare blood clotting known as vaccine-induced immune thrombotic thrombocytopenia," said Chief Medical Officer David Williams.

"We are reviewing the data to consider options for the use of AstraZeneca for second doses and more broadly moving forward." (Reporting by Moira Warburton in Vancouver and Allison Martell in Toronto, Editing by Rosalba O'Brien)